Eumelanin-like Poly(levodopa) Nanoscavengers for Inflammation Disease Therapy

In the current years, polydopamine nanoparticles (PDA NPs) have been extensively investigated as an eumelanin mimic. However, unlike natural eumelanin, PDA NPs contain no 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-derived units and may be limited in certain intrinsic properties; superior eumelanin-like nanomaterials are still actively being sought. Levodopa (l-DOPA) is a natural eumelanin precursor and expected to convert into DHICA and further remain within the final product through covalent or physical interactions. Herein, poly(levodopa) nanoparticles [P(l-DOPA) NPs] were synthesized with the assistance of zinc oxide as a supplement to synthetic eumelanin. This study found that P(l-DOPA) NPs had ∼90% DHICA-derived subunits on their surface and exhibited superior antioxidant activity compared to PDA NPs due to their looser polymeric microstructure. Benefitting from a stronger ROS scavenging ability, P(l-DOPA) NPs outperformed PDA NPs in treating cellular oxidative stress and acute inflammation. This research opens up new possibilities for the development and application of novel melanin-like materials.

Designing bioinspired multifunctional nanoplatforms to support wound healing and skin regeneration: Mg-hydroxyapatite meets melanins

Melanin is a multifunctional biological pigment that recently emerged as endowed with anti-inflammatory, antioxidant, and antimicrobial properties and with high potentialities in skin protection and regenerative medicine. Here, a biomimetic magnesium-doped nano-hydroxyapatite (MgHA) was synthesized and decorated with melanin molecules starting from two different monomeric precursors, i.e. 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and dopamine (DA), demonstrating to be able to polymerize on the surface of MgHA nanostructures, thus leading to a melanin coating. This functionalization was realized by a simple and green preparation method requiring mild conditions in an aqueous medium and room temperature. Complementary spectroscopy and electron imaging analyses were carried out to define the effective formation of a stable coating, the percentage of the organic compounds, and the structural properties of resulting melanin-coated nanostructures, which showed good antioxidant activity. The in vitro interaction with a cell model, i.e. mouse fibroblasts, was investigated. The excellent biocompatibility of all bioinspired nanostructures was confirmed from a suitable cell proliferation. Finally, the enhanced biological performances of the nanostructures coated with melanin from DHICA were confirmed by scratch assays. Jointly our findings indicated that low crystalline MgHA and melanin pigments can be efficiently combined, and the resulting nanostructures are promising candidates as multifunctional platforms for a more efficient approach for skin regeneration and protection.

Structure prediction from spectra amidst dynamical heterogeneity in melanin

Melanin is a biopolymer pigment that plays a central role in skin photoprotection. Its extensive chemical and dynamical heterogeneity imparts this property through a broad featureless ultraviolet/visible absorption spectrum. Conventionally, the rational design of synthetic photoprotective pigments revolves around establishing the structure-spectra correlation and developing biomimetic materials with desired optical properties. This approach fails to explain the mechanistic details of melanin's absorption spectrum because it arises from an ensemble of structures rather than a local minimum on the potential energy surface. Here, we propose an inverse design approach to elucidate the contributions of dominant chromophoric units in various wavelength domains of the melanin spectrum.

The dual nature of biomimetic melanin

Melanin-inspired nanomaterials offer unique photophysical, electronic and radical scavenging properties that are widely explored for health and environmental preservation, or energy conversion and storage. The incorporation of functional melanin building blocks in more complex nanostructures or surfaces is typically achieved via a bottom-up approach starting from a molecular precursor, in most cases dopamine. Here we demonstrate that indeed, the oxidative polymerization of dopamine, for the synthesis of melanin-like polydopamine (PDA), leads to the simultaneous formation of more than one nanosized species with different compositions, morphologies and properties. In particular, a low-density polymeric structure and dense nanoparticles (NP) are simultaneously formed. The two populations could be separated and analyzed in real time using a chromatographic technique free of any stationary phase (flow field fractionation, FFF). The results following the synthesis of melanin-like PDA showed that the NP are formed only during the first 6 hours as a result of a supramolecular self-assembly-driven polymerization, while the formation of the polymer continues for about 36 hours. The two populations were also separated and characterized using TEM, UV-vis absorption spectroscopy, fluorescence and light scattering spectroscopy, DLS, FTIR, ζ-potential measurements, gel electrophoresis and pH titrations. Interestingly, very different properties between the two populations were observed: in particular the polymer contains a higher number of catechol units (8 mmol g-1 -OH) with respect to the NP (1 mmol g-1 -OH) and presents a much higher antioxidant activity. The attenuation of light by NP is more efficient than that by the polymer especially in the Vis-NIR region. Moreover, while the NP scatter light with an efficiency up to 27% they are not fluorescent, and the polymer does not scatter light but shows an excitation wavelength-dependent fluorescence typical of multi-fluorophoric uncoupled systems.

Catechol-Amyloid Interactions

This review introduces multifaceted mutual interactions between molecules containing a catechol moiety and aggregation-prone proteins. The complex relationships between these two molecular species have previously been elucidated primarily in a unidirectional manner, as demonstrated in cases involving the development of catechol-based inhibitors for amyloid aggregation and the elucidation of the role of functional amyloid fibers in melanin biosynthesis. This review aims to consolidate scattered clues pertaining to catechol-based amyloid inhibitors, functional amyloid scaffold of melanin biosynthesis, and chemically designed peptide fibers for providing chemical insights into the role of the local three-dimensional orientation of functional groups in manifesting such interactions. These orientations may play crucial, yet undiscovered, roles in various supramolecular structures.

Enhanced Photoacoustic Response by Synergistic Ag-Melanin Interplay at the Core of Ternary Biocompatible Hybrid Silica-Based Nanoparticles

Photoacoustics (PA) is gaining increasing credit among biomolecular imaging methodologies by virtue of its poor invasiveness, deep penetration, high spatial resolution, and excellent endogenous contrast, without the use of any ionizing radiation. Recently, we disclosed the excellent PA response of a self-structured biocompatible nanoprobe, consisting of ternary hybrid nanoparticles with a silver core and a melanin component embedded into a silica matrix. Although preliminary evidence suggested a crucial role of the Ag sonophore and the melanin-containing nanoenvironment, whether and in what manner the PA response is controlled and affected by the self-structured hybrid nanosystems remained unclear. Because of their potential as multifunctional platforms for biomedical applications, a detailed investigation of the metal-polymer-matrix interplay underlying the PA response was undertaken to understand the physical and chemical factors determining the enhanced response and to optimize the architecture, composition, and performance of the nanoparticles for efficient imaging applications. Herein, we provide the evidence for a strong synergistic interaction between eumelanin and Ag which suggests an important role in the in situ-generated metal-organic interface. In particular, we show that a strict ratio between melanin and silver precursors and an accurate choice of metal nanoparticle dimension and the kind of metal are essential for achieving strong enhancements of the PA response. Systematic variation of the metal/melanin component is thus shown to offer the means of tuning the stability and intensity of the photoacoustic response for various biomedical and theranostic applications.

Taphonomic experiments reveal authentic molecular signals for fossil melanins and verify preservation of phaeomelanin in fossils

Melanin pigments play a critical role in physiological processes and shaping animal behaviour. Fossil melanin is a unique resource for understanding the functional evolution of melanin but the impact of fossilisation on molecular signatures for eumelanin and, especially, phaeomelanin is not fully understood. Here we present a model for the chemical taphonomy of fossil eumelanin and phaeomelanin based on thermal maturation experiments using feathers from extant birds. Our results reveal which molecular signatures are authentic signals for thermally matured eumelanin and phaeomelanin, which signatures are artefacts derived from the maturation of non-melanin molecules, and how these chemical data are impacted by sample preparation. Our model correctly predicts the molecular composition of eumelanins in diverse vertebrate fossils from the Miocene and Cretaceous and, critically, identifies direct molecular evidence for phaeomelanin in these fossils. This taphonomic framework adds to the geochemical toolbox that underpins reconstructions of melanin evolution and of melanin-based coloration in fossil vertebrates.

Melanin: insights into structure, analysis, and biological activities for future development

Melanin, a widely distributed pigment found in various organisms, possesses distinct structures that can be classified into five main types: eumelanin (found in animals and plants), pheomelanin (found in animals and plants), allomelanin (found in plants), neuromelanin (found in animals), and pyomelanin (found in fungi and bacteria). In this review, we present an overview of the structure and composition of melanin, as well as the various spectroscopic identification methods that can be used, such as Fourier transform infrared (FTIR) spectroscopy, electron spin resonance (ESR) spectroscopy, and thermogravimetric analysis (TGA). We also provide a summary of the extraction methods of melanin and its diverse biological activities, including antibacterial properties, anti-radiation effects, and photothermal effects. The current state of research on natural melanin and its potential for further development is discussed. In particular, the review provides a comprehensive summary of the analysis methods used to determine melanin species, offering valuable insights and references for future research. Overall, this review aims to provide a thorough understanding of the concept and classification of melanin, its structure, physicochemical properties, and structural identification methods, as well as its various applications in the field of biology.

The rate and quality of post-mortem hair root changes in relation to melanin content

Hairs is a relatively environmentally resistant biological material that is often found at crime scenes. Human hair is more durable than other biological traces such as blood or urine, and its collection and storage does not require specific preservation procedures. Melanin is the hair pigment, which is the main determinant of hair colour. There are two pigments present in human hair: eumelanin, predominant in dark hair, and pheomelanin, responsible for red colour. Eumelanin is more resistant and has photoprotective properties, while pheomelanin is phototoxic and shows lower resistance to environmental factors. The differences in the properties of eu- and pheomelanin are the basis of the present study, which aimed to examine the rate and quality of taphonomic changes in hair roots in relation to the predominant melanin type, under the influence of selected environmental factors, such as soil pH, degree of exposure to solar radiation, temperature and water from a natural watercourse (river) and chemically pure water. Therefore, changes in blonde, dark, grey, red and dyed hair roots were microscopically documented for six months under the influence of the above factors. The results of the study indicated the strongest degradation potential among acidic soil and a riverine environment, as well as the protective role of eumelanin against environmental taphonomic factors. Degradation occurred most rapidly in the river environment, where microbial activity was additionally observed. Distilled water, exposure to sunlight and low temperature did not lead to decomposition changes. The results of our team's research provide the basis for an extended analysis of the changes occurring in hair under the influence of environmental factors in relation to melanin content.

Amyloid Fibers Increase Free Radicals of Synthetic Melanin

Functional amyloid fibers are crucial in melanogenesis, but their roles are incompletely understood. In particular, their relationship with intrinsic spin characters of melanin remains unexplored. Here, we show that adding an amyloid scaffold greatly augments the spin density in synthetic melanin. It also brings about concurrent alterations in water dispersibility, bandgaps, and radical scavenging properties of the synthetic melanin, which facilitates its applications in solar water remediation and protection of human keratinocytes from UV irradiation. This work provides implications in the unrevealed role of functional amyloid in melanogenesis and in the origin of the superiority of natural melanin toward its synthetic variants in terms of the spin-related properties.

Melanin biopolymers from microbial world with future perspectives-a review

Melanin is an amorphous polymer made of heterogeneous functional groups synthesized by diverse organisms including fungi, bacteria, animals, and plants. It was widely acknowledged for its biological processes and its key role in the protection of organisms from environmental stress. Recently, melanin clutches attention in the field of nanobiotechnology, drug delivery, organic semiconductors and bioelectronics, environmental bioremediation, photoprotection, etc., Furthermore, melanin from natural sources like microbial community shows antimicrobial, fighting cancer, radical scavenging, cosmeceuticals, and many therapeutic areas as well. Though the multipotentiality nature of melanin has been put forth, real-world applications still flag fall behind, which might be anticipated to the inadequate and high price essence of natural melanin. However, current bioprocess technologies have paved for the large-scale or industrial production of microbial melanin, which could help in the replacement of synthetic melanin. Thus, this review emphasizes the various sources for melanin, i.e., types-based on its pathways and its chemical structures, functional efficiency, physical properties, and conventional and modern methods of both extraction and characterization. Moreover, an outlook on how it works in the field of medicine, bioremediation, and other related areas provides perspectives on the complete exploitation of melanin in practical applications of medicine and the environment.

D-A Structured High-Performance Photothermal/Photodynamic Thionin-Synthetic Melanin Nanoparticles for Rapid Bactericidal and Wound Healing Effects

Synthesized melanin nanoparticles (SMNPs) are used as advanced photothermal materials. However, their internal structures are complex and disordered, and tuning the photothermal performance of nanoparticles is still a hot spot of concern. This article presents thionin (Th)-doped SMNPs, namely Th-SMNPs, which are the first SMNPs formed using the one-pot polymerization of Th with Levodopa. Th can undergo Michael addition and Schiff base reaction between indole dihydroxy/indolequinone and their oligomers to form donor-acceptor pairs in the structure to modulate the photothermal performance of SMNPs. Structural and spectroscopic analyses and density functional theory simulations further confirm the existence of the donor-acceptor structure. Th-SMNPs exhibit excellent total photothermal efficiency (34.49%) in the near-infrared region (808 nm), which is a 60% improvement compared to SMNPs. This allows Th-SMNPs to exhibit excellent photothermal performance at low power 808 nm laser irradiation. Meanwhile, Th not only enhances the photothermal properties of SMNPs, but also imparts photodynamic effects to SMNPs. Th-SMNPs can produce 1 O2 under 660 nm laser irradiation. A dual-function photothermal and photodynamic textile named Th-SMNPs@cotton is constructed based on Th-SMNPs, which can act as a rapid photothermal/photodynamic sterilization and is promising for wound healing treatment of bacterial infections under low-power dual laser irradiation.

Functionalization of and through Melanin: Strategies and Bio-Applications

A unique feature of nanoparticles for bio-application is the ease of achieving multi-functionality through covalent and non-covalent functionalization. In this way, multiple therapeutic actions, including chemical, photothermal and photodynamic activity, can be combined with different bio-imaging modalities, such as magnetic resonance, photoacoustic, and fluorescence imaging, in a theragnostic approach. In this context, melanin-related nanomaterials possess unique features since they are intrinsically biocompatible and, due to their optical and electronic properties, are themselves very efficient photothermal agents, efficient antioxidants, and photoacoustic contrast agents. Moreover, these materials present a unique versatility of functionalization, which makes them ideal for the design of multifunctional platforms for nanomedicine integrating new functions such as drug delivery and controlled release, gene therapy, or contrast ability in magnetic resonance and fluorescence imaging. In this review, the most relevant and recent examples of melanin-based multi-functionalized nanosystems are discussed, highlighting the different methods of functionalization and, in particular, distinguishing pre-functionalization and post-functionalization. In the meantime, the properties of melanin coatings employable for the functionalization of a variety of material substrates are also briefly introduced, especially in order to explain the origin of the versatility of melanin functionalization. In the final part, the most relevant critical issues related to melanin functionalization that may arise during the design of multifunctional melanin-like nanoplatforms for nanomedicine and bio-application are listed and discussed.

Chemiexcitation: Mammalian Photochemistry in the Dark†

Light is one way to excite an electron in biology. Another is chemiexcitation, birthing a reaction product in an electronically excited state rather than exciting from the ground state. Chemiexcited molecules, as in bioluminescence, can release more energy than ATP. Excited states also allow bond rearrangements forbidden in ground states. Molecules with low-lying unoccupied orbitals, abundant in biology, are particularly susceptible. In mammals, chemiexcitation was discovered to transfer energy from excited melanin, neurotransmitters, or hormones to DNA, creating the lethal and carcinogenic cyclobutane pyrimidine dimer. That process was initiated by nitric oxide and superoxide, radicals triggered by ultraviolet light or inflammation. Several poorly understood chronic diseases share two properties: inflammation generates those radicals across the tissue, and cells that die are those containing melanin or neuromelanin. Chemiexcitation may therefore be a pathogenic event in noise- and drug-induced deafness, Parkinson's disease, and Alzheimer's; it may prevent macular degeneration early in life but turn pathogenic later. Beneficial evolutionary selection for excitable biomolecules may thus have conferred an Achilles heel. This review of recent findings on chemiexcitation in mammalian cells also describes the underlying physics, biochemistry, and potential pathogenesis, with the goal of making this interdisciplinary phenomenon accessible to researchers within each field.

A Model Eumelanin from 5,6-Dihydroxyindole-2-Carboxybutanamide Combining Remarkable Antioxidant and Photoprotective Properties with a Favourable Solubility Profile for Dermo-Cosmetic Applications

The search for new synthetic melanin-related pigments that maintain the antioxidant and photoprotective properties of naturally occurring dark eumelanins, while overcoming their unfavorable solubility, and molecular heterogeneity is presently a very active issue for dermo-cosmetic purposes. In this work, we explored the potential of a melanin obtained from the carboxybutanamide of a major eumelanin biosynthetic precursor, 5,6-dihydroxyindole-2-carboxylic acid (DHICA), by aerobic oxidation under slightly alkaline conditions. Analysis of the pigment by EPR, ATR-FTIR and MALDI MS indicated a substantial structural similarity to DHICA melanin, while investigation of the early intermediates confirmed unchanged regiochemistry of the oxidative coupling. The pigment exhibited a UVA-visible absorption even more intense than that of DHICA melanin, and a noticeable solubility in polar solvents of dermo-cosmetic relevance. The hydrogen- and/or electron-donor ability, and the iron (III) reducing power as determined by conventional assays provided evidence for marked antioxidant properties not merely ascribable to the more favorable solubility profile, while the inhibitory action of the radical- or photosensitized solar light-induced lipid peroxidation was more marked compared to that of DHICA melanin. Overall, these results hint at this melanin, which remarkable properties are, in part, due to the electronic effects of the carboxyamide functionality as a promising functional ingredient for dermo-cosmetic formulations.

Fungi as a source of eumelanin: current understanding and prospects

Melanins represent a diverse collection of pigments with a variety of structures and functions. One class of melanin, eumelanin, is recognizable to most as the source of the dark black color found in cephalopod ink. Sepia officinalis is the most well-known and sought-after source of non-synthetic eumelanin, but its harvest is limited by the availability of cuttlefish, and its extraction from an animal source brings rise to ethical concerns. In recent years, these limitations have become more pressing as more applications for eumelanin are developed-particularly in medicine and electronics. This surge in interest in the applications of eumelanin has also fueled a rise in the interest of alternative, bio-catalyzed production methods. Many culinarily-utilized fungi are ideal candidates in this production scheme, as examples exist which have been shown to produce eumelanin, their growth at large scales is well understood, and they can be cultivated on recaptured waste streams. However, much of the current research on the fungal production of eumelanin focuses on pathogenic fungi and eumelanin's role in virulence. In this paper, we will review the potential for culinary fungi to produce eumelanin and provide suggestions for new research areas that would be most impactful in the search for improved fungal eumelanin producers.

Characterization of pyomelanin secreted by Shewanella sp. and their application in metal recovery

Melanin is a biopolymer with versatile structural and functional properties and diverse applications in recovering toxic chemicals from water and wastewater, biomedical imaging, and as theragnostic agent. We report the structural characterization and biosynthetic pathway of an extracellular pyomelanin secreted by a sponge-associated bacterium, Shewanella sp. (Shewanella-melanin), and their potential application in metal recovery from liquid. Pyomelanin particles of > 50 µm size were found in the culture medium within 48 h of growth, which were formed through the self-polymerization of benzoquinone molecule produced through homogentisic acid pathway. The aspC and hppD genes involved in the biosynthetic pathway of pyomelanin were detected in the whole genome sequence of Shewanella sp. The FT-IR spectra of Shewanella-melanin, at ~ 3300-3420 cm^-1 corresponding to the stretching vibration of -NH and -OH, was in good agreement with that of Sepia melanin, while its elemental composition (C/N/H/S of 29.2:8.23:6.41:1.58) was unique. Shewanella-melanin showed ~ 300 and ~ 950 times increased chelation of manganese and iron from a liquid medium supplemented with 2 mM of MnSO₄ and FeSO₄, respectively, compared to a control. The FT-IR spectrum showed the binding of metal ions to the carboxylic acid, hydroxyl, and amine groups of Shewanella-melanin. The Shewanella-melanin, with its excellent metal biosorption, could be a potential candidate for removing toxic compounds from water, in turn contributing to the fulfillment of sustainable development goal (SDG) 6.

Hydrophobic Melanin via Post-Synthetic Modification for Controlled Self-Assembly

Allomelanin is a class of nitrogen-free melanin mostly found in fungi and, like all naturally occurring melanins, is hydrophilic. Herein, we develop a facile method to modify synthetic hydrophilic allomelanin to yield hydrophobic derivatives through post-synthetic modifications. Amine-functionalized molecules of various kinds can be conjugated to allomelanin nanoparticles under mild conditions with high loading efficiencies. Hydrophobicity is conferred by introducing amine-terminated alkyl groups with different chain lengths. We demonstrate that the resulting hydrophobic allomelanin nanoparticles undergo air/water interfacial self-assembly in a controlled fashion, which enables the generation of large-scale and uniform structural colors. This work provides an efficient and tunable surface chemistry modification strategy to broaden the scope of synthetic melanin structure and function beyond the known diversity found in nature.

Guaiacol as a natural melanin biosynthesis inhibitor to control northern corn leaf blight

BACKGROUND

The natural 1,8-dihydroxynaphthalene (DHN) melanin biosynthesis inhibitors (MBIs) are one of the promising approaches to the integrated management of plant diseases but have received scarce attention until now. Herein, to explore the natural DHN MBIs used in the control of northern corn leaf blight (NCLB), a library of 53 essential oil compounds was used to screen the MBIs against Exserohilum turcicum, the causal pathogen of NCLB, using tricyclazole as a reference compound.

RESULTS

The results of morphological change in the colony, thermogravimetric analysis, ultraviolet-visible spectroscopy, and transmission electron microscopy confirmed that guaiacol could effectively inhibit the melanin production at 50 μg/mL under in vitro conditions. The in vitro bioassay results indicated that this inhibition effect was concentration-dependent and the minimum inhibition concentration of guaiacol was 50 μg/mL. The in vivo experimental results demonstrated that guaiacol significantly inhibited appressorium formation and penetration on corn leaf sheaths at the concentration of 500 μg/mL. The pot experiment results revealed that there were no differences between guaiacol (500 μg/mL) and tricyclazole (100 μg/mL) in control efficacy. The enzymatic assay suggested that guaiacol might exert the activity through inhibiting DHN polymerization to form melanins, which was distinct from tricyclazole.

CONCLUSIONS

Taken together, this study screened out guaiacol as a natural MBI from 53 essential oil compounds and verified its effectiveness in the control of NCLB at 500 μg/mL. Above all, this research opened an avenue for exploring natural DHN MBIs in the integrated management of plant diseases. © 2022 Society of Chemical Industry.

Melanin of fungi: from classification to application

Melanin is a secondary metabolite composed of complex heterogeneous polymers. Fungal melanin is considered to be a sustainable and biodegradable natural pigment and has a variety of functional properties and biological activities. On one hand, due to its own specific properties it can play the role of antioxidant, anti-radiation, adsorption, and photoprotection. On the other hand, it has good biological activities such as hepatoprotective effect, hypolipidemic effect and anti-cancer. Therefore, it is widely used in various fields of daily life, including dyeing, food, biomedical and commercial industry. It is conducive to environmental protection and human health. However, the insolubility of fungal melanin in water, acids and organic solvents has been an obstacle to its commercial applications. Thus, the chemical modification methods of fungal melanin are summarized to increase its solubility and expand the application fields. Although fungal melanin has been used in many industries, as the structure and function of fungal melanin and modified melanin are further studied, more functional properties and bioactivities are expected to be discovered for a wide range of applications in the future.

Contrast mechanisms in pump-probe microscopy of melanin

Pump-probe microscopy of melanin in tumors has been proposed to improve diagnosis of malignant melanoma, based on the hypothesis that aggressive cancers disaggregate melanin structure. However, measured signals of melanin are complex superpositions of multiple nonlinear processes, which makes interpretation challenging. Polarization control during measurement and data fitting are used to decompose signals of melanin into their underlying molecular mechanisms. We then identify the molecular mechanisms that are most susceptible to melanin disaggregation and derive false-coloring schemes to highlight these processes in biological tissue. We demonstrate that false-colored images of a small set of melanoma tumors correlate with clinical concern. More generally, our systematic approach of decomposing pump-probe signals can be applied to a multitude of different samples.

A Straightforward Access to New Amides of the Melanin Precursor 5,6-Dihydroxyindole-2-carboxylic Acid and Characterization of the Properties of the Pigments Thereof

We report herein an optimized procedure for preparation of carboxamides of 5,6-dihydroxyindole-2-carboxylic acid (DHICA), the main biosynthetic precursor of the skin photoprotective agents melanins, to get access to pigments with more favorable solubility properties with respect to the natural ones. The developed procedure was based on the use of a coupling agent (HATU/DIPEA) and required protection of the catechol function by easily removable acetyl groups. The O-acetylated compounds could be safely stored and taken to the reactive o-diphenol form just before use. Satisfactorily high yields (>85%) were obtained for all amides. The oxidative polymerization of the synthesized amides carried out in air in aqueous buffer at pH 9 afforded melanin-like pigmented materials that showed chromophores resembling those of DHICA-derived pigments, with a good covering of the UVA and the visible region, and additionally exhibited a good solubility in alcoholic solvents, a feature of great interest for the exploitation of these materials as ingredients of dermocosmetic formulations.

Biomimetic Chiral Photonic Materials with Tunable Metallic Colorations Prepared from Chiral Melanin-like Nanorods for UV Shielding, Humidity Sensing, and Cosmetics

Many biological species combine the helical organization of cellulose or chitin microfibrils with broadband light absorption of black melanin to produce brilliant structural colors with metallic and glossy effects and other diverse functions. In this work, based on core-shell CNC@PDA chiral nanorods consisting of cellulose nanocrystals (CNCs) as the core and melanin-like polydopamine (PDA) as the shell that can form well-defined chiral liquid crystal phases, we report chiral photonic materials that closely mimic the unique coloration mechanisms and functionalities mastered by several biological species. The photonic films formed by such single CNC@PDA nanorods have brilliant iridescent structural colors originating from selective reflection of circularly polarized lights by the helical organization of CNC@PDAs across the films. Furthermore, the colors of such films have background-independent brightness, high visibility, and metallic effects that arise from the light absorption of the PDA component. Especially, the color ranges and metallic effects of the films can be conveniently tuned by varying the thickness of the PDA shell. In addition, the UV absorption and hygroscopic properties of PDA endow these CNC@PDA films with efficient broadband UV shielding and sensitive humidity-induced dynamic color changes. Due to the mussel-like superior adhesion of PDA, CNC@PDA-based photonic coatings can be formed conformably onto diverse kinds of substrates. A shiny eye shadow with viewing angle-dependent colorful patterns was used to demonstrate the potential applications. With combinations of multiple unique properties in one photonic material fabricated from a single building block, these CNC@PDA-based films are expected to have potential applications in cosmetics, UV protection, anticounterfeiting, chiral reflectors, etc.

Designed Amyloid Fibers with Emergent Melanosomal Functions

Short peptides designed to self-associate into amyloid fibers with metal ion-binding ability have been used to catalyze various types of chemical reactions. This manuscript demonstrates that one of these short-peptide fibers coordinated with Cu^II can exhibit melanosomal functions. The coordinated Cu^II and the amyloid structure itself are differentially functional in accelerating oxidative self-association of dopamine into melanin-like species and in regulating their material properties (e.g., water dispersion, morphology, and the density of unpaired electrons). The results have implications for the role of functional amyloids in melanin biosynthesis and for designing peptide-based supramolecular structures with various emergent functions.

Benchtop X-band electron paramagnetic resonance detection of melanin and Nitroxyl spin probe in zebrafish

EPR spectroscopy is a technique that provides direct information about free radicals in biological systems. So far, X-band EPR was seldomly used for in vivo studies as the small resonator size and high power used to detect EPR signals were unsuitable for living organisms. Here, we report new solutions which lift some limitations and make X-band EPR suitable for an in vivo detection of free radicals in zebrafish - a small laboratory animal that is often used as a model for various studies related to free radicals. We designed specially-shaped glass and quartz capillaries to ensure the zebrafish's safety during the experiments. The optimal EPR spectrometer parameters, safe for zebrafish embryos and sufficient to obtain EPR spectrum, were 4 scans by 20s, 100G sweep, and 0.8 mW power. Combining the specially-shaped capillary with a multi-harmonic analyzer for the EPR spectrometer allowed increasing the time up to 16 scans by 11s and lowering the power to 0.25 mW. As a proof of principle, we demonstrate the detection of melanin radicals and the 5-DSA spin probe in zebrafish larvae. As fish survive the EPR scans, the possibility of performing multiple measurements of free radicals in living zebrafish offers new tools for studies aiming to understand redox biology and membrane-dependent functions in both health and disease.

RGD-functionalised melanin nanoparticles for intraoperative photoacoustic imaging-guided breast cancer surgery

PURPOSE

Obtaining tumour-free margins is critical for avoiding re-excision and reducing local recurrence following breast-conserving surgery; however, it remains challenging. Imaging-guided surgery provides precise detection of residual lesions and assists surgical resection. Herein, we described water-soluble melanin nanoparticles (MNPs) conjugated with cyclic Arg-Gly-Asp (cRGD) peptides for breast cancer photoacoustic imaging (PAI) and surgical navigation.

METHODS

The cRGD-MNPs were synthesised and characterized for morphology, photoacoustic characteristics and stability. Tumour targeting and toxicity of cRGD-MNPs were determined by using either breast cancer cells, MDA-MB-231 tumour-bearing mice or the FVB/N-Tg (MMTV-PyVT) 634Mul/J mice model. PAI was used to locate the tumour and guide surgical resection in MDA-MB-231 tumour-bearing mice.

RESULTS

The cRGD-MNPs exhibited excellent in vitro and in vivo tumour targeting with low toxicity. Intravenous administration of cRGD-MNPs to MDA-MB-231 tumour-bearing mice showed an approximately 2.1-fold enhancement in photoacoustic (PA) intensity at 2 h, and the ratio of the PA intensity at the tumour site to that in the surrounding normal tissue was 3.2 ± 0.1, which was higher than that using MNPs (1.7 ± 0.3). Similarly, the PA signal in the spontaneous breast cancer increased ~ 2.5-fold at 2 h post-injection of cRGD-MNPs in MMTV-PyVT transgenic mice. Preoperative PAI assessed tumour volume and offered three-dimensional (3D) reconstruction images for accurate surgical planning. Surgical resection following real-time PAI showed high consistency with histopathological analysis.

CONCLUSION

These results highlight that cRGD-MNP-mediated PAI provide a powerful tool for breast cancer imaging and precise tumour resection. cRGD-MNPs with fine PA properties have great potential for clinical translation.

Characterization of an Insoluble and Soluble Form of Melanin Produced by Streptomyces cavourensis SV 21, a Sea Cucumber Associated Bacterium

Melanin is a widely distributed and striking dark-colored pigment produced by countless living organisms. Although a wide range of bioactivities have been recognized, there are still major constraints in using melanin for biotechnological applications such as its fragmentary known chemical structure and its insolubility in inorganic and organic solvents. In this study, a bacterial culture of Streptomyces cavourensis SV 21 produced two distinct forms of melanin: (1) a particulate, insoluble form as well as (2) a rarely observed water-soluble form. The here presented novel, acid-free purification protocol of purified particulate melanin (PPM) and purified dissolved melanin (PDM) represents the basis for an in-depth comparison of their physicochemical and biological properties, which were compared to the traditional acid-based precipitation of melanin (AM) and to a synthetic melanin standard (SM). Our data show that the differences in solubility between PDM and PPM in aqueous solutions may be a result of different adjoining cation species, since the soluble PDM polymer is largely composed of Mg²⁺ ions and the insoluble PPM is dominated by Ca²⁺ ions. Furthermore, AM shared most properties with SM, which is likely attributed to a similar, acid-based production protocol. The here presented gentler approach of purifying melanin facilitates a new perspective of an intact form of soluble and insoluble melanin that is less chemical altered and thus closer to its original biological form.

Growth modulation and metabolic responses of Ganoderma boninense to salicylic acid stress

Various phenolic compounds have been screened against Ganoderma boninense, the fungal pathogen causing basal stem rot in oil palms. In this study, we focused on the effects of salicylic acid (SA) on the growth of three G. boninense isolates with different levels of aggressiveness. In addition, study on untargeted metabolite profiling was conducted to investigate the metabolomic responses of G. boninense towards salicylic acid. The inhibitory effects of salicylic acid were both concentration- (P < 0.001) and isolate-dependent (P < 0.001). Also, growth-promoting effect was observed in one of the isolates at low concentrations of salicylic acid where it could have been utilized by G. boninense as a source of carbon and energy. Besides, adaptation towards salicylic acid treatment was evident in this study for all isolates, particularly at high concentrations. In other words, inhibitory effect of salicylic acid treatment on the fungal growth declined over time. In terms of metabolomics response to salicylic acid treatment, G. boninense produced several metabolites such as coumarin and azatyrosine, which suggests that salicylic acid modulates the developmental switch in G. boninense towards the defense mode for its survival. Furthermore, the liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) analysis showed that the growth of G. boninense on potato dextrose agar involved at least four metabolic pathways: amino acid metabolism, lipid pathway, tryptophan pathway and phenylalanine pathway. Overall, there were 17 metabolites that contributed to treatment separation, each with P<0.005. The release of several antimicrobial metabolites such as eudistomin I may enhance G. boninense's competitiveness against other microorganisms during colonisation. Our findings demonstrated the metabolic versatility of G. boninense towards changes in carbon sources and stress factors. G. boninense was shown to be capable of responding to salicylic acid treatment by switching its developmental stage.

Xanthurenic Acid in the Shell Purple Patterns of Crassostrea gigas: First Evidence of an Ommochrome Metabolite in a Mollusk Shell

Ommochromes are one of the least studied groups of natural pigments, frequently confused with melanin and, so far, exclusively found in invertebrates such as cephalopods and butterflies. In this study focused on the purple color of the shells of a mollusk, Crassostrea gigas, the first evidence of a metabolite of ommochromes, xanthurenic acid (XA), was obtained by liquid chromatography combined with mass spectrometry (UPLC-MS). In addition to XA and various porphyrins previously identified, a second group of high molecular weight acid-soluble pigments (HMASP) has been identified with physicochemical and structural characteristics similar to those of ommochromes. In addition, fragmentation of HMASP by tandem mass spectrometry (MS/MS) has revealed a substructure common to XA and ommochromes of the ommatin type. Furthermore, the presence of melanins was excluded by the absence of characteristic by-products among the oxidation residues of HMASP. Altogether, these results show that the purple color of the shells of Crassostrea gigas is a complex association of porphyrins and ommochromes of potentially ommatin or ommin type.

A triple-stimulus responsive melanin-based nanoplatform with an aggregation-induced emission-active photosensitiser for imaging-guided targeted synergistic phototherapy/hypoxia-activated chemotherapy

Multimodal synergistic therapy has gained increasing attention in cancer treatment to overcome the limitations of monotherapy and achieve high anticancer efficacy. In this study, a synergistic phototherapy and hypoxia-activated chemotherapy nanoplatform based on natural melanin nanoparticles (MPs) loaded with the bioreduction prodrug tirapazamine (TPZ) and decorated with hyaluronic acid (HA) was developed. A self-reporting aggregation-induced emission (AIE)-active photosensitizer (PS) (BATTMN) was linked to the prepared nanoparticles by boronate ester bonds. The MPs and BATTMN-HA played roles as quenchers for PS and cancer targeting/photodynamic moieties, respectively. As a pH sensitive bond, the borate ester bonds between HA and BATTMN are hydrolysed in the acidic cancer environment, thereby separating BATTMN from the nanoparticles and leading to the induction of fluorescence for imaging-guided synergistic phototherapy/hypoxia-activated chemotherapy under dual irradiation. TPZ can be released upon activation by pH, near-infrared (NIR) and hyaluronidase (Hyal). Particularly, the hypoxia-dependent cytotoxicity of TPZ was amplified by oxygen consumption in the tumor intracellular environment induced by the AIE-active PS in photodynamic therapy (PDT). The nanoparticles developed in our research showed favorable photothermal conversion efficiency (η = 37%), desired cytocompatibility, and excellent synergistic therapeutic efficacy. The proposed nanoplatform not only extends the application scope of melanin materials with AIE-active PSs, but also offers useful insights into developing multistimulus as well as multimodal synergistic tumor treatment.

Natural Melanin/Alginate Hydrogels Achieve Cardiac Repair through ROS Scavenging and Macrophage Polarization

The efficacy of cardiac regenerative strategies for myocardial infarction (MI) treatment is greatly limited by the cardiac microenvironment. The combination of reactive oxygen species (ROS) scavenging to suppress the oxidative stress damage and macrophage polarization to regenerative M2 phenotype in the MI microenvironment can be desirable for MI treatment. Herein, melanin nanoparticles (MNPs)/alginate (Alg) hydrogels composed of two marine-derived natural biomaterials, MNPs obtained from cuttlefish ink and alginate extracted from ocean algae, are proposed. Taking advantage of the antioxidant property of MNPs and mechanical support from injectable alginate hydrogels, the MNPs/Alg hydrogel is explored for cardiac repair by regulating the MI microenvironment. The MNPs/Alg hydrogel is found to eliminate ROS against oxidative stress injury of cardiomyocytes. More interestingly, the macrophage polarization to regenerative M2 macrophages can be greatly promoted in the presence of MNPs/Alg hydrogel. An MI rat model is utilized to evaluate the feasibility of the as-prepared MNPs/Alg hydrogel for cardiac repair in vivo. The antioxidant, anti-inflammatory, and proangiogenesis effects of the hydrogel are investigated in detail. The present study opens up a new way to utilize natural biomaterials for MI treatment and allows to rerecognize the great value of natural biomaterials in cardiac repair.

A Biomimetic Aggregation-Induced Emission Photosensitizer with Antigen-Presenting and Hitchhiking Function for Lipid Droplet Targeted Photodynamic Immunotherapy

Photodynamic therapy (PDT) is a promising alternative approach for effective cancer treatment that is associated with an antitumor immune response. However, immunosuppression of the tumor microenvironment limits the immune response induced by PDT. Stimulation and proliferation of T cells is a critical step for generating immune responses and depends on the efficient presentation of tumor antigens and co-stimulatory molecules by antigen-presenting cells (APCs). Here, biomimetic aggregation-induced emission (AIE) photosensitizers with antigen-presenting and hitchhiking abilities (DC@AIEdots) are developed by coating dendritic cell (DC) membranes on the nanoaggregates of the AIEgens. Notably, the inner AIE molecules can selectively accumulate in lipid droplets of tumor cells, and the outer cell membrane can facilitate the hitchhiking of DC@AIEdots onto the endogenous T cells and enhance the tumor delivery efficiency by about 1.6 times. Furthermore, DC@AIEdots can stimulate the in vivo proliferation and activation of T cells and trigger the immune system. The potential applications of therapeutic agents targeting lipid droplets for immunotherapy are indicated and a new hitchhiking approach for drug delivery is provided. Lastly, the study presents a photoactive and artificial antigen-presenting platform for effective T cell stimulation and cancer photodynamic immunotherapy.

Site-Specific Biomimicry of Antioxidative Melanin Formation and Its Application for Acute Liver Injury Therapy and Imaging

Biocompatible nano-antioxidants composed of natural molecules/materials, such as dopamine and melanin, are of great interest for diverse biomedical applications. However, the lack of understanding of the precise structure of these biomaterials and thus the actual dose of effective components impedes their advancement to translation. Herein, a strategy to mimic in situ melanin formation and explore its antioxidative applications is reported, by developing a PEGylated, phenylboronic-acid-protected L-DOPA precursor (PAD) that can self-assemble into well-defined nanoparticles (PADN). Exposure to oxidative species leads to deprotection of phenylboronic acids, transforming PADN to PEG-L-DOPA, which, similar to the biosynthetic pathway of melanin, can be oxidized and polymerized into an antioxidative melanin-like structure. With ultrahigh stability and superior antioxidative activity, the PADN shows remarkable efficacy in prevention and treatment of acute liver injury/failure. Moreover, the in situ structure transformation enables PADN to visualize damaged tissue noninvasively by photoacoustic imaging. Overall, a bioinspired antioxidant with precise structure and site-specific biological activity for theranostics of oxidative stress-related diseases is described.

Biodegradation of bio-sourced and synthetic organic electronic materials towards green organic electronics

Ubiquitous use of electronic devices has led to an unprecedented increase in related waste as well as the worldwide depletion of reserves of key chemical elements required in their manufacturing. The use of biodegradable and abundant organic (carbon-based) electronic materials can contribute to alleviate the environmental impact of the electronic industry. The pigment eumelanin is a bio-sourced candidate for environmentally benign (green) organic electronics. The biodegradation of eumelanin extracted from cuttlefish ink is studied both at 25 °C (mesophilic conditions) and 58 °C (thermophilic conditions) following ASTM D5338 and comparatively evaluated with the biodegradation of two synthetic organic electronic materials, namely copper (II) phthalocyanine (Cu-Pc) and polyphenylene sulfide (PPS). Eumelanin biodegradation reaches 4.1% (25 °C) in 97 days and 37% (58 °C) in 98 days, and residual material is found to be without phytotoxic effects. The two synthetic materials, Cu-Pc and PPS, do not biodegrade; Cu-Pc brings about the inhibition of microbial respiration in the compost. PPS appears to be potentially phytotoxic. Finally, some considerations regarding the biodegradation test as well as the disambiguation of "biodegradability" and "bioresorbability" are highlighted.

Gold Nanorod-Melanin Hybrids for Enhanced and Prolonged Photoacoustic Imaging in the Near-Infrared-II Window

Photoacoustic (PA) imaging holds great promise as a noninvasive imaging modality. Gold nanorods (GNRs) with absorption in the second near-infrared (NIR-II) window have emerged as excellent PA probes because of their tunable optical absorption, surface modifiability, and low toxicity. However, pristine GNRs often undergo shape transition upon laser illumination due to thermodynamic instability, leading to a reduced PA signal after a few seconds of imaging. Here, we report monodisperse GNR-melanin nanohybrids where a tunable polydopamine (PDA) coating was conformally coated on GNRs. GNR@PDAs showed a threefold higher PA signal than pristine GNRs due to the increased optical absorption, cross-sectional area, and thermal confinement. More importantly, the PA signal of GNR@PDAs only decreased by 29% during the 5 min of laser illumination in the NIR-II window, while significant attenuation (77%) was observed for GNRs. The GNR@PDAs maintained 87% of its original PA signal in vivo even after 10 min of laser illumination. This PDA-enabled strategy affords a rational design for robust PA imaging probes and provides more opportunities for other types of photomediated biomedicines, such as photothermal and photodynamic regimens.

Regulation of Blood Glucose Using Islets Encapsulated in a Melanin-Modified Immune-Shielding Hydrogel

Islet transplantation is currently a promising treatment for type 1 diabetes mellitus. However, the foreign body reaction and retrieval difficulty often lead to transplantation failure and hinder the clinical application. To address these two challenges, we propose a balanced charged sodium alginate-polyethyleneimine-melanin (SA-PEI-Melanin) threadlike hydrogel with immune shielding and retrievable properties. The attractiveness of this study lies in that the introduction of melanin can stimulate insulin secretion, especially under near-infrared (NIR) irradiation. After demonstrating a good immune-shielding effect, we performed an in vivo transplantation experiment. The results showed that the blood glucose level in the SA-PEI-Melanin group was stably controlled below the diabetic blood glucose criterion, and this blood glucose level could be further adjusted after NIR irradiation. In addition, the evaluation after retrieving the SA-PEI-Melanin hydrogel indicated that the islets still maintained a normal physiological function, further proving its excellent immunological protection. This study provides a new approach for the accurate regulation of blood glucose in patients with type 1 diabetes mellitus and contributes to developing a promising transplant system to reconcile real-time and precise light-defined insulin secretion regulation.

COVID-19 and the ethnicity link - is there a photochemical link?

A hypothesis is proposed to explain the increased detrimental effect of COVID-19 for Black, Asian and Minority Ethnic (BAME) men and women compared to Caucasian individuals. This is based on the differing photochemistry of phaeomelanin in fair skin and eumelanin in dark/black skin. It is suggested that a range of reactive oxygen species, including, singlet oxygen and the superoxide radical anion, derived via direct photolysis of phaeomelanin, may escape the melanocyte and cause subsequent damage to the SARS-CoV-2 virus. It is further suggested that (large) carbon and sulphur peroxy radicals, from oxygen addition to radicals formed by carbon–sulphur bond cleavage, may assist via damage to the cell membranes. It is also speculated that light absorption by phaeomelanin and the subsequent C-S bond cleavage, leads to release of pre-absorbed reactive oxygen species, such as singlet oxygen and free radicals, which may also contribute to an enhanced protective effect for fair-skinned people.

Chemical Evaluation of Eumelanin Maturation by ToF-SIMS and Alkaline Peroxide Oxidation HPLC Analysis

Residual melanins have been detected in multimillion-year-old animal body fossils; however, confident identification and characterization of these natural pigments remain challenging due to loss of chemical signatures during diagenesis. Here, we simulate this post-burial process through artificial maturation experiments using three synthetic and one natural eumelanin exposed to mild (100 °C/100 bar) and harsh (250 °C/200 bar) environmental conditions, followed by chemical analysis employing alkaline hydrogen peroxide oxidation (AHPO) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Our results show that AHPO is sensitive to changes in the melanin molecular structure already during mild heat and pressure treatment (resulting, e.g., in increased C-C cross-linking), whereas harsh maturation leads to extensive loss of eumelanin-specific chemical markers. In contrast, negative-ion ToF-SIMS spectra are considerably less affected by mild maturation conditions, and eumelanin-specific features remain even after harsh treatment. Detailed analysis of ToF-SIMS spectra acquired prior to experimental treatment revealed significant differences between the investigated eumelanins. However, systematic spectral changes upon maturation reduced these dissimilarities, indicating that intense heat and pressure treatment leads to the formation of a common, partially degraded, eumelanin molecular structure. Our findings elucidate the complementary nature of AHPO and ToF-SIMS during chemical characterization of eumelanin traces in fossilized organismal remains.

Advances in the Design of Genuine Human Tyrosinase Inhibitors for Targeting Melanogenesis and Related Pigmentations

Human tyrosinase (hsTYR) is the key enzyme ensuring the conversion of l-tyrosine to dopaquinone, thereby initiating melanin synthesis, i.e., melanogenesis. Although the protein has long been familiar, knowledge about its three-dimensional structure and efficient overexpression protocols emerged only recently. Consequently, for decades medicinal chemistry studies aiming at developing skin depigmenting agents relied almost exclusively on biological assays performed using mushroom tyrosinase (abTYR), producing a plethoric literature, often of little useful purpose. Indeed, several recent reports have pointed out spectacular differences in terms of interaction patterns and inhibition values between hsTYR and abTYR, including for widely used standard tyrosinase inhibitors. In this review, we summarize the last developments regarding the potential role of hsTYR in human pathologies, the advances in recombinant expression systems and structural data retrieving, and the pioneer generation of true hsTYR inhibitors. Finally, we present suggestions for the design of future inhibitors of this highly attractive target in pharmacology and dermocosmetics.

Carvone Decreases Melanin Content by Inhibiting Melanoma Cell Proliferation via the Cyclic Adenosine Monophosphate (cAMP) Pathway

Melanin, which determines the color of the skin and hair, is initially synthesized to protect the skin from ultraviolet light; however, excessive melanin pigmentation caused by abnormal cell proliferation can result in various melanocytic lesions. Cyclic adenosine monophosphate (cAMP) is known to regulate cell cycle progression and consequently to inhibit the division of abnormally proliferating cells. In this work, we aimed to test whether carvone, a scent compound from plants, inhibits proliferation and subsequently reduces melanin content of melanoma cells and to determine whether its beneficial effects are mediated by the cAMP pathway. We found that carvone decreases melanin content and inhibits melanoma cell proliferation in a concentration-dependent manner. Meanwhile, it inhibited the activation of cell cycle-associated proteins such as cyclin-dependent kinase 1 (CDK1). Of note, the beneficial effects of carvone were abrogated by cAMP inhibition. Our findings indicate potential benefits of carvone for the treatment of melanomas and presumably other hyperpigmentation-related dermatological disorders such as melasmas, lentigines, and excessive freckles.

Proton-Conductive Melanin-Like Fibers through Enzymatic Oxidation of a Self-Assembling Peptide

Melanin pigments have various properties that are of technological interest including photo- and radiation protection, rich coloration, and electronic functions. Nevertheless, laboratory-based synthesis of melanin and melanin-like materials with morphologies and chemical structures that are specifically optimized for these applications, is currently not possible. Here, melanin-like materials that are produced by enzymatic oxidation of a supramolecular tripeptide structures that are rich in tyrosine and have a 1D morphology are demonstrated, that are retained during the oxidation process while conducting tracks form through oxidative tyrosine crosslinking. Specifically, a minimalistic self-assembling peptide, Lys-Tyr-Tyr (KYY) with strong propensity to form supramolecular fibers, is utilized. Analysis by Raman spectroscopy shows that the tyrosines are pre-organized inside these fibers and, upon enzymatic oxidation, result in connected catechols. These form 1D conducting tracks along the length of the fiber, which gives rise to a level of internal disorder, but retention of the fiber morphology. This results in highly conductive structures demonstrated to be dominated by proton conduction. This work demonstrates the ability to control oxidation but retain a well-defined fibrous morphology that does not have a known equivalent in biology, and demonstrate exceptional conductivity that is enhanced by enzymatic oxidation.

Probing the heterogeneous structure of eumelanin using ultrafast vibrational fingerprinting

Eumelanin is a brown-black biological pigment with sunscreen and radical scavenging functions important to numerous organisms. Eumelanin is also a promising redox-active material for energy conversion and storage, but the chemical structures present in this heterogeneous pigment remain unknown, limiting understanding of the properties of its light-responsive subunits. Here, we introduce an ultrafast vibrational fingerprinting approach for probing the structure and interactions of chromophores in heterogeneous materials like eumelanin. Specifically, transient vibrational spectra in the double-bond stretching region are recorded for subsets of electronic chromophores photoselected by an ultrafast excitation pulse tuned through the UV-visible spectrum. All subsets show a common vibrational fingerprint, indicating that the diverse electronic absorbers in eumelanin, regardless of transition energy, contain the same distribution of IR-active functional groups. Aggregation of chromophores diverse in oxidation state is the key structural property underlying the universal, ultrafast deactivation behavior of eumelanin in response to photoexcitation with any wavelength.

Melanin distribution from the dermal-epidermal junction to the stratum corneum: non-invasive in vivo assessment by fluorescence and Raman microspectroscopy

The fate of melanin in the epidermis is of great interest due to its involvement in numerous physiological and pathological processes in the skin. Melanin localization can be assessed ex vivo and in vivo using its distinctive optical properties. Melanin exhibits a characteristic Raman spectrum band shape and discernible near-infrared excited (NIR) fluorescence. However, a detailed analysis of the capabilities of depth-resolved confocal Raman and fluorescence microspectroscopy in the evaluation of melanin distribution in the human skin is lacking. Here we demonstrate how the fraction of melanin at different depths in the human skin in vivo can be estimated from its Raman spectra (bands at 1,380 and 1,570 cm-1) using several procedures including a simple ratiometric approach, spectral decomposition and non-negative matrix factorization. The depth profiles of matrix factorization components specific to melanin, collagen and natural moisturizing factor provide information about their localization in the skin. The depth profile of the collagen-related matrix factorization component allows for precise determination of the dermal-epidermal junction, i.e. the epidermal thickness. Spectral features of fluorescence background originating from melanin were found to correlate with relative intensities of the melanin Raman bands. We also hypothesized that NIR fluorescence in the skin is not originated solely from melanin, and the possible impact of oxidized species should be taken into account. The ratio of melanin-related Raman bands at 1,380 and 1,570 cm-1 could be related to melanin molecular organization. The proposed combined analysis of the Raman scattering signal and NIR fluorescence could be a useful tool for rapid non-invasive in vivo diagnostics of melanin-related processes in the human skin.

Detection of a pheomelanin-like pigment by EPR spectroscopy in the mycelium of Plenodomus biglobosus

Melanin occurrence in Plenodomus biglobosus was investigated using electron paramagnetic (spin) resonance (EPR, ESR) spectroscopy. The fungus was isolated from living and dead leaves of European ash (Fraxinus excelsior L.). Dark pigmentation of P. biglobosus mycelium in vitro, especially on the reverse, was observed. The black coloration intensified with the age of the culture and inspired us to check if the analyzed fungus species synthesizes melanin. Melanin contains unpaired electrons, thus, EPR spectroscopy was applied, as a specific technique, to verify its presence in P. biglobosus. The EPR spectrum of the mycelium showed a very strong melanin signal, revealing pheomelanin-like features. Thus, the black pigment of P. biglobosus was clearly identified as melanin. However, no melanin was detected in the apparently dark culture medium even when zinc (II) acetate was added to increase the sensitivity of detection. Pheomelanin has many unusual biological functions but it is not commonly found in fungi. Detection of this type of melanin in P. biglobosus, which can be both endophytic or pathogenic, suggests a closer examination of the potential role of this melanin in host-parasite interaction.

Conidial Melanin of the Human-Pathogenic Fungus Aspergillus fumigatus Disrupts Cell Autonomous Defenses in Amoebae

The human-pathogenic fungus Aspergillus fumigatus is a ubiquitous saprophyte that causes fatal lung infections in immunocompromised individuals. Following inhalation, conidia are ingested by innate immune cells and can arrest phagolysosome maturation. How this virulence trait could have been selected for in natural environments is unknown. Here, we found that surface exposure of the green pigment 1,8-dihydroxynaphthalene-(DHN)-melanin can protect conidia from phagocytic uptake and intracellular killing by the fungivorous amoeba Protostelium aurantium and delays its exocytosis from the nonfungivorous species Dictyostelium discoideum To elucidate the antiphagocytic properties of the surface pigment, we followed the antagonistic interactions of A. fumigatus conidia with the amoebae in real time. For both amoebae, conidia covered with DHN-melanin were internalized at far lower rates than were seen with conidia lacking the pigment, despite high rates of initial attachment to nonkilling D. discoideum When ingested by D. discoideum, the formation of nascent phagosomes was followed by transient acidification of phagolysosomes, their subsequent neutralization, and, finally, exocytosis of the conidia. While the cycle was completed in less than 1 h for unpigmented conidia, the process was significantly prolonged for conidia covered with DHN-melanin, leading to an extended intracellular residence time. At later stages of this cellular infection, pigmented conidia induced enhanced damage to phagolysosomes and infected amoebae failed to recruit the ESCRT (endosomal sorting complex required for transport) membrane repair machinery or the canonical autophagy pathway to defend against the pathogen, thus promoting prolonged intracellular persistence in the host cell and the establishment of a germination niche in this environmental phagocyte.IMPORTANCE Infections with Aspergillus fumigatus are usually acquired by an inhalation of spores from environmental sources. How spores of a saprophytic fungus have acquired abilities to withstand and escape the phagocytic attacks of innate immune cells is not understood. The fungal surface pigment dihydroxynaphtalene-melanin has been shown to be a crucial factor for the delay in phagosome maturation. Here, we show that this pigment also has a protective function against environmental phagocytes. Pigmented conidia escaped uptake and killing by the fungus-eating amoeba Protostelium aurantium When ingested by the nonfungivorous phagocyte Dictyostelium discoideum, the pigment attenuated the launch of cell autonomous defenses against the fungal invader, such as membrane repair and autophagy, leading to prolonged intracellular retention. Membrane damage and cytoplasmic leakage may result in an influx of nutrients and thus may further promote intracellular germination of the fungus, indicating that A. fumigatus has acquired some of the basic properties of intracellular pathogens.

Photoprotection and Skin Pigmentation: Melanin-Related Molecules and Some Other New Agents Obtained from Natural Sources

Direct sun exposure is one of the most aggressive factors for human skin. Sun radiation contains a range of the electromagnetic spectrum including UV light. In addition to the stratospheric ozone layer filtering the most harmful UVC, human skin contains a photoprotective pigment called melanin to protect from UVB, UVA, and blue visible light. This pigment is a redox UV-absorbing agent and functions as a shield to prevent direct UV action on the DNA of epidermal cells. In addition, melanin indirectly scavenges reactive oxygenated species (ROS) formed during the UV-inducing oxidative stress on the skin. The amounts of melanin in the skin depend on the phototype. In most phenotypes, endogenous melanin is not enough for full protection, especially in the summertime. Thus, photoprotective molecules should be added to commercial sunscreens. These molecules should show UV-absorbing capacity to complement the intrinsic photoprotection of the cutaneous natural pigment. This review deals with (a) the use of exogenous melanin or melanin-related compounds to mimic endogenous melanin and (b) the use of a number of natural compounds from plants and marine organisms that can act as UV filters and ROS scavengers. These agents have antioxidant properties, but this feature usually is associated to skin-lightening action. In contrast, good photoprotectors would be able to enhance natural cutaneous pigmentation. This review examines flavonoids, one of the main groups of these agents, as well as new promising compounds with other chemical structures recently obtained from marine organisms.

Electron Paramagnetic Resonance Imaging of Melanin in Honey Bee

Honey bees play a crucial role in the nature by pollinating wild flowers. Over the past years, there has been an increasing concern regarding the honey bee colony decline. Pesticides or environmental effects targeting the biochemistry of insect chitin and cuticle coating may be in part responsible for honey bee pathologies. We here propose the use of electron paramagnetic resonance imaging (EPRI) as a tool to image the melanin-chitin complexes as part of the exoskeleton of the honey bee. EPRI at 9.65 GHz was applied on intact freeze-dried bees. The imaging data were collected on the melanin peak. High-resolution images revealed that this compound is extensively distributed in the periphery of the animal, data consistent with the localization in the cuticle of the bee. While EPR of melanin has been so far explored in the context of melanoma characterization, it may offer new opportunities in research on honey bees and other insects.

Adsorptive removal of trivalent and pentavalent arsenic from aqueous solutions using iron and copper impregnated melanin extracted from the marine bacterium Pseudomonas stutzeri

The metalloid arsenic is one of the most conspicuous groundwater contaminants in the Indian subcontinent and its removal from aqueous medium is the main focus of this study. The study aims at functionalising melanin using iron and copper for the efficient removal of arsenic and rendering water fit for consumption. Melanin obtained from the marine bacteria Pseudomonas stutzeri was functionalised by iron impregnation (Fe-melanin) and copper impregnation (Cu-melanin). Morphological studies using FESEM portrayed the impregnated iron and copper granules on the surface of melanin, while XRD analysis confirmed the presence of Fe₂O₃ and CuO on melanin. Adsorption studies on As (V) and As (III) were conducted using Fe-melanin and Cu-melanin for different operating variables like pH, temperature and contact time. More than 99% per cent of As (III) and As (V) from water was removed at a pH range between 4 and 6 within 50 min in the case of Fe-melanin and 80 min for Cu-melanin. Adsorption equilibrium studies showed better fit with Langmuir adsorption isotherm and had good agreement with Redlich-Peterson's three-parameter model. The maximum adsorption capacities of Fe-melanin and Cu-melanin obtained from Langmuir adsorption model are 50.12 and 20.39 mg/g, respectively, for As (V) and similarly 39.98 and 19.52 mg/g, respectively, for As (III). Arsenic-binding to the functionalised melanin was confirmed using FT-IR and the XPS analysis. Reuse of the adsorbent was effectively done by desorbing the iron and copper together with the bound As (III) and As (V) and further re-impregnation of iron and copper in melanin. Re-functionalised melanin showed 99% adsorption efficiency up to four cycles of adsorption/desorption.

The Guanidine Pseudoalkaloids 10-Methoxy-Leonurine and Leonurine Act as Competitive Inhibitors of Tyrosinase

Tyrosinase plays a key role in the production of melanin. A variety of industrial fields have shown interest in the development of tyrosinase inhibitors from plants. In this study, compounds 1–5 derived from Leonurus japonicas were evaluated to determine their ability to inhibit tyrosinase. Of these, 10-methoxy-leonurine (1) and leonurine (2) exhibited IC₅₀ values of 7.4 ± 0.4 and 12.4 ± 0.8 μM, respectively, and acted as competitive inhibitors of tyrosinase, with Ki values in the micromolar range. In silico modeling revealed a guanidine group located in the inner cavity and a benzene ring docked within the active site of these compounds. These guanidine pseudoalkaloids show potential not only as tyrosinase inhibitors but also as lead compounds in new scaffolds for the development of novel inhibitors.

A Dual-Modality MR/PA Imaging Contrast Agent Based on Ultrasmall Biopolymer Nanoparticles for Orthotopic Hepatocellular Carcinoma Imaging

BACKGROUND

Hepatocellular carcinoma (HCC) is the second leading cause of cancer death and early stage diagnosis can greatly increase the survival rate of patient. However, the accurate detection of HCC remains an urgent challenge in medical diagnosis. The combination of magnetic resonance imaging (MRI) and photoacoustic imaging (PAI) are conducive for accurate locating of cancerous tissue. Therefore, it is necessary to explore a more facile and biosafe dual-modal contrast agent for orthotopic HCC detection.

METHODS

In this study, a promising contrast agent had been identified based on gadolinium chelated melanin nanoparticles and evaluated its usage as a dual-modal T1 MRI and PAI contrast agent for orthotopic HCC detection.

RESULTS

The gadolinium-based melanin nanoparticles presented ultrasmall size, high chelation stability and negligible cytotoxicity estimated by CCK-8 assay. Moreover, the nanoparticle exhibited higher r1 relaxivity (45.762 mM-1 s-1) than clinically approved Gadodiamide (4.975 mM-1 s-1) at 1.5 T MR scanning. A linear regression analysis confirmed that the nanoparticles were ideal candidates for PAI in vitro. After the nanoparticles were injected into vein in mice with orthotopic HCC, a dramatic increase in signal of the liver was observed at 0.5 hr by MRI and PAI, while the tumor exerted remarkable signal enhancement at 7 hrs, showing excellent detection sensitivity. In addition, the nanoparticles exhibited excellent biocompatibility and they can be excreted through both hepatobiliary and renal pathways after diagnosis.

CONCLUSION

These results indicate that the ultrasmall gadolinium chelated melanin nanoparticles is a promising candidate as a dual-modal MRI/PAI contrast agent for the detection of orthotopic HCC.