Exciton Annihilation and Energy Transfer in Self-Assembled Peptide−Porphyrin Complexes Depends on Peptide Secondary Structure

Bioinspired porphyrin-peptide supramolecular assemblies and their applications

Inspired by the hierarchical chiral assembly of porphyrin-proteins in photosynthetic systems, the hierarchical self-assembly of porphyrin-amino acids/peptides provides a novel strategy for constructing functional materials. How to artificially simulate the assembly of porphyrins, proteins, and other cofactors in the photosynthesis system to obtain persistent strong light capture, charge separation and catalytic reactions has become an important concern in the construction of biomimetic photosynthesis systems. This paper summarizes the different assembly strategies adopted in recent years, the effects of driving forces on self-assembly, and the application of porphyrin-peptides in catalysis and biomedicine, and briefly discusses the challenges and prospects for future research.

Characteristics and therapeutic applications of antimicrobial peptides

The demand for novel antimicrobial compounds is rapidly growing due to the phenomenon of antibiotic resistance in bacteria. In response, numerous alternative approaches are being taken including use of polymers, metals, combinatorial approaches, and antimicrobial peptides (AMPs). AMPs are a naturally occurring part of the immune system of all higher organisms and display remarkable broad-spectrum activity and high selectivity for bacterial cells over host cells. However, despite good activity and safety profiles, AMPs have struggled to find success in the clinic. In this review, we outline the fundamental properties of AMPs that make them effective antimicrobials and extend this into three main approaches being used to help AMPs become viable clinical options. These three approaches are the incorporation of non-natural amino acids into the AMP sequence to impart better pharmacological properties, the incorporation of AMPs in hydrogels, and the chemical modification of surfaces with AMPs for device applications. These approaches are being developed to enhance the biocompatibility, stability, and/or bioavailability of AMPs as clinical options.

Light polarization dependency existing in the biological photosystem and possible implications for artificial antenna systems

The processes of biological photosynthesis provide inspiration and valuable lessons for artificial energy collection, transfer, and conversion systems. The extraordinary efficiency of each sequential process of light to biomass conversion originates from the unique architecture and mechanism of photosynthetic proteins. Near 100% quantum efficiency of energy transfer in biological photosystems is achieved by the chlorophyll assemblies in antenna complexes, which also exhibit a significant degree of light polarization. The three-dimensional chiral assembly of chlorophylls is an optimized biological architecture that enables maximum energy transfer efficiency with precisely designed coupling between chlorophylls. In this review, we summarize the key lessons from the photosynthetic processes in biological photosystems, and move our focus to energy transfer mechanisms and the chiral structure of the chlorophyll assembly. Then, we introduce recent approaches and possible implications to realize the biological energy transfer processes on bioinspired scaffold-based artificial antenna systems.

The sequential structure of tripyridiniumylporphyrin pendants in water-soluble copolymers and their association behaviour with tetrasulfonatophenylporphyrin guests: UV-vis absorption and fluorescence emission spectra study

A novel cationic tripyridiniumylporphyrin monomer, 5-[4-[2-(acryloyloxy)ethoxy]phenyl]-l0,l5,20-tris(N-methyl-4-pyridiniumyl)porphyrinate zinc(ii) (ZnTrMPyP), was synthesized, and its self-aggregation in water was studied by UV-vis absorption. The monomer was copolymerized with acrylamide in water and DMSO, respectively, to prepare the water-soluble polymers P-W and P-D. The aggregation behaviour of the copolymers in aqueous solution was investigated by UV-vis absorption and fluorescence emission spectra. The polymer P-D displayed very similar absorption and emission spectra to those of ZnTrMPyP in water, indicating that the polymer chains in P-D have no significant effect on the aggregate structure of ZnTrMPyP in aqueous media. In comparison, two new absorption bands appeared in the Q band range of polymer P-W and its fluorescence spectra red shifted and the fluorescence quantum yield decreased obviously. These characteristics remained unchanged even in a good solvent for the monomer, suggesting that a new aggregation structure for the porphyrin pendants fixed by the covalent bond was formed. According to the different dispersed states of the porphyrin monomer in water and DMSO, the porphyrin pendants should distribute randomly in the P-D polymer chains while having micro-blocky sequences in polymer P-W. The association behaviour between the copolymers and tetra(p-sulfonatophenyl)porphyrin, TSPP, bearing opposite charged substituents were studied by absorption and emission Spectra and further analyzed by the Benesi–Hildebrand and the Stern–Volmer methods. The results showed that relatively discrete porphyrin pendants in P-D formed a 1 : 1 stoichiometric complex with TSPP and both static and dynamic mechanisms were active in this quenching process, while the tightly associated porphyrin pendants in P-W interacted with TSPP as an entirety and static quenching was dominant in this process. This observation was in accordance with their sequential structure. The polymer P-W has a wider absorption range and higher absorption intensity in the long wavelength region than the porphyrin monomer, which can more efficiently absorb light to accomplish light harvesting in water.

Water-soluble random and micro-blocky copolymers P(ZnTrMPyP-AM) were prepared and the porphyrin pendants on them associate with anionic porphyrins in different manner.

Antimicrobial activity of a porphyrin binding peptide

Amphiphilic alpha-helices are common motifs used in numerous biological systems including membrane channels/pores and antimicrobial peptides (AMPs), and binding proteins, and a variety of synthetic biomaterials. Previously, an amphiphilic peptide with lysine-containing motifs was shown to reversibly bind the anionic porphyrin meso-Tetra(4-sulfonatophenyl)porphyrin (TPPS₄ ^2-) and promote the formation of excitonically coupled conductive J-aggregates. The work presented here focuses on the use of this amphiphilic peptide and derivatives as a potential antimicrobial agent. AMPs are naturally occurring components of the innate immune system, which selectively target and kill bacteria. Sequence derivatives were synthesized in which the position of the Trp, used as a fluorescence reporter, was changed. Additional variants were synthesized where the hydrophobic amino acids were replaced with Ala to reduce net hydrophobicity or where the cationic Lys residues were replaced with diaminopropionic acid (Dap). All peptide sequences retained the ability to bind TPPS₄ ^2- and promote the formation of J-aggregates. The peptides all exhibited a preference for binding anionic lipid vesicles compared to zwitterionic bilayers. The Trp position did not impact antimicrobial activity, but the substituted peptides exhibited markedly lower efficacy. The Dap-containing peptide was only active against E. coli and P. aeruginosa, while the Ala-substituted peptide was inactive at the concentrations tested. This trend was also evident in bacterial membrane permeabilization. The results indicate that the amphiphilic porphyrin binding peptides can also be used as antimicrobial peptides. The cationic nature is a driver in binding to lipid bilayers, but the overall hydrophobicity is important for antimicrobial activity and membrane disruption.

Microbial nanowires - Electron transport and the role of synthetic analogues

Electron transfer is central to cellular life, from photosynthesis to respiration. In the case of anaerobic respiration, some microbes have extracellular appendages that can be utilised to transport electrons over great distances. Two model organisms heavily studied in this arena are Shewanella oneidensis and Geobacter sulfurreducens. There is some debate over how, in particular, the Geobacter sulfurreducens nanowires (formed from pilin nanofilaments) are capable of achieving the impressive feats of natural conductivity that they display. In this article, we outline the mechanisms of electron transfer through delocalised electron transport, quantum tunnelling, and hopping as they pertain to biomaterials. These are described along with existing examples of the different types of conductivity observed in natural systems such as DNA and proteins in order to provide context for understanding the complexities involved in studying the electron transport properties of these unique nanowires. We then introduce some synthetic analogues, made using peptides, which may assist in resolving this debate. Microbial nanowires and the synthetic analogues thereof are of particular interest, not just for biogeochemistry, but also for the exciting potential bioelectronic and clinical applications as covered in the final section of the review.

STATEMENT OF SIGNIFICANCE

Some microbes have extracellular appendages that transport electrons over vast distances in order to respire, such as the dissimilatory metal-reducing bacteria Geobacter sulfurreducens. There is significant debate over how G. sulfurreducens nanowires are capable of achieving the impressive feats of natural conductivity that they display: This mechanism is a fundamental scientific challenge, with important environmental and technological implications. Through outlining the techniques and outcomes of investigations into the mechanisms of such protein-based nanofibrils, we provide a platform for the general study of the electronic properties of biomaterials. The implications are broad-reaching, with fundamental investigations into electron transfer processes in natural and biomimetic materials underway. From these studies, applications in the medical, energy, and IT industries can be developed utilising bioelectronics.

Tailorable Exciton Transport in Doped Peptide-Amphiphile Assemblies

Light-harvesting biomaterials are an attractive target in photovoltaics, photocatalysis, and artificial photosynthesis. Through peptide self-assembly, complex nanostructures can be engineered to study the role of chromophore organization during light absorption and energy transport. To this end, we demonstrate the one-dimensional transport of excitons along naturally occurring, light-harvesting, Zn-protoporphyrin IX chromophores within self-assembled peptide-amphiphile nanofibers. The internal structure of the nanofibers induces packing of the porphyrins into linear chains. We find that this peptide assembly can enable long-range exciton diffusion, yet it also induces the formation of excimers between adjacent molecules, which serve as exciton traps. Electronic coupling between neighboring porphyrin molecules is confirmed by various spectroscopic methods. The exciton diffusion process is then probed through transient photoluminescence and absorption measurements and fit to a model for one-dimensional hopping. Because excimer formation impedes exciton hopping, increasing the interchromophore spacing allows for improved diffusivity, which we control through porphyrin doping levels. We show that diffusion lengths of over 60 nm are possible at low porphyrin doping, representing an order of magnitude improvement over the highest doping fractions.

Peptide-Modulated Self-Assembly of Chromophores toward Biomimetic Light-Harvesting Nanoarchitectonics

Elegant self-assembling complexes by the combination of proteins/peptides with functional chromophores are decisively responsible for highly efficient light-harvesting and energy transfer in natural photosynthetic systems. Mimicking natural light-harvesting complexes through synthetic peptides is attractive due to their advantanges of programmable primary structure, tunable self-assembly architecture and easy availability in comparison to naturally occuring proteins. Here, an overview of recent progresses in the area of biomimetic light-harvesting nanoarchitectonics based on peptide-modulated self-assembly of chromophores is provided. Adjusting the organization of chromophores, either by creating peptide-chromophore conjugates or by the non-covalent assembly of peptides and chromophores are highlighted. The light-harvesting properties, especially the energy transfer of the biomimetic complexes are critically discussed. The applications of such complexes in the mineralization of inorganic nanoparticles, generation of molecular hydrogen and oxygen, and photosynthesis of bioactive molecules are also included.

Solvent-Tuned Self-Assembled Nanostructures of Chiral l/d-Phenylalanine Derivatives of Protoporphyrin IX

Protoporphyrin IX is a naturally occurring amphiphilic porphyrin with a rigid hydrophobic nonpolar core and two polar propionic acid substitutions on the porphyrin ring. This molecule can be modified on the hydrophilic group, which can lead to strengthened π-π-stacking and spontaneous self-assembly into novel nanostructures. Herein, we use l- phenylalanine and d-phenylalanine to modify protoporphyrin IX, and use the two derivatives for solvophobic-controlled self-assembly. Both derivatives possess two important features: 1) the aromatic core of the porphyrin for dispersive interactions and 2) a chiral amino acid to maximize the influence of chirality on selfassembly. These derivatives lead to the formation of a variety of nanostructure morphologies, such as spheres, nanofibers, lamellar structures, and thread-like and spherical shells. Solution-based self-assembly was determined by UV/Vis, fluorescence, and circular dichroism spectroscopy, and the formed nanostructures were characterized by scanning electron microscopy (SEM). Such engineered porphyrin derivatives could have potential applications in energy transport and storage, supramolecular chemistry, materials science, and medicine.

Testing the role of charge and structure on the stability of peptide-porphyrin complexes

This study aims to extend a structural and biophysical understanding of a coiled-coil based peptide model system that serves as a scaffold for the anionic porphyrin, TPPS4. This is part of an ongoing biomaterials effort to create photoelectronically active mesoscale fibrils for surface deposition and characterization of conductivity properties. The goals are two-fold: (1) to explore optimal basic side-chain moieties for tight binding to TPPS4 and (2) to test the binding of various metalated TPPS4 derivatives to our peptide model system. The latter goal is to control the electronic and redox properties of the fibrillar biomaterials. A soluble version of the peptide biomaterial was used in order to probe binding and to extract thermodynamically rigorous equilibrium binding constants. UV-visible spectroscopy and circular dichroism spectropolarimtery are used to measure the effects of binding on the Soret band of the porphyrin and the helical signal of the peptide, respectively. For the first study, it was found that lysine, ornithine, and arginine are equally robust at engaging TPPS4 with low micromolar binding affinity. In the case of the metalated porphyrins, submicromolar binding affinity was observed for Cu(II), Ni(II), and Pd(II). The ability of these metalated porphyrins to bind with high affinity is dependent largely on structural perturbations of the porphyrin molecule, rather than on induced electronic effects.

Self-assembly of highly ordered peptide amphiphile metalloporphyrin arrays

Long fibers assembled from peptide amphiphiles capable of binding the metalloporphyrin zinc protoporphyrin IX ((PPIX)Zn) have been synthesized. Rational peptide design was employed to generate a peptide, c16-AHL(3)K(3)-CO(2)H, capable of forming a β-sheet structure that propagates into larger fibrous structures. A porphyrin-binding site, a single histidine, was engineered into the peptide sequence in order to bind (PPIX)Zn to provide photophysical functionality. The resulting system indicates control from the molecular level to the macromolecular level with a high order of porphyrin organization. UV/visible and circular dichroism spectroscopies were employed to detail molecular organization, whereas electron microscopy and atomic force microscopy aided in macromolecular characterization. Preliminary picosecond transient absorption data are also reported. Reduced hemin, (PPIX)Fe(II), was also employed to highlight the material's versatility and tunability.

Characterization of mesoscale coiled-coil peptide-porphyrin complexes

Photoelectronically conductive self-assembling peptide-porphyrin assemblies have great potential in their use as biomaterials, owing largely to their environmentally responsive properties. We have successfully designed a coiled-coil peptide that can self-assemble to form mesoscale filaments and serve as a scaffold for porphyrin interaction. In our earlier work, peptide-porphyrin-based biomaterials were formed at neutral pH, but the structures were irregular at the nano- to microscale size range, as judged by atomic force microscopy. We identified a pH in which mesoscale fibrils were formed, taking advantage of the types of porphyrin interactions that are present in well-characterized J-aggregates. We used UV-visible spectroscopy, circular dichroism spectropolarimetry, fluorescence spectroscopy, and atomic force microscopy to characterize these self-assembling biomaterials. We propose a new assembly paradigm that arises from a set of unique porphyrin-porphyrin and porphyrin-peptide interactions whose structure may be readily modulated by changes in pH or peptide concentration.