Melanin-Inspired Chromophoric Microparticles Composed of Polymeric Peptide Pigments

Bioinspired Three-Component System to Prepare Full-Color Functional Biomimetic Pigments

Nature provides a great source of inspiration for the development of sustainable materials with excellent properties, among which melanin with optical, electronic, and radiation protection properties are considered to be promising coloring materials. However, compared to chemical pigments, the single color, complex oxidation process, and poor solubility of natural melanin strongly limit their further applications. Here, we introduce a series of melanin-like polymeric pigments with amino acid-encoded physicochemical properties by a simple three-component reaction system. Our protocol enables artificial control of the chromophore structures through the rational design of the substrates and dopants, thereby combining the safety and functionality of biopigments with the color richness of chemical dyes. Similar to the photoprotective effect of natural melanin, the polymeric pigments showed excellent antioxidant activity in reducing free radicals and have the advantages of iridescent color, strong tinting strength, stability, and affordability. Furthermore, due to their ability to dye substrates, these biomimetic are expected to become new low-cost bioactive chromophores and find various biochemical applications such as in clothing and hair dyeing, food addition, and anticounterfeiting detection.

Interactions of Melanin with Electromagnetic Radiation: From Fundamentals to Applications

Melanin, especially integumentary melanin, interacts in numerous ways with electromagnetic radiation, leading to a set of critical functions, including radiation protection, UV-protection, pigmentary and structural color productions, and thermoregulation. By harnessing these functions, melanin and melanin-like materials can be widely applied to diverse applications with extraordinary performance. Here we provide a unified overview of the melanin family (all melanin and melanin-like materials) and their interactions with the complete electromagnetic radiation spectrum (X-ray, Gamma-ray, UV, visible, near-infrared), which until now has been absent from the literature and is needed to establish a solid fundamental base to facilitate their future investigation and development. We begin by discussing the chemistries and morphologies of both natural and artificial melanin, then the fundamentals of melanin-radiation interactions, and finally the exciting new developments in high-performance melanin-based functional materials that exploit these interactions. This Review provides both a comprehensive overview and a discussion of future perspectives for each subfield of melanin that will help direct the future development of melanin from both fundamental and applied perspectives.

Aqueous Graphene Dispersion and Biofunctionalization via Enzymatic Oxidation of Tripeptides

Graphene, a 2D carbon material, possesses extraordinary mechanical, electrical, and thermal properties, making it highly attractive for various biological applications such as biosensing, biotherapeutics, and tissue engineering. However, the tendency of graphene sheets to aggregate and restack hinders its dispersion in water, limiting these applications. Peptides, with their defined amino acid sequences and versatile functionalities, are compelling molecules with which to modify graphene-aromatic amino acids can strengthen interactions through π-stacking and charged groups can be chosen to make the sheets dispersible and stable in water. Here, a facile and green method for covalently functionalizing and dispersing graphene using amphiphilic tripeptides, facilitated by a tyrosine phenol side chain, through an aqueous enzymatic oxidation process is demonstrated. The presence of a second aromatic side chain group enhances this interaction through non-covalent support via π-π stacking with the graphene surface. Futhermore, the addition of charged moieties originating from either ionizable amino acids or terminal groups facilitates profound interactions with water, resulting in the dispersion of the newly functionalized graphene in aqueous solutions. This biofunctionalization method resulted in ≈56% peptide loading on the graphene surface, leading to graphene dispersions that remain stable for months in aqueous solutions outperforming currently used surfactants.

Light-triggered AND logic tetrapeptide dynamic covalent assembly

We demonstrate light-triggered dynamic covalent assembly of a linear short tetrapeptide containing two terminal cysteine residues in an AND logic manner. A photobase generator is introduced to accomplish light-mediated pH regulation to increase the reduction potential of thiols in the tetrapeptide, which activates its oxidative polymerization through disulfide bonds. Interestingly, it is elucidated that under light irradiation, mere co-existence of photobase generator and the oxidizing agent permits the polymerization performance of this tetrapeptide. Hence, a light-triggered AND logic dynamic covalent assembly of a tetrapeptide is achieved. Further, upon redox response, the reversible aggregation and disaggregation can be transformed for numerous times due to the dynamic covalent feature of disulfide bond. As a comparison, no assembly occurs for a short peptide containing one terminal cysteine residue under the same stimuli condition. This work offers a new approach to remotely control programmable molecular assembly of short linear peptides based on dynamic covalent bond, holding great potential in wide bioapplications.

Topochemistry for Difficult Peptide-Polymer Synthesis: Single-Crystal-to-Single-Crystal Synthesis of an Isoleucine-Based Polymer, a Hydrophobic Coating Material

Polymers of hydrophobic amino acids are predicted to be potential coating materials for the creation of hydrophobic surfaces. The oligopeptides of hydrophobic amino acids are called "difficult peptides"; as the name suggests, it is difficult to synthesize them by conventional methods. We circumvented this synthetic challenge by adopting topochemical azide-alkyne cycloaddition (TAAC) polymerization of a hydrophobic dipeptide monomer. We designed an Ile-based dipeptide, decorated with azide and alkyne, which arrange in the crystal in a head-to-tail fashion with the azide and alkyne of the adjacent molecules in a ready-to-react orientation. The monomer, on mild heating of its crystals, undergoes regiospecific TAAC polymerization to yield a 1,4-disubstituted-triazole-linked polymer in a single-crystal-to-single-crystal fashion. The solid obtained after evaporation of the monomer solution also maintained crystallinity and underwent regiospecific topochemical polymerization as in the case of crystals. This topochemical polymerization could be studied using different techniques such as FTIR, NMR, DSC, GPC, MALDI, PXRD, and SCXRD. Since the polymer is insoluble in common solvents and hence difficult to coat surfaces, the monomer was first sprayed and evaporated on various surfaces and polymerized on the surface. Such polymer-coated surfaces exhibited water contact angles of up to 134°, showing that this Ile-derived polymer is very hydrophobic and can potentially be used as a coating material for various applications.

Localized and regulated peptide pigment formation inside liquid droplets through confined enzymatic oxidation

Melanin pigments are found in most life forms, where they are responsible for coloration and ultraviolet (UV) light protection. Natural melanin is a poorly soluble and complex biosynthesis product produced through confined and templated enzymatic oxidation of tyrosine. It has been challenging to create water-soluble synthetic mimics. This study demonstrates the enzymatic synthesis of oxidized phenols confined inside liquid droplets. We use an amphiphilic, bifunctional peptide, DYFR9, that combines a tyrosine tripeptide previously shown to undergo enzymatic oxidation to form peptide pigments with broad absorbance, and polyarginine to facilitate complex coacervation in the presence of ATP. When ATP, DYFR9 are mixed and exposed to tyrosinase, pigmented liquid droplets result, while no appreciable oxidation is observed in the bulk.

Emergent properties of melanin-inspired peptide/RNA condensates

Most biocatalytic processes in eukaryotic cells are regulated by subcellular microenvironments such as membrane-bound or membraneless organelles. These natural compartmentalization systems have inspired the design of synthetic compartments composed of a variety of building blocks. Recently, the emerging field of liquid-liquid phase separation has facilitated the design of biomolecular condensates composed of proteins and nucleic acids, with controllable properties including polarity, diffusivity, surface tension, and encapsulation efficiency. However, utilizing phase-separated condensates as optical sensors has not yet been attempted. Here, we were inspired by the biosynthesis of melanin pigments, a key biocatalytic process that is regulated by compartmentalization in organelles, to design minimalistic biomolecular condensates with emergent optical properties. Melanins are ubiquitous pigment materials with a range of functionalities including photoprotection, coloration, and free radical scavenging activity. Their biosynthesis in the confined melanosomes involves oxidation-polymerization of tyrosine (Tyr), catalyzed by the enzyme tyrosinase. We have now developed condensates that are formed by an interaction between a Tyr-containing peptide and RNA and can serve as both microreactors and substrates for tyrosinase. Importantly, partitioning of Tyr into the condensates and subsequent oxidation-polymerization gives rise to unique optical properties including far-red fluorescence. We now demonstrate that individual condensates can serve as sensors to detect tyrosinase activity, with a limit of detection similar to that of synthetic fluorescent probes. This approach opens opportunities to utilize designer biomolecular condensates as diagnostic tools for various disorders involving abnormal enzymatic activity.

The Multifaceted Opportunities Provided by the Pheomelanin-Inspired 1,4-Benzothiazine Chromophore: A Still-Undervalued Issue

1,4-Benzothiazines are the main building blocks of the naturally occurring pheomelanin pigments, and their chromophoric properties have been strongly related to the well-known phototoxicity of these pigments, partly responsible for the high incidence of melanoma and other skin cancers in red-haired people. However, some peculiar features of the 1,4-benzothiazine chromophore could be functionally exploited in several sectors. Within this context, in this perspective, an overview of the very recently reported applications of the 1,4-benzothiazine chromophore in pH sensing, filter permeability control, smart packaging, electrochromic device fabrication, bioimaging, photocatalysis, and HPLC detection systems is provided, together with a brief presentation of recently developed synthetic approaches to the 1,4-benzothiazine scaffold, with the aim of emphasizing the still-undervalued multifunctional opportunities offered by this class of compounds.

In Situ Synthesis of Keratin and Melanin Chromophoric Submicron Particles

In humans, melanin plays an esthetic role, dictating hair and skin color and traits, while keratin is the protein that comprises most of the epidermis layer. Eumelanin and pheomelanin are types of melanin synthesized from the same building blocks via enzymatic oxidation. Pheomelanin has an additional building block, cysteine amino acid, which affects its final structure. Keratin contains high cysteine content, and by exploiting free thiols in hydrolyzed keratin, we demonstrate the formation of keratin-melanin (KerMel) chromophoric submicron particles. Cryo-TEM analyses found KerMel particle sizes to be 100-300 nm and arranged in the form of a central keratin particle with polymerized l-dopa chains. Attenuated total reflection (ATR)-FTIR, UV-vis, and fluorescence measurements identified new chemical bonds, indicating the formation of KerMel particles. Finally, KerMel replicated natural skin tones and proved cytocompatibility for human epidermal keratinocytes at concentrations below 0.1 mg/mL. Taken together, KerMel is a novel, tunable material that has the potential to integrate into the cosmetic industry.

Real-time fluorescence dynamics in one-step synthesis of gold nanoclusters coupling with peptide motifs

The molecule-like electronic structure endows gold nanoclusters (AuNCs) a most intriguing property, fluorescence, thereby AuNCs offer a great potential for biomedical applications. Recent efforts to improve the fluorescence of AuNCs mainly focus on tailoring size, structure and chemical environments. Herein, with the help of molecular dynamics simulation, we designed tyrosine-containing peptide motifs as the reducing agents, protecting ligands to synthesis P (peptide)-AuNCs in one-step reaction, which was developed to real-time monitor the fluorescence evolution of P-AuNCs. P-AuNCs with a quantum yield of ∼ 18 % were synthesized and further demonstrated for multiple biomedical applications, such as sensing of temperature (10-55 ℃) and metal ions (with a limit of detection of 5 nM for Hg2+), as well as cell labeling and imaging. With the excellent biocompatibility, wide spectral range and potential capacity for bio-recognition, this study provides a useful one-step synthesis strategy for screening out peptide motifs to real-time modulate the optical properties of peptide-containing hybrid nanomaterials.

Magnetic Control of Cells by Chemical Fabrication of Melanin

Melanin is an organic material biosynthesized from tyrosine in pigment-producing cells. The present study reports a simple method to generate tailored functional materials in mammalian cells by chemically fabricating intracellular melanin. Our approach exploits synthetic tyrosine derivatives to hijack the melanin biosynthesis pathway in pigment-producing cells. Its application was exemplified by synthesizing and using a paramagnetic tyrosine derivative, m-YR, which endowed melanoma cells with responsiveness to external magnetic fields. The mechanical force generated by the magnet-responsive melanin forced the cells to elongate and align parallel to the magnetic power lines. Critically, even non-pigment cells were similarly remote-controlled by external magnetic fields once engineered to express tyrosinase and treated with m-YR, suggesting the versatility of the approach. The present methodology may potentially provide a new avenue for mechanobiology and magnetogenetic studies and a framework for magnetic control of specific cells.

Sequence-Dependent Tyrosine-Containing Peptide Nanoassemblies for Sensing Tyrosinase and Melanoma

Tyrosine-containing peptide nanoassemblies have received tremendous attention because of their potential applications in biomedicine and nanomaterial fields. However, a current outstanding challenge is to direct the equilibrium between oxidative polymerization of precursors and the noncovalent assembly to precisely tune their specific nanostructures and functionalities through the rational design of peptide sequences. With a simple library of tripeptides containing tyrosine, glycine, and lysine, here we demonstrate how amino acid sequence encodes the property of tripeptide nanoassemblies by modulating the enzymatic oxidation of tyrosinase with the accompanied self-assembly, and thus select the pathways toward fluorescent or melanin-like nanoassemblies. The fluorescence of tripeptide nanoassemblies has been demonstrated in sensing both tyrosinase and melanoma. Our findings will provide inspiration of peptide sequence design for generating the complex bioactive peptide nanomaterials for biomedical applications.

Spatiotemporal Control of Melanin Synthesis in Liquid Droplets

Melanins are natural biopolymers that have remarkable properties including UV-protection, coloration, and antioxidant activity. Their biosynthesis is regulated both spatially and temporally and involves supramolecular templating and compartmentalization of enzymes and reactants within specialized organelles called melanosomes. In contrast, the laboratory-based bulk synthesis of melanin by tyrosine or dopamine oxidation is a poorly controlled process, resulting in materials with undefined properties. Inspired by the pigment's biosynthesis, we developed a methodology to spatiotemporally regulate melanin formation in liquid droplets. The spatial control is achieved by sequestration of the reaction in dextran-rich droplets of a polyethylene glycol/dextran aqueous two-phase system, where the use of a photocleavable protected tyrosine provides a temporal control over its enzymatic oxidation-polymerization. We show that the liquid droplets allow for confined local reactivity as they serve as reaction centers for melanin synthesis and compartmentalize the melanin product. This methodology opens tremendous opportunities for applications in skincare and biomedicine.

Development of Universal and Clickable Film by Mimicking Melanogenesis: On-Demand Oxidation of Tyrosine-Based Azido Derivative by Tyrosinase

In this study, we synthesized a tyrosine-based azido derivative (TBAD) that permits both substrate-independent surface coating and clickable film functionalization by mimicking natural melanogenesis. In contrast to catechol derivatives, which are generally susceptible to oxidation by air under ambient conditions, the monophenol-based TBAD remains stable under alkaline and neutral conditions, and is activated to oxidized quinone in situ by tyrosinase to initiate melanin-like polymerization. The resulting poly(TBAD) film can be formed on various substrates including noble metals, metal oxides, and synthetic polymers, which can undergo click reaction with terminal alkyne moieties on the entire surface or a specific region through Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The enzyme-mediated coating can rapidly form thin films (∼10 nm) and produce a uniform film morphology, which are important aspects in surface chemistry. This on-demand, clickable coating may become a significant tool for bioconjugation, soft lithography, and labeling techniques. This article is protected by copyright. All rights reserved.

Peptide-Based Supramolecular Systems Chemistry

Peptide-based supramolecular systems chemistry seeks to mimic the ability of life forms to use conserved sets of building blocks and chemical reactions to achieve a bewildering array of functions. Building on the design principles for short peptide-based nanomaterials with properties, such as self-assembly, recognition, catalysis, and actuation, are increasingly available. Peptide-based supramolecular systems chemistry is starting to address the far greater challenge of systems-level design to access complex functions that emerge when multiple reactions and interactions are coordinated and integrated. We discuss key features relevant to systems-level design, including regulating supramolecular order and disorder, development of active and adaptive systems by considering kinetic and thermodynamic design aspects and combinatorial dynamic covalent and noncovalent interactions. Finally, we discuss how structural and dynamic design concepts, including preorganization and induced fit, are critical to the ability to develop adaptive materials with adaptive and tunable photonic, electronic, and catalytic properties. Finally, we highlight examples where multiple features are combined, resulting in chemical systems and materials that display adaptive properties that cannot be achieved without this level of integration.

Colorful Pigments for Hair Dyeing Based on Enzymatic Oxidation of Tyrosine Derivatives

Melanin exists widely in nature and can afford a variety of colors from black to brown and red according to chemical structure differences and specific mixtures. Inspired by nature, this work reports that tyrosine derivatives with different protecting groups at its N- or C-terminal can be enzymatically oxidized into melanin-like pigments with a wide range of colors. The emergence of colorful pigments can be attributed to the incomplete enzymatic oxidation and polymerization caused by the chemical premodification of the tyrosine molecule. The pigments can be deposited on the surface of the hair to obtain a series of colorful and saturated hair dye effects. Moreover, after the pigments were coated on the hair, we can further deposit silver nanoparticles through in situ reduction, making these coatings have anti-inflammatory and antibacterial potential, thereby expanding their potential use for people with low immunity or those who work in hospitals. This work proposes a green and effective way to synthesize colorful pigments with great potential applications in the hair dying and cosmetic industries.

Green Fluorescent Tripeptide Nanostructures: Synergetic Effects of Oxidation and Hierarchical Assembly

Peptide-based fluorescent materials hold promise for applications in energy harvesting and biomedicine. One remaining challenge is to overcome the barrier of fluorescence red shift, especially for peptide probes merely made of natural amino acids. Here we demonstrated an about 100 nm fluorescence red shift using a tripeptide GYK. Under UV illumination or in Fenton reactions, the hydroxyl free radical-based oxidation crosslinks GYK tripeptides into dimers, which can further act as building blocks to hierarchically assemble into nanostructures of different sizes and finally can shift the fluorescence from blue to green color (∼510 nm). Such assemblies can form core-shell-like nanostructures through further crosslinking of their surface with additional GYK monomers, which can not only make the nanostructures more robust but also efficiently improve their quantum yields. This research will deepen our understanding of bioluminescence, which sheds light on various biomedical applications of peptide-based fluorescent probes.