Pulmonary delivery of LNP-mRNAs aerosolised by vibrating mesh nebulizer: An emphasis on variations and in-depth analyses of physicochemical properties

The delivery of lipid nanoparticle (LNP)-mRNAs to the lungs attracts fast increasing interests for vaccination, as the mucosal immunity in the airway can prevent the establishment of an infection rather than only reduce the level of infection associated with systemic immunity triggered via intramuscular injection. The vibrating mesh nebuliser was well utilized to atomize inhalation solutions/suspensions for pulmonary delivery hence employed in this study for aerosolising LNP-mRNAs. In comparison with pre-aerosolised LNP-mRNAs, the post-aerosolised vectors demonstrated a significant increase (t-test, unpaired, p < 0.05) in particle size (215-363 nm vs. 116-130 nm), polydispersity index (PDI: > 0.33 vs. < 0.27), zeta potential (ZP: 11-14 mV vs. 2.6-7.7 mV), and encapsulation efficiency (EE: ∼99 % w/w vs. ∼91 % w/w), indicating a structural alteration upon high-frequency mesh vibration (HFMV). The particle sizes of LNP-mRNAs were further enlarged upon inertial impaction, and the size increments were dependent on the velocities of airflow for impaction and the N/P ratios. The aerosolised mists were fine, with >54 % w/w deposited in lower respiratory tract and >28.5 % w/w further delivered to alveolar regions. Further, a model was created to elucidate the variations of physicochemical properties for LNP-mRNAs upon HFMV and inertial impaction, and it disclosed that the fluidity and shear-induced fusion of LNPs were the fundamental reasons to cause these unfavourable changes particularly the size enlargement. These insights reveal that the effective development of inhaled LNP-mRNAs will rely on shear-less devices, formulation optimizations, inhalable dry powders, and their potential combinations.

Advanced Carriers for Precise Delivery and Therapeutic Mechanisms of Traditional Chinese Medicines: Integrating Spatial Multi-Omics and Delivery Visualization

The complex composition of traditional Chinese medicines (TCMs) has posed challenges for in-depth study and global application, despite their abundance of bioactive compounds that make them valuable resources for disease treatment. To overcome these obstacles, it is essential to modernize TCMs by focusing on precise disease treatment. This involves elucidating the structure-activity relationships within their complex compositions, ensuring accurate in vivo delivery, and monitoring the delivery process. This review discusses the research progress of TCMs in precision disease treatment from three perspectives: spatial multi-omics technology for precision therapeutic activity, carrier systems for precise in vivo delivery, and medical imaging technology for visualizing the delivery process. The aim is to establish a novel research paradigm that advances the precision therapy of TCMs.

In Situ Synthesis and Visualization of Membrane SNAP25 Nano-Organization

Cryo-electron tomography (cryo-ET) can provide insights into the structure and states of natural membrane environments to explore the role of SNARE proteins at membrane fusion and understand the relationship between their subcellular localization/formation and action mechanism. Nevertheless, the identification of individual molecules in crowded and low signal-to-noise ratio membrane environments remains a significant challenge. In this study, cryo-ET is employed to image near-physiological state 293T cell membranes, specifically utilizing in situ synthesized gold nanoparticles (AuNPs) bound with cysteine-rich protein tags to single-molecularly labeled synaptosomal-associated protein 25 (SNAP25) on the membrane surface. The high-resolution images reveal that SNAP25 is predominantly located in regions of high molecular density within the cell membrane and aggregates into smaller clusters, which may increase the fusion efficiency. Remarkably, a zigzag arrangement of SNAP25 is observed on the cell membrane. These findings provide valuable insights into the functional mechanisms of SNARE proteins.

Control of artificial membrane fusion in physiological ionic solutions beyond the limits of electroformation

Membrane fusion, merging two lipid bilayers, is crucial for fabricating artificial membrane structures. Over the past 40 years, in contrast to precise and controllable membrane fusion in-vivo through specific molecules such as SNAREs, controlling the fusion in-vitro while fabricating artificial membrane structures in physiological ionic solutions without fusion proteins has been a challenge, becoming a significant obstacle to practical applications. We present an approach consisting of an electric field and a few kPa hydraulic pressure as an additional variable to physically control the fusion, enabling tuning of the shape and size of the 3D freestanding lipid bilayers in physiological ionic solutions. Mechanical model analysis reveals that pressure-induced parallel/normal tensions enhance fusion among membranes in the microwell. In-vitro peptide-membrane assay, mimicking vesicular transport via pressure-assisted fusion, and stability of 38 days with in-chip pressure control via pore size-regulated hydrogel highlight the potential for diverse biological applications.

Towards a biomimetic cellular structure and physical morphology with liposome-encapsulated agarose sol systems

The cytoplasm, serving as the primary hub of cellular metabolism, stands as a pivotal cornerstone for the harmonious progression of life. The ideal artificial cell should not only have a biomembrane structure system similar to that of a cell and the function of carrying genetic information, but also should have an intracellular environment. In this pursuit, we employed a method involving the incorporation of glycerol into agarose, resulting in the formation of agarose-glycerol mixed sol (AGs). This dynamic sol exhibited fluidic properties at ambient temperature, closely mimicking the viscosity of authentic cytoplasm. Harnessing the electroformation technique, AGs was encapsulated within liposomes, enabling the efficient creation of artificial cells that closely resembled native cellular dimensions through meticulous parameter adjustments of the alternating current (AC) field. Subsequently, artificial cells harboring AGs were subjected to diverse electrolyte and non-electrolyte solutions, enabling a comprehensive exploration of their deformation phenomena, encompassing both inward and outward budding. This study represents a significant stride forward in addressing one of the most fundamental challenges in the construction of artificial cytoplasm. It is our fervent aspiration that this work shall offer invaluable insights and guidance for future endeavors in the realm of artificial cell construction.

Protocol to produce plant-based hybrid nanovesicles from fresh turmeric and pepper using polyethylene glycol

In addition to proteins, microRNAs, and lipids, plant-derived exosome-like nanovesicles (ENVs) are also enriched with host plant bioactives. Both curcumin and piperine are water insoluble, lack bioavailability, and are extracted by non-ecofriendly solvents. Herein, we present an eco-friendly protocol for co-isolating both curcumin and piperine in the form of hybrid ENVs. We describe steps for sample pre-processing, combined homogenization of plant materials, filtration, and differential centrifugation. We then detail procedures for polyethylene glycol-based fusion and precipitation of hybrid ENVs. For complete details on the use and execution of this protocol, please refer to Kumar et al.1.

Nanotherapeutics for prostate cancer treatment: A comprehensive review

Prostate cancer (PCa) is the most prevalent solid organ malignancy and seriously affects male health. The adverse effects of prostate cancer therapeutics can cause secondary damage to patients. Nanotherapeutics, which have special targeting abilities and controlled therapeutic release profiles, may serve as alternative agents for PCa treatment. At present, many nanotherapeutics have been developed to treat PCa and have shown better treatment effects in animals than traditional therapeutics. Although PCa nanotherapeutics are highly attractive, few successful cases have been reported in clinical practice. To help researchers design valuable nanotherapeutics for PCa treatment and avoid useless efforts, herein, we first reviewed the strategies and challenges involved in prostate cancer treatment. Subsequently, we presented a comprehensive review of nanotherapeutics for PCa treatment, including their targeting methods, controlled release strategies, therapeutic approaches and mechanisms. Finally, we proposed the future prospects of nanotherapeutics for PCa treatment.

A Peptide-Based Oscillator

Living organisms are replete with rhythmic and oscillatory behavior at all levels, to the extent that oscillations have been termed as a defining attribute of life. Recent studies of synthetic oscillators that mimic such functions have shown decayed cycles in batch-mode reactions or sustained oscillatory kinetics under flow conditions. Considering the hypothesized functionality of peptides in early chemical evolution and their central role in current bio-nanotechnology, we now reveal a peptide-based oscillator. Oscillatory behavior was achieved by coupling coiled-coil-based replication processes as positive feedback to controlled initiation and inhibition pathways in a continuously stirred tank reactor (CSTR). Our results stress that assembly into the supramolecular structure and specific interactions with the replication substrates are crucial for oscillations. The replication-inhibition processes were first studied in batch mode, which produced a single damped cycle. Thereafter, combined experimental and theoretical characterization of the replication process in a CSTR under different flow and environmental (pH, redox) conditions demonstrated reasonably sustained oscillations. We propose that studies in this direction might pave the way to the design of robust oscillation networks that mimic the autonomous behavior of proteins in cells (e.g., in the cyanobacterial circadian clock) and hence hint at feasible pathways that accelerated the transition from simple peptides to extant enzymes.

A Self-Regenerating Artificial Cell, that is One Step Closer to Living Cells: Challenges and Perspectives

Controllable, self-regenerating artificial cells (SRACs) can be a vital advancement in the field of synthetic biology, which seeks to create living cells by recombining various biological molecules in the lab. This represents, more importantly, the first step on a long journey toward creating reproductive cells from rather fragmentary biochemical mimics. However, it is still a difficult task to replicate the complex processes involved in cell regeneration, such as genetic material replication and cell membrane division, in artificially created spaces. This review highlights recent advances in the field of controllable, SRACs and the strategies to achieve the goal of creating such cells. Self-regenerating cells start by replicating DNA and transferring it to a location where proteins can be synthesized. Functional but essential proteins must be synthesized for sustained energy generation and survival needs and function in the same liposomal space. Finally, self-division and repeated cycling lead to autonomous, self-regenerating cells. The pursuit of controllable, SRACs will enable authors to make bold advances in understanding life at the cellular level, ultimately providing an opportunity to use this knowledge to understand the nature of life.

Three Compartment Liposome Fusion: Functional Protocells for Biocatalytic Cascades and Operation of Dynamic DNA Machineries

Nucleic acid‐functionalized liposomes modified at their boundaries with o‐nitrobenzyl phosphate‐caged hairpin units and pH‐responsive C‐G·C+ triplex forming strands are used for the concomitant light and pH‐triggered fusion of three types of loaded liposomes. The fusion processes are followed by light‐scattering size enlargement measurements, optical methods, and biocatalytic cascades activated upon the mixing of the liposomes loaded with enzymes and their substrates and their fusion into the cell‐like containments. The fused liposomes act as functional protocells for the integration of biocatalytic machineries. This is exemplified by the operation of an autonomous polymerization/nickase machinery synthesizing a Mg2+‐ion‐dependent DNAzyme and of a transcription machinery yielding the Malachite Green‐RNA aptamer product.

Enhanced Liposomal Drug Delivery Via Membrane Fusion Triggered by Dimeric Coiled-Coil Peptides

An ideal nanomedicine system improves the therapeutic efficacy of drugs. However, most nanomedicines enter cells via endosomal/lysosomal pathways and only a small fraction of the cargo enters the cytosol inducing therapeutic effects. To circumvent this inefficiency, alternative approaches are desired. Inspired by fusion machinery found in nature, synthetic lipidated peptide pair E4/K4 is used to induce membrane fusion previously. Peptide K4 interacts specifically with E4, and it has a lipid membrane affinity and resulting in membrane remodeling. To design efficient fusogens with multiple interactions, dimeric K4 variants are synthesized to improve fusion with E4-modified liposomes and cells. The secondary structure and self-assembly of dimers are studied; the parallel PK4 dimer forms temperature-dependent higher-order assemblies, while linear K4 dimers form tetramer-like homodimers. The structures and membrane interactions of PK4 are supported by molecular dynamics simulations. Upon addition of E4, PK4 induced the strongest coiled-coil interaction resulting in a higher liposomal delivery compared to linear dimers and monomer. Using a wide spectrum of endocytosis inhibitors, membrane fusion is found to be the main cellular uptake pathway. Doxorubicin delivery results in efficient cellular uptake and concomitant antitumor efficacy. These findings aid the development of efficient delivery systems of drugs into cells using liposome-cell fusion strategies.

Anti-phagocytosis-blocking repolarization-resistant membrane-fusogenic liposome (ARMFUL) for adoptive cell immunotherapy

Equipping multiple functionalities on adoptive effector cells is essential to overcome the complex immunological barriers in solid tumors for superior antitumor efficacy. However, current cell engineering technologies cannot endow these functionalities to cells within a single step because of the different spatial distributions of targets in one cell. Here, we present a core-shell anti-phagocytosis-blocking repolarization-resistant membrane-fusogenic liposome (ARMFUL) to achieve one-step multiplexing cell engineering for multifunctional cell construction. Through fusing with the M1 macrophage membrane, ARMFUL inserts an anti-CD47 (aCD47)-modified lipid shell onto the surface and simultaneously delivers colony-stimulating factor 1 receptor inhibitor BLZ945-loaded core into the cytoplasm. The surface-presenting aCD47 boosts macrophage's phagocytosis against the tumor by blocking CD47. The cytoplasm-located BLZ945 prompts its polarization resistance to M2 phenotype in the immunosuppressive microenvironment via inactivating the intracellular M2 polarization signaling pathway. This ARMFUL provides a versatile cell engineering platform to customize multimodal cellular functions for enhanced adoptive cell therapy.

Dynamic exchange controls the assembly structure of nucleic-acid-peptide chimeras

Recent attempts to develop the next generation of functional biomaterials focus on systems chemistry approaches exploiting dynamic networks of hybrid molecules. This task is often found challenging, but we herein present ways for profiting from the multiple interaction interfaces forming Nucleic-acid-Peptide assemblies and tuning their formation. We demonstrate that the formation of well-defined structures by double-stranded DNA-peptide conjugates (dsCon) is restricted to a specific range of environmental conditions and that precise DNA hybridization, satisfying the interaction interfaces, is a crucial factor in this process. We further reveal the impact of external stimuli, such as competing free DNA elements or salt additives, which initiate dynamic interconversions, resulting in hybrid structures exhibiting spherical and fibrillar domains or a mixture of spherical and fibrillar particles. This extensive analysis of the co-assembly systems chemistry offers new insights into prebiotic hybrid assemblies that may now facilitate the design of new functional materials. We discuss the implications of these findings for the emergence of function in synthetic materials and during early chemical evolution.

Membrane-Fusion-Mediated Multiplex Engineering of Tumor Cell Surface Glycans for Enhanced NK Cell Therapy

Natural killer (NK) cell therapies show potential for tumor treatment but are immunologically resisted by the overexpressed immunosuppressing tumor cell surface glycans. To reverse this glycan-mediated immunosuppression, the surface NK-inhibitory glycan expressions need to be downregulated and NK-activating glycan levels should be elevated synchronously with optimal efficiency. Here, a core-shell membrane-fusogenic liposome (MFL) is designed to simultaneously achieve the physical modification of NK-activating glycans and biological inhibition of immunosuppressing glycans on the tumor cell surface via a membrane-fusion manner. Loaded into a tumor-microenvironment-triggered-degradable thermosensitive hydrogel, MFLs could be conveniently injected and controllably released into local tumor. Through fusion with tumor cell membrane, the released MFLs could simultaneously deliver sialyltransferase-inhibitor-loaded core into cytoplasm, and anchor NK-activating-glycan-modified shell onto tumor surface. This spatially-differential distribution of core and shell in one cell ensures the effective inhibition of intracellular sialyltransferase to downregulate immunosuppressing sialic acid, and direct presentation of NK-activating Lewis X trisaccharide (LeX) on tumor surface simultaneously. Consequentially, the sialic acid-caused immunosuppression of tumor surface is reprogrammed to be LeX-induced NK activation, resulting in sensitive susceptibility to NK-cell-mediated recognition and lysis for improved tumor elimination. This MFL provides a novel platform for multiplex cell engineering and personalized regulation of intercellular interactions for enhanced cancer immunotherapy.

Understanding a protein fold: the physics, chemistry, and biology of α-helical coiled coils

Protein science is being transformed by powerful computational methods for structure prediction and design: AlphaFold2 can predict many natural protein structures from sequence, and other AI methods are enabling the de novo design of new structures. This raises a question: how much do we understand the underlying sequence-to-structure/function relationships being captured by these methods? This perspective presents our current understanding of one class of protein assembly, the α-helical coiled coils. At first sight, these are straightforward: sequence repeats of hydrophobic (h) and polar (p) residues, (hpphppp)_n, direct the folding and assembly of amphipathic α helices into bundles. However, many different bundles are possible: they can have two or more helices (different oligomers); the helices can have parallel, antiparallel or mixed arrangements (different topologies); and the helical sequences can be the same (homomers) or different (heteromers). Thus, sequence-to-structure relationships must be present within the hpphppp repeats to distinguish these states. I discuss the current understanding of this problem at three levels: First, physics gives a parametric framework to generate the many possible coiled-coil backbone structures. Second, chemistry provides a means to explore and deliver sequence-to-structure relationships. Third, biology shows how coiled coils are adapted and functionalized in nature, inspiring applications of coiled coils in synthetic biology. I argue that the chemistry is largely understood; the physics is partly solved, though the considerable challenge of predicting even relative stabilities of different coiled-coil states remains; but there is much more to explore in the biology and synthetic biology of coiled coils.

Sticking the Landing: Enhancing Liposomal Cell Delivery using Reversible Covalent Chemistry and Caged Targeting Groups

Liposomes are highly effective nanocarriers for encapsulating and delivering a wide range of therapeutic cargo. While advancements in liposome design have improved several pharmacological characteristics, an important area that would benefit from further progress involves cellular targeting and entry. In this concept article, we will focus on recent progress utilizing strategies including reversible covalent bonding and caging groups to activate liposomal cell entry. These approaches take advantage of advancements that have been made in complementary fields including molecular sensing and chemical biology and direct this technology toward controlling liposome cell delivery properties. The decoration of liposomes with groups including boronic acids and cyclic disulfides is presented as a means for driving delivery through reaction with functional groups on cell surfaces. Additionally, caging groups can be exploited to activate cell delivery only upon encountering a target stimulus. These approaches provide promising new avenues for controlling cell delivery in the development of next-generation liposomal therapeutic nanocarriers.

Membrane Fusion Mediated by Non-covalent Binding of Re-engineered Cholera Toxin Assemblies to Glycolipids

Membrane fusion is essential for the transport of macromolecules and viruses across membranes. While glycan-binding proteins (lectins) often initiate cellular adhesion, subsequent fusion events require additional protein machinery. No mechanism for membrane fusion arising from simply a protein binding to membrane glycolipids has been described thus far. Herein, we report that a biotinylated protein derived from cholera toxin becomes a fusogenic lectin upon cross-linking with streptavidin. This novel reengineered protein brings about hemifusion and fusion of vesicles as demonstrated by mixing of fluorescently labeled lipids between vesicles as well as content mixing of liposomes filled with fluorescently labeled dextran. Exclusion of the complex at vesicle-vesicle interfaces could also be observed, indicating the formation of hemifusion diaphragms. Discovery of this fusogenic lectin complex demonstrates that new emergent properties can arise from simple changes in protein architecture and provides insights into new mechanisms of lipid-driven fusion.

Sum frequency spectroscopy studies on cell membrane fusion induced by divalent cations

Cell membrane fusion is a fundamental biological process involved in a number of cellular living functions. Regarding this, divalent cations can induce fusion of the lipid bilayers through binding and bridging of divalent cations to the charged lipids, thus leading to the cell membrane fusion. How-ever, the elaborate mechanism of cell membrane fusion induced by divalent cations is still needed to be elucidated. Here, surface/interface sensitive sum frequency generation vibrational spectroscopy (SFG-VS) and dynamic light scattering (DLS) were applied in this research to study the responses of phospholipid monolayer to the exposure of divalent metal ions i.e. Ca2+ and Mg2+. According to the particle size distribution results measured by DLS experiments, it was found that Ca2+ could induce inter-vesicular fusion while Mg2+ could not. An octadecyltrichlorosilane self-assembled monolayer (OTS SAM)-lipid monolayer system was designed to model the cell membrane for the SFG-VS experiment. Ca2+ could interact with the lipid PO2− head groups more strongly, resulting in cell membrane fusion more easily, in comparison with Mg2+. No specific interaction between the two metal cations and the C=O groups was observed. However, the C=O orientations changed more after Ca2+-PO2− binding than Mg2+ mediation on lipid monolayer. Meanwhile, Ca2+ could induce dehydration of the lipids (which should be related to the strong Ca2+-PO2− interaction), leading to the reduced hindrance for cell membrane fusion.

Malaria parasites release vesicle subpopulations with signatures of different destinations

Malaria is the most serious mosquito-borne parasitic disease, caused mainly by the intracellular parasite Plasmodium falciparum. The parasite invades human red blood cells and releases extracellular vesicles (EVs) to alter its host responses. It becomes clear that EVs are generally composed of sub-populations. Seeking to identify EV subpopulations, we subject malaria-derived EVs to size-separation analysis, using asymmetric flow field-flow fractionation. Multi-technique analysis reveals surprising characteristics: we identify two distinct EV subpopulations differing in size and protein content. Small EVs are enriched in complement-system proteins and large EVs in proteasome subunits. We then measure the membrane fusion abilities of each subpopulation with three types of host cellular membranes: plasma, late and early endosome. Remarkably, small EVs fuse to early endosome liposomes at significantly greater levels than large EVs. Atomic force microscope imaging combined with machine-learning methods further emphasizes the difference in biophysical properties between the two subpopulations. These results shed light on the sophisticated mechanism by which malaria parasites utilize EV subpopulations as a communication tool to target different cellular destinations or host systems.

Concluding remarks: discussion on natural and artificial enzymes including synthetic models

This paper overviews the final remarks lecture delivered (by K. D. K.) at the end of this bioinorganic chemistry Faraday Discussion, held online for a worldwide audience from January 31 - February 3, 2022. This paper provides discussion in six sections: (1) the Introductory lecture, from Ed Solomon, emphasized past and present uses of advanced spectroscopic methods and theoretical approaches to elucidate metalloenzyme active site structure, physical properties and function. (2) The discussion topics are divided into groups having similar research themes, as seen from this author's perspective. Emphasis is given to the non-heme iron group of articles with dioxygen activation research. (3) Small molecule activation (e.g., N2, CO2 and O2 reduction; CH4 or H2O oxidation) is widely covered in this discussion; this authors' view of the important reactions in bioinorganic chemistry is discussed. (4) We discuss current practice and vision for employing materials chemistry to widely apply to electrocatalytic methods to effect small molecule activation (as above) to fulfill societal energy demands. (5) A discussion is given on the topic of synthetic models and the approach utilized therein. (6) New research on the authors' synthetic modeling is presented; preliminary results are given in the area of copper mediated peroxide activation.

Membrane-fusogenic biomimetic particles: a new bioengineering tool learned from nature

Membrane fusion, a fundamental biological process of the fusion of the membrane composition between cells within natural organisms, is vital for cell-cell communication and cargo transport between the living cells....

Synthetic Cell as a Platform for Understanding Membrane-Membrane Interactions

In the pursuit of understanding life, model membranes made of phospholipids were envisaged decades ago as a platform for the bottom-up study of biological processes. Micron-sized lipid vesicles have gained great acceptance as their bilayer membrane resembles the natural cell membrane. Important biological events involving membranes, such as membrane protein insertion, membrane fusion, and intercellular communication, will be highlighted in this review with recent research updates. We will first review different lipid bilayer platforms used for incorporation of integral membrane proteins and challenges associated with their functional reconstitution. We next discuss different methods for reconstitution of membrane fusion and compare their fusion efficiency. Lastly, we will highlight the importance and challenges of intercellular communication between synthetic cells and synthetic cells-to-natural cells. We will summarize the review by highlighting the challenges and opportunities associated with studying membrane-membrane interactions and possible future research directions.

Dynamic Surface Layer Coiled Coil Proteins Processing Analog-to-Digital Information

Surface layer proteins perform multiple functions in prokaryotic cells, including cellular defense, cell-shape maintenance, and regulation of import and export of materials. However, mimicking the complex and dynamic behavior of such two-dimensional biochemical systems is challenging, and hence research has so far focused mainly on the design and manipulation of the structure and functionality of protein assemblies in solution. Motivated by the new opportunities that dynamic surface layer proteins may offer for modern technology, we herein demonstrate that immobilization of coiled coil proteins onto an inorganic surface facilitates complex behavior, manifested by reversible chemical reactions that can be rapidly monitored as digital surface readouts. Using multiple chemical triggers as inputs and several surface characteristics as outputs, we can realize reversible switching and logic gate operations that are read in parallel. Moreover, using the same coiled coil protein monolayers for derivatization of nanopores drilled into silicon nitride membranes facilitates control over ion and mass transport through the pores, thereby expanding the applicability of the dynamic coiled coil system for contemporary stochastic biosensing applications.

Liposome fusion with orthogonal coiled coil peptides as fusogens: the efficacy of roleplaying peptides

Biological membrane fusion is a highly specific and coordinated process as a multitude of vesicular fusion events proceed simultaneously in a complex environment with minimal off-target delivery. In this study, we develop a liposomal fusion model system with specific recognition using lipidated derivatives of a set of four de novo designed heterodimeric coiled coil (CC) peptide pairs. Content mixing was only obtained between liposomes functionalized with complementary peptides, demonstrating both fusogenic activity of CC peptides and the specificity of this model system. The diverse peptide fusogens revealed important relationships between the fusogenic efficacy and the peptide characteristics. The fusion efficiency increased from 20% to 70% as affinity between complementary peptides decreased, (from K_F ≈ 10⁸ to 10⁴ M⁻¹), and fusion efficiency also increased due to more pronounced asymmetric role-playing of membrane interacting ‘K’ peptides and homodimer-forming ‘E’ peptides. Furthermore, a new and highly fusogenic CC pair (E₃/P1_K) was discovered, providing an orthogonal peptide triad with the fusogenic CC pairs P2_E/P2_K and P3_E/P3_K. This E₃/P1_k pair was revealed, via molecular dynamics simulations, to have a shifted heptad repeat that can accommodate mismatched asparagine residues. These results will have broad implications not only for the fundamental understanding of CC design and how asparagine residues can be accommodated within the hydrophobic core, but also for drug delivery systems by revealing the necessary interplay of efficient peptide fusogens and enabling the targeted delivery of different carrier vesicles at various peptide-functionalized locations.

We developed a liposomal fusion model system with specific recognition using a set of heterodimeric coiled coil peptide pairs. This study unravels important structure–fusogenic efficacy relationships of peptide fusogens.

Maltoheptaose-Presenting Nanoscale Glycoliposomes for the Delivery of Rifampicin to E. coli

Liposomes, a nanoscale drug delivery system, are well known for their ability to improve pharmacokinetics and reduce drug toxicity. In this work, maltoheptaose (G7)-presenting glycoliposomes were synthesized and evaluated in the delivery of the antibiotic rifampicin. Two types of liposomes were prepared: nonfluid liposomes from l-α-phosphatidylcholine (PC) and cholesterol, and fluid liposomes from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol). G7-derivatized glycolipid, G7-DPPE (DPPE: 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine), was incorporated into the liposomes at 21 and 14 μmol/mg to form nanoparticles of 75 ± 12 and 146 ± 14 nm for the nonfluid and fluid G7-glycoliposomes, respectively. The multivalent G7-glycoliposomes were characterized by lectin binding with concanavalin A (Con A). The dissociation constant K _d between Con A and the nonfluid or fluid G7-glycoliposomes was 0.93 or 0.51 μM, which represented ~900- or 1600-fold stronger affinity than the binding between Con A and G7. The G7-glycoliposomes were loaded with rifampicin at 6.6 and 16 wt % encapsulation for the nonfluid and fluid G7-glycoliposomes, respectively. Introducing a carbohydrate in the liposomes slowed down the release of rifampicin, with the G7-glycoliposomes having the slowest release rate and the lowest permeability coefficient among the liposome formulations. The fluid G7-glycoliposomes lowered the minimal inhibitory concentration (MIC) of rifampicin against E. coli ORN208 by about 3 times, whereas liposomes without G7 or Man (d-mannose)-glycoliposomes showed no improvement in MIC. The rifampicin-loaded fluid G7-glycoliposomes demonstrated the best sustained antibacterial activity against E. coli, with up to 2 log reduction in the colony forming units at 4 × MIC after 24 h. Fluorescence resonance energy transfer and confocal fluorescence microscopy revealed stronger interactions of the bacterium with the fluid G7-glycoliposomes than other liposome formulations.

Current problems and future avenues in proteoliposome research

Membrane proteins (MPs) are the gatekeepers between different biological compartments separated by lipid bilayers. Being receptors, channels, transporters, or primary pumps, they fulfill a wide variety of cellular functions and their importance is reflected in the increasing number of drugs that target MPs. Functional studies of MPs within a native cellular context, however, is difficult due to the innate complexity of the densely packed membranes. Over the past decades, detergent-based extraction and purification of MPs and their reconstitution into lipid mimetic systems has been a very powerful tool to simplify the experimental system. In this review, we focus on proteoliposomes that have become an indispensable experimental system for enzymes with a vectorial function, including many of the here described energy transducing MPs. We first address long standing questions on the difficulty of successful reconstitution and controlled orientation of MPs into liposomes. A special emphasis is given on coreconstitution of several MPs into the same bilayer. Second, we discuss recent progress in the development of fluorescent dyes that offer sensitive detection with high temporal resolution. Finally, we briefly cover the use of giant unilamellar vesicles for the investigation of complex enzymatic cascades, a very promising experimental tool considering our increasing knowledge of the interplay of different cellular components.

Membrane protein density determining membrane fusion revealed by dynamic fluorescence imaging

Membrane fusion is fundamental to biological activity of cells, so disclosingits relevant mechanism is very important for understanding various cell functions. Although artificial model systems have been developed to uncover the mechanism of membrane fusion, key factors determining the mode of membrane fusion remain unclear. Based on the construction of different types of liposome vesicles, we used a dynamic fluorescence imaging method to investigate the effect of membrane protein distribution density on membrane fusion. Time-resolved imaging revealed that protein-free pure phospholipid vesicles themselves occurred full membrane fusion. Moreover, we prepared proteoliposomes with increasing protein-to-lipid ratio to better reflect the characteristic of membrane structure in vivo. Our data showed that pure phospholipid vesicles no longer fused with the proteoliposomes that in a higher protein proportion, indicating dense membrane proteins may hinder membrane fusion. A further comparative analysis of the interactions of pure phospholipid vesicles with the cell membrane / giant plasma membrane vesicles (GPMVs) / protein-free giant unilamellar vesicles (GUVs) confirmed the inhibitory effect of dense membrane proteins on membrane fusion. Our work demonstrates the membrane protein density influences the mode of membrane fusion and lays a foundation for constructing quasi-native membrane fusion models in vitro.

Intriguing Biomedical Applications of Synthetic and Natural Cell-Derived Vesicles: A Comparative Overview

The significant role of a vesicle is well recognized; however, only lately has the advancement in biomedical applications started to uncover their usefulness. Although the concept of vesicles originates from cell biology, it later transferred to chemistry and material science to develop nanoscale artificial vesicles for biomedical applications. Herein, we examine different synthetic and biological vesicles and their applications in the biomedical field in general. As our understanding of biological vesicles increases, more suitable biomimicking synthetic vesicles will be developed. The comparative discussion between synthetic and natural vesicles for biomedical applications is a relevant topic, and we envision this could enable the development of a proper approach to realize the next-generation treatment goals.

In vitro single vesicle fusion assays based on pore-spanning membranes: merits and drawbacks

Neuronal fusion mediated by soluble N-ethylmaleimide-sensitive-factor attachment protein receptors (SNAREs) is a fundamental cellular process by which two initially distinct membranes merge resulting in one interconnected structure to release neurotransmitters into the presynaptic cleft. To get access to the different stages of the fusion process, several in vitro assays have been developed. In this review, we provide a short overview of the current in vitro single vesicle fusion assays. Among those assays, we developed a single vesicle assay based on pore-spanning membranes (PSMs) on micrometre-sized pores in silicon, which might overcome some of the drawbacks associated with the other membrane architectures used for investigating fusion processes. Prepared by spreading of giant unilamellar vesicles with reconstituted t-SNAREs, PSMs provide an alternative tool to supported lipid bilayers to measure single vesicle fusion events by means of fluorescence microscopy. Here, we discuss the diffusive behaviour of the reconstituted membrane components as well as that of the fusing synthetic vesicles with reconstituted synaptobrevin 2 (v-SNARE). We compare our results with those obtained if the synthetic vesicles are replaced by natural chromaffin granules under otherwise identical conditions. The fusion efficiency as well as the different fusion states observable in this assay by means of both lipid mixing and content release are illuminated.

Membrane fusion studied by colloidal probes

Membrane-coated colloidal probes combine the benefits of solid-supported membranes with a more complex three-dimensional geometry. This combination makes them a powerful model system that enables the visualization of dynamic biological processes with high throughput and minimal reliance on fluorescent labels. Here, we want to review recent applications of colloidal probes for the study of membrane fusion. After discussing the advantages and disadvantages of some classical vesicle-based fusion assays, we introduce an assay using optical detection of fusion between membrane-coated glass microspheres in a quasi two-dimensional assembly. Then, we discuss free energy considerations of membrane fusion between supported bilayers, and show how colloidal probes can be combined with atomic force microscopy or optical tweezers to access the fusion process with even greater detail.

Selected tools to visualize membrane interactions

In the past decade, we developed various fluorescence-based methods for monitoring membrane fusion, membrane docking, distances between membranes, and membrane curvature. These tools were mainly developed using liposomes as model systems, which allows for the dissection of specific interactions mediated by, for example, fusion proteins. Here, we provide an overview of these methods, including two-photon fluorescence cross-correlation spectroscopy and intramembrane Förster energy transfer, with asymmetric labelling of inner and outer membrane leaflets and the calibrated use of transmembrane energy transfer to determine membrane distances below 10 nm. We discuss their application range and their limitations using examples from our work on protein-mediated vesicle docking and fusion.

Liposome Fusion Mediated by Hydrophobic Magnetic Nanoparticles Stabilized with Oleic Acid and Modulated by an External Magnetic Field

Membrane fusion is considered relevant in countless scientific areas and biotechnological processes, ranging from vital life events to biomedicine, pharmaceuticals, and materials engineering, among others. In this study, we employed hydrophobic oleic acid (OA)-coated magnetite (Fe₃O₄) nanoparticles (MNP-OA) as a platform to induce the fusion of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine liposomes [large unilamellar vesicles (LUVs)] in a colloidal dispersion. This fusion was monitored through dynamic light scattering, turbidimetry, and fluorescence assay using the well-known Tb/dipicolinic acid (DPA) complex formation assay. MNP-OA have shown to be able to induce fusion with the mixing of liposomal inner content with direct dependence on the nanoparticle concentration added to the LUVs. Moreover, changes in the permeability of the liposome bilayer, upon the addition of MNP-OA to liposomes, were evaluated by studying the leakage of carboxyfluorescein and of the co-encapsulated Tb/DPA complex. These assays allowed us to determine that MNP-OA did not significantly modify liposome permeability during the fusion process. Transmission electron microscopy and confocal microscopy revealed that MNP-OA remained embedded in the lipid bilayer without producing membrane rupture, liposome deformation, or destruction. In addition, we evaluated the effect of applying a low-intensity magnetic field to the LUVs/MNP-OA system and observed that the nanoparticles considerably increased their fusogenic activity under this external stimulus, as well as they are capable of responding to low magnetic fields of around 0.45 mT. These results revealed the potential of hydrophobic magnetic nanoparticles, stabilized with OA, to act as a fusogen, thus representing a valuable tool for biotechnological applications.

Versatile Phospholipid Assemblies for Functional Synthetic Cells and Artificial Tissues

The bottom-up construction of a synthetic cell from nonliving building blocks capable of mimicking cellular properties and behaviors helps to understand the particular biophysical properties and working mechanisms of a cell. A synthetic cell built in this way possesses defined chemical composition and structure. Since phospholipids are native biomembrane components, their assemblies are widely used to mimic cellular structures. Here, recent developments in the formation of versatile phospholipid assemblies are described, together with the applications of these assemblies for functional membranes (protein reconstituted giant unilamellar vesicles), spherical and nonspherical protoorganelles, and functional synthetic cells, as well as the high-order hierarchical structures of artificial tissues. Their biomedical applications are also briefly summarized. Finally, the challenges and future directions in the field of synthetic cells and artificial tissues based on phospholipid assemblies are proposed.

Synthesis of PNA-Peptide Conjugates as Functional SNARE Protein Mimetics

PNA-peptide conjugates are versatile tools in chemical biology, which are employed in a variety of applications. Here, we present the synthesis of PNA-peptide conjugates that serve as SNARE protein-mimicking biooligomers. They resemble the structure of native SNARE proteins but exhibit a much simpler architecture. Incorporated into liposomes, they induce lipid mixing, so that they can be used to study the SNARE-mediated membrane fusion in a simplified setting in vitro. They consist of artificial SNARE recognition units made out of PNA oligomers, which are attached to the native linker and transmembrane domains of two neuronal SNAREs. The PNA-peptide conjugates are synthesized via solid-phase peptide synthesis in a continuous fashion starting with the peptide part, followed by assembly of the PNA recognition unit. On top, we describe a strategy to synthesize PNA-peptide conjugates in a fully automated fashion by using a peptide synthesizer.

Lipid-Modified Peptide Nucleic Acids: Synthesis and Application to Programmable Liposome Fusion

Peptide nucleic acids (PNAs) can be modified with aliphatic lipid chains and designed to be water soluble and able to spontaneously insert into phospholipid bilayers. Liposomes with 1.5% negatively charged POPG can be driven to fuse and mix their inner content volumes via functionalization with such lipidated peptide nucleic acids (LiPNAs). During fusion, only low amounts of leakage occur (<5%). We describe here the synthesis and purification of such LiPNAs using an automated peptide synthesizer and the preparation of LiPNA functionalized liposomes. Further, we describe the measurement of LiPNA-induced fusion using a fluorescence-based assay for the content mixing between a liposome population with an encapsulated self-quenching fluorescent dye (SRB) and a buffer-filled liposome population.

FRET-Based Assay for the Quantification of Extracellular Vesicles and Other Vesicles of Complex Composition

Research in the field of extracellular vesicles is rapidly expanding and finding footholds in many areas of medical science. However, the availability of methodologies to quantify the concentration of membrane material present in a sample remains limited. Herein, we present a novel approach for the quantification of vesicle material, specifically the quantification of the total lipid membrane surface area, found in a sample using Förster resonance energy transfer (FRET). In this assay, sonication is used to drive the fusion between vesicles in the sample to be quantified and liposomes containing a pair of FRET fluorophores. The change in emission spectrum upon vesicle fusion is directly related to the total membrane surface area of the sample added, and a calibration curve allows for the quantification of a variety of vesicle species, including enveloped viruses, bacterial outer membrane vesicles, and mammalian extracellular vesicles. Without extensive optimization of experimental parameters, we were able to quantify down to ∼10⁹ vesicles/mL, using as little as 60 μL of the sample. The assay precision was comparable to that of a commercial nanoparticle tracking analysis system. While its limit of detection was slightly higher, the FRET assay is superior for the detection of small vesicles, as its performance is vesicle-size-independent. Taken together, the FRET assay is a simple, robust, and versatile method for the quantification of a variety of purified vesicle samples.

Nanocarrier‐Mediated Cytosolic Delivery of Biopharmaceuticals

Biopharmaceuticals have emerged to play a vital role in disease treatment and have shown promise in the rapidly expanding pharmaceutical market due to their high specificity and potency. However, the delivery of these biologics is hindered by various physiological barriers, owing primarily to the poor cell membrane permeability, low stability, and increased size of biologic agents. Since many biological drugs are intended to function by interacting with intracellular targets, their delivery to intracellular targets is of high relevance. In this review, the authors summarize and discuss the use of nanocarriers for intracellular delivery of biopharmaceuticals via endosomal escape and, especially, the routes of direct cytosolic delivery by means including the caveolae‐mediated pathway, contact release, intermembrane transfer, membrane fusion, direct translocation, and membrane disruption. Strategies with high potential for translation are highlighted. Finally, the authors conclude with the clinical translation of promising carriers and future perspectives.

Near-infrared light-activated membrane fusion for cancer cell therapeutic applications

The spatiotemporal stimulation of liposome-liposome or liposome-membrane fusion processes attracts growing interest as a means to mimic cell-cell interactions in nature and for using these processes for biomedical applications. We report the use of o-nitrobenzyl phosphate functionalized-cholesterol tethered nucleic acid-modified liposomes as functional photoresponsive units for inducing, by NIR-irradiation, spatiotemporal liposome-liposome or liposome-membrane fusion processes. The liposomes are loaded with upconversion nanoparticles (UCNPs) and their NIR irradiation (λ = 980 nm) yields luminescence at λ = 365 nm, providing a localized light-source to deprotect the o-nitrobenzyl phosphate groups and resulting in the fragmentation of the nucleic acid structures. In one system, the NIR-triggered fusion of two liposomes, L1 and L2, is exemplified. Liposome L1 is loaded with UCNPs and Tb3+ ions, and the liposome boundary is functionalized with a cholesterol-tethered, o-nitrobenzyl phosphate caged hairpin nucleic acid structure. Liposome L2 is loaded with 2,6-pyridinedicarboxylic acid, DPA, and its boundary is modified with a cholesterol-tethered nucleic acid, complementary to a part of the caged hairpin, associated with L1. NIR-irradiation of the L1/L2 mixture resulted in the photocleavage of the hairpin structure, associated with L1, and the resulting fragmented nucleic acid associated with L1 hybridized with the nucleic acid linked to L2, leading to the fusion of the two liposomes. The fusion process was followed by dynamic light scattering, and by monitoring the fluorescence of the Tb3+-DPA complex generated upon the fusion of the liposomes and their exchange of contents (fusion efficiency 30%). In a second system, the fusion of the liposomes L1, loaded with UCNPs and doxorubicin (DOX), with HeLa cancer cells functionalized with nucleic acid tethers, complementary to the hairpin units associated with the boundary of L1, and linked to the MUC-1 receptor sites associated with the HeLa cells, through a MUC-1 aptamer unit is exemplified. The effect of DOX-loaded L1/HeLa cell fusion on the cytotoxicity towards HeLa cells is addressed. The NIR UCNP-stimulated cleavage of the o-nitrobenzyl phosphate caged hairpin units associated with L1 leads to the fragmentation of the hairpin units and the resulting nucleic acid tethers hybridize with the nucleic acid-modified HeLa cells, resulting in the liposome-HeLa cell fusion and the release of DOX into the HeLa cells. Selective spatiotemporal cytotoxicity towards HeLa cells is demonstrated (ca. 40% cell killing within two days). The study presents a comprehensive stepwise set of experiments directed towards the development of NIR-driven liposome-liposome or liposome-membrane fusion processes.

Modulation of Coiled-Coil Binding Strength and Fusogenicity through Peptide Stapling

Peptide stapling is a technique which has been widely employed to constrain the conformation of peptides. One of the effects of such a constraint can be to modulate the interaction of the peptide with a binding partner. Here, a cysteine bis-alkylation stapling technique was applied to generate structurally isomeric peptide variants of a heterodimeric coiled-coil forming peptide. These stapled variants differed in the position and size of the formed macrocycle. C-terminal stapling showed the most significant changes in peptide structure and stability, with calorimetric binding analysis showing a significant reduction of binding entropy for stapled variants. This entropy reduction was dependent on cross-linker size and was accompanied by a change in binding enthalpy, illustrating the effects of preorganization. The stapled peptide, along with its binding partner, were subsequently employed as fusogens in a liposome model system. An increase in both lipid- and content-mixing was observed for one of the stapled peptide variants: this increased fusogenicity was attributed to increased coiled-coil binding but not to membrane affinity, an interaction theorized to be a primary driving force in this fusion system.

DNA-Mediated Liposome Fusion Observed by Fluorescence Spectrometry

DNA-programmed and controlled fusion of lipid membranes have recently been optimized to reliably mix the contents between two populations of liposomes, each functionalized with complementary lipidated DNA (LiNA) oligomer. In this chapter we describe a procedure for DNA-controlled fusion of liposomes mediated by LiNAs that are designed to force bilayers into close proximity. Using a self-quenching fluorescent dye (Sulforhodamine B) to monitor both the mixing of the internal volumes and leakage of the dye into the outer volume we measure the efficiency of content mixing in the bulk population, allowing for direct comparison between different LiNA designs. By generating samples for calibration corresponding to different amounts of content mixing, the average number of fusion events per labeled liposome can be estimated.

Lipidated Polyaza Crown Ethers as Membrane Anchors for DNA-Controlled Content Mixing between Liposomes

The ability to manipulate and fuse nano-compartmentalized volumes addresses a demand for spatiotemporal control in the field of synthetic biology, for example in the bottom-up construction of (bio)chemical nanoreactors and for the interrogation of enzymatic reactions in confined space. Herein, we mix entrapped sub-attoliter volumes of liposomes (~135 nm diameter) via lipid bilayer fusion, facilitated by the hybridization of membrane-anchored lipidated oligonucleotides. We report on an improved synthesis of the membrane-anchor phosphoramidites that allows for a flexible choice of lipophilic moiety. Lipid-nucleic acid conjugates (LiNAs) with and without triethylene glycol spacers between anchor and the 17 nt binding sequence were synthesized and their fusogenic potential evaluated. A fluorescence-based content mixing assay was employed for kinetic monitoring of fusion of the bulk liposome populations at different temperatures. Data obtained at 50 °C indicated a quantitative conversion of the limiting liposome population into fused liposomes and an unprecedently high initial fusion rate was observed. For most conditions and designs only low leakage during fusion was observed. These results consolidate LiNA-mediated membrane fusion as a robust platform for programming compartmentalized chemical and enzymatic reactions.

Antibiotic interactions using liposomes as model lipid membranes

Lipidomics and proteomics have undergone a tremendous revolution, and the knowledge about drugs' mechanism of action in biological membranes has been deepened. Methods to study the interactions of drugs with biological membranes have opened new perspectives to rational drug design, based not only in the pharmacological target of the drugs but also on the interaction with biological membranes. These methods expand our ability to acquire the ADME-Tox profile of drugs, simplifying the complexity of biological membranes. Particularly, antibiotic resistance is considered one of the greatest threats to human health, being the prospects for replacing current antimicrobial drugs extremely scarce. With the decline of the discovery and the emergence of multidrug resistant pathogens to the existing arsenal, the objective in the development of new drugs to combat the resistance to antibiotics has been replaced by the modification of existing antibiotics. Therefore, drug-membrane interaction studies using membrane models of the eukaryotic and prokaryotic cell membranes, associated with a broad of complementary methods, may contribute to a deep picture concerning the effect of antibiotics upon their intake until their pharmacological target. This critical review will discuss the relevance of a range of different methods to study the interaction of antibiotic drugs using liposomes as biological membranes models. The advantages and the limitations of these methods will be discussed and future perspectives in this field will be proposed.

Effect of Interfacial Water on the Nanomechanical Properties of Negatively Charged Floating Bilayers Supported on Gold Electrodes

Floating lipid bilayers composed of phosphatidylglycerols and cardiolipin were deposited on gold electrodes premodified with 1-thio-β-d-glucose monolayer by spreading of small unilamellar vesicles. The resulting lipid membrane was homogeneous, and its thickness was ∼5.0 nm. Electrochemical characterization combined with surface-enhanced infrared absorption spectroscopy revealed that negative polarization of the electrode leads to accumulation of water molecules in the interfacial region between lipid membrane and the thioglucose film. Moreover, the buildup of water layer was demonstrated to affect the nanomechanical properties of the membrane. The latter was manifested by well-pronounced decrease of Young's modulus of the lipid bilayer correlating with increasing hydration. This effect was ascribed to the decoupling of the membrane from supporting thioglucose film due to the accumulation of interfacial water. As a result, the effective stiffness of the supporting layer is lower and it alters the nanomechanical behavior of lipid membrane. Our results provide strong experimental proof for the correlation between elastic properties of floating lipid membrane and the amount of water accumulated in the submembrane region.

Tumor Microenvironment and Cell Fusion

Cell fusion is a highly regulated biological process that occurs under both physiological and pathological conditions. The cellular and extracellular environment is critical for the induction of the cell-cell fusion. Aberrant cell fusion is initiated during tumor progression. Tumor microenvironment is a complex dynamic system formed by the interaction between tumor cells and their surrounding cells. Cell-cell fusion mediates direct interaction between tumor cells and their surrounding cells and is associated with tumor initiation and progression. Various microenvironmental factors affect cell fusion in tumor microenvironment and generate hybrids that acquire genomes of both parental cells and exhibit novel characteristics, such as tumor stem cell-like properties, radioresistance, drug resistance, immune evasion, and enhanced migration and invasion abilities, which are closely related to the initiation, invasion, and metastasis of tumor. The phenotypic characteristics of hybrids are based on the phenotypes of parental cells, and the fusion of tumor cells with diverse types of microenvironmental fusogenic cells is concomitant with phenotypic heterogeneity. This review highlights the types of fusogenic cells in tumor microenvironment that can fuse with tumor cells and their specific significance and summarizes the various microenvironmental factors affecting tumor cell fusion. This review may be used as a reference to develop strategies for future research on tumor cell fusion and the exploration of cell fusion-based antitumor therapies.

Formation and Reactivity of New Isoporphyrins: Implications for Understanding the Tyr-His Cross-Link Cofactor Biogenesis in Cytochrome c Oxidase

Cytochrome c oxidase (CcO) catalyzes the reduction of dioxygen to water utilizing a heterobinuclear active site composed of a heme moiety and a mononuclear copper center coordinated to three histidine residues, one of which is covalently cross-linked to a tyrosine residue via a post-translational modification (PTM). Although this tyrosine-histidine moiety has functional and structural importance, the pathway behind this net oxidative C-N bond coupling is still unknown. A novel route employing an iron(III) meso-substituted isoporphyrin derivative, isoelectronic with Cmpd-I ((Por^•+)Fe^IV═O), is for the first time proposed to be a key intermediate in the Tyr-His cofactor biogenesis. Newly synthesized iron(III) meso-substituted isoporphyrins were prepared with azide, cyanide, and substituted imidazole functionalities, by adding nucleophiles to an iron(III) π-dication species formed via addition of trifluoroacetic acid to F₈Cmpd-I (F₈ = (tetrakis(2,6-difluorophenyl)porphyrinate)). Isoporphyrin derivatives were characterized at cryogenic temperatures via ESI-MS and UV-vis, ²H NMR, and EPR spectroscopies. Addition of 1,3,5-trimethoxybenzene or 4-methoxyphenol to the imidazole-substituted isoporphyrin led to formation of the organic product containing the imidazole coupled to aromatic substrate via a new C-N bond, as detected via cryo-ESI-MS. Experimental evidence for the formation of an imidazole-substituted isoporphyrin and its promising reactivity to form the imidazole-phenol coupled product yields viability to the herein proposed pathway behind the PTM (i.e., biogenesis) leading to the key covalent Tyr-His cross-link in CcO.