Natural Melanins Research Articles

Photoreactivity and phototoxicity of experimentally photodegraded hair melanosomes from individuals of different skin phototypes.

Even though melanin is commonly viewed as natural photoprotectant, the pigment demonstrates residual photoreactivity, which under certain conditions could contribute to UVA-dependent melanomagenesis. Skin melanin is constantly exposed to external stressors, including solar radiation, which could induce photodegradation of the pigment. Although photodegradation of melanin pigments was studied in synthetic models and RPE melanosomes, photochemical and photobiological effects of experimental photodegradation of human skin melanin of different chemical composition remain unknown. In this work, melanosomes isolated from hair of individuals of different skin phototypes (I-III, V) were exposed to high-intensity violet light and its impact on physical and chemical properties of the pigments were analyzed using electron paramagnetic resonance (EPR), spectrophotometry and dynamic light scattering (DLS). Photoreactivity of photodegraded melanins was examined by EPR oximetry, EPR spin-trapping and time-resolved singlet oxygen phosphorescence. Antioxidant potential of the pigments was measured using the EPR DPPH assay. Cellular effect of the exposure of melanosome-loaded HaCaT cells to UV-Vis light was determined by MTT assay, JC-10 assay, and iodometric assay. The data revealed that experimental photodegradation increased photoreactivity of natural melanins, while decreasing their antioxidant capacity. Photodegraded melanin was responsible for higher cell death, a decrease in mitochondrial membrane potential and elevated levels of lipid hydroperoxides.

Eumelanin decorated poly(lactic acid) electrospun substrates as a new strategy for spinal cord injury treatment

Spinal cord injury (SCI) is characterized by neuroinflammatory processes that are marked by an uncontrolled activation of microglia, which directly damages neurons. Natural and synthetic melanins represent an effective tool to treat neuroinflammation because they possess immunomodulatory properties. Here, the main objective was to evaluate the effect of eumelanin-coated poly(lactic acid) (EU@PLA) aligned microfibers on in vitro model of neuroinflammation related to spinal cord injury in terms of inflammatory mediators' modulation. Aligned fibers were chosen to provide physical cues to guide axonal growth in a specific direction thus restoring the synaptic connection. Eumelanin decorated PLA electrospun substrates were produced combining electrospinning, spin coating and solid-state polymerization processes (oxidative coupling under oxygen atmosphere). Biological response in terms of antioxidant and anti-inflammatory activity was analyzed on an in vitro model of neuroinflammation [microglial cells stimulated with lipopolysaccharide (LPS)]. Cell morphology and EU@PLA mechanism of action, in terms of toll-like receptor-4 (TLR-4) involvement were assessed. The results show that EU@PLA fibers were able to decrease reactive oxygen species, nuclear factor kappa-light-chain-enhancer of activated B cells (NF-кB) expression >50 % compared to PLA + LPS and interleukin 6 (IL-6) secretion about 20 %. Finally, the mechanism of action of EU@PLA in microglia was found to be dependent on the TLR-4 signaling. Protein expression analysis revealed a decreased in TLR-4 production induced by LPS stimulation in presence of EU@PLA. Overall, our results show that EU@PLA represents an innovative and effective strategy for the control of inflammatory response in central nervous system.

Photoreactivity of Hair Melanin from Different Skin Phototypes-Contribution of Melanin Subunits to the Pigments Photoreactive Properties.

Photoreactivity of melanin has become a major focus of research due to the postulated involvement of the pigment in UVA-induced melanoma. However, most of the hitherto studies were carried out using synthetic melanin models. Thus, photoreactivity of natural melanins is yet to be systematically analyzed. Here, we examined the photoreactive properties of natural melanins isolated from hair samples obtained from donors of different skin phototypes (I, II, III, and V). X-band and W-band electron paramagnetic resonance (EPR) spectroscopy was used to examine the paramagnetic properties of the pigments. Alkaline hydrogen peroxide degradation and hydroiodic acid hydrolysis were used to determine the chemical composition of the melanins. EPR oximetry and spin trapping were used to examine the oxygen photoconsumption and photo-induced formation of superoxide anion, and time-resolved near infrared phosphorescence was employed to determine the singlet oxygen photogeneration by the melanins. The efficiency of superoxide and singlet oxygen photogeneration was related to the chemical composition of the studied melanins. Melanins from blond and chestnut hair (phototypes II and III) exhibited highest photoreactivity of all examined pigments. Moreover, melanins of these phototypes showed highest quantum efficiency of singlet oxygen photogeneration at 332 nm and 365 nm supporting the postulate of the pigment contribution in UVA-induced melanoma.

Unraveling the Structure and Function of Melanin through Synthesis.

Melanin is ubiquitous in living organisms across different biological kingdoms of life, making it an important, natural biomaterial. Its presence in nature from microorganisms to higher animals and plants is attributed to the many functions of melanin, including pigmentation, radical scavenging, radiation protection, and thermal regulation. Generally, melanin is classified into five types-eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin-based on the various chemical precursors used in their biosynthesis. Despite its long history of study, the exact chemical makeup of melanin remains unclear, and it moreover has an inherent diversity and complexity of chemical structure, likely including many functions and properties that remain to be identified. Synthetic mimics have begun to play a broader role in unraveling structure and function relationships of natural melanins. In the past decade, polydopamine, which has served as the conventional form of synthetic eumelanin, has dominated the literature on melanin-based materials, while the synthetic analogues of other melanins have received far less attention. In this perspective, we will discuss the synthesis of melanin materials with a special focus beyond polydopamine. We will emphasize efforts to elucidate biosynthetic pathways and structural characterization approaches that can be harnessed to interrogate specific structure-function relationships, including electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (ssNMR) spectroscopy. We believe that this timely Perspective will introduce this class of biopolymer to the broader chemistry community, where we hope to stimulate new opportunities in novel, melanin-based poly-functional synthetic materials.

Applications of Natural and Synthetic Melanins as Biosorbents and Adhesive Coatings

Applications of Natural and Synthetic Melanins as Biosorbents and Adhesive Coatings

The influence of iron on selected properties of synthetic pheomelanin

It is believed that while eumelanin plays photoprotective and antioxidant role in pigmented tissues, pheomelanin being more photoreactive could behave as a phototoxic agent. Although the metal ion-sequestering ability of melanin might be protective, transition metal ions present in natural melanins could affect their physicochemical properties. The aim of this research was to study iron binding by pheomelanin and analyze how such a binding affects selected properties of the melanin. Synthetic pheomelanin (CDM), prepared by enzymatic oxidation of DOPA in the presence of cysteine was analyzed by electron paramagnetic resonance (EPR) spectroscopy, spectrophotometry, chemical analysis, and time-resolved measurements of singlet oxygen phosphorescence. Iron broadened EPR signal of melanin and increased its optical absorption. Iron bound to melanin exhibited EPR signal at g = 4.3, typical for high-spin iron (III). Iron bound to melanin significantly altered the kinetics of melanin photodegradation, which in turn modified the accessibility and stability of the melanin–iron complexes as indicated by the release of iron from melanin induced by diethylenetriaminepentaacetic acid and KCN. Although bound to melanin iron little affects initial stages of photodegradation of CDM, the effect of iron becomes more pronounced at later stages of melanin photolysis.

Melanin.

Melanins are a unique class of pigments found throughout the biosphere with a wide variety of functions, structures, and presentations. Cordero and Casadevall highlight the wide range of places melanins are found and the diverse functions they play in nature.

Impairment of mixed melanin-based pigmentation in parrots.

Parrots and allies (Order Psittaciformes) have evolved an exclusive capacity to synthesize polyene pigments called psittacofulvins at feather follicles, which allows them to produce a striking diversity of pigmentation phenotypes. Melanins are polymers constituting the most abundant pigments in animals, and the sulphurated form (pheomelanin) produces colors that are similar to those produced by psittacofulvins. However, the differential contribution of these pigments to psittaciform phenotypic diversity has not been investigated. Given the color redundancy, and physiological limitations associated to pheomelanin synthesis, we hypothesized that the latter would be avoided by psittaciform birds. Here we test this by using Raman spectroscopy to identify pigments in feathers exhibiting colors suspicious of being produced by pheomelanin (i.e., dull red, yellow and grey- and green-brownish) in 26 species from the three main lineages of Psittaciformes. We detected the non-sulphurated melanin form (eumelanin) in black, grey and brown plumage patches, and psittacofulvins in red, yellow and green patches, but no evidence of pheomelanin. As natural melanins are assumed to be composed of eumelanin and pheomelanin in varying ratios, our results represent the first report of impairment of mixed melanin-based pigmentation in animals. Given that psittaciforms also avoid the uptake of circulating carotenoid pigments, these birds seem to have evolved a capacity to avoid functional redundancy between pigments, likely by regulating follicular gene expression. Ours study provides the first vibrational characterization of different psittacofulvin-based colors and thus helps to determine the relative polyene chain length in these pigments, which is related to their antireductant protection activity.

Tailoring Synthetic Melanin Nanoparticles for Enhanced Photothermal Therapy.

Melanin and its synthetic analogs (i.e., polydopamine nanomaterials) are able to transform a near-infrared (NIR) light energy source to heat for the selective killing of cancer cells. Although many of the effects on these nontoxic photothermal agents have been well documented, a concern has arisen that the extended usage of these natural and synthetic melanins might be hindered by their limited photothermal effects under low-density light irradiation. To address this issue, herein, we propose a rational and green fabrication strategy toward a new class of synthetic melanin nanoparticles (SMNPs) with superior photothermal effects via the one-pot copolymerization of two kinds of naturally occurring monomers (arginine and dopamine). The total photothermal efficiencies of these arginine-doped SMNPs could be significantly improved (i.e., ∼60% increase) by enhancing 808 nm NIR light absorption via the construction of donor-acceptor microstructures within SMNPs and decreasing nonthermal radiative transition processes via the increase of free radical concentrations within SMNPs. The resulting SMNPs demonstrated higher photothermal therapy efficiencies in both killing 4T1 cancer cells in vitro and suppressing tumor growth and recurrence compared with conventional agents. This work offers new opportunities in the structural and functional tailoring of melanin-inspired nanomaterials for cancer treatment via green fabrication strategies.

Solution-processed cellulose/melanin films and fibers

Melanins constitute a class of biomacromolecules that are abundant in nature. Although they exhibit a unique range of physical and chemical properties, their intractable nature has precluded their dissolution in common solvents and facile processing into functional materials. The authors found that commercially available 40 wt% tetra-n-butylammonium hydroxide (TBAH) solution could yield stable solutions of synthetic and natural melanins at ambient conditions and the dissolution rate could be accelerated by microwave radiation. Transmission electron microscopy images clearly show the disruption of the aggregated structure of melanins in TBAH. Scanning electron microscopy images of melanin/TBAH solutions precipitated in acetone revealed that melanins are dissolved and regenerated and not merely dispersed. To the best of the authors’ knowledge, this is the first report demonstrating a facile approach to solution processing of melanins with other biopolymers, which allows leverage of many unique properties of melanins. As an illustrative example, the authors demonstrate here the successful fabrication of cellulose/melanin composite films and fibers and their significant dye absorption capacity (62% reduction) with methylene blue.

Nanoarchitecturing of Natural Melanin Nanospheres by Layer-by-Layer Assembly: Macroscale Anti-inflammatory Conductive Coatings with Optoelectronic Tunability.

Natural melanins are biocompatible conductors with versatile functionalities. Here, we report fabrication of multifunctional poly(vinyl alcohol)/melanin nanocomposites by layer-by-layer (LBL) assembly using melanin nanoparticles (MNPs) directly extracted from sepia officinalis inks. The LBL assembly offers facile manipulation of nanotextures as well as nm-thickness control of the macroscale film by varying solvent qualities. The time-resolved absorption was monitored during the process and quantitatively studied by fractal dimension and lacunarity analysis. The capability of nanoarchitecturing provides confirmation of complete monolayer formation and leads to tunable iridescent reflective colors of the MNP films. In addition, the MNP films have durable electrochemical conductivities as evidenced by enhanced charge storage capacities for 1000 cycles. Moreover, the MNP covered ITO (indium tin oxide) substrates significantly reduced secretion of inflammatory cytokines, TNF-α, by raw 264.7 macrophage cells compared to bare ITO, by a factor of 5 and 1.8 with and without lipopolysaccharide endotoxins, respectively. These results highlight the optoelectronic device-level tunability along with the anti-inflammatory biocompatibility of the MNP LBL film. This combination of performance should make these films particularly interesting for bioelectronic device applications such as electroceuticals, artificial bionic organs, biosensors, and implantable devices.

Theoretical insights into the photo-protective mechanisms of natural biological sunscreens: building blocks of eumelanin and pheomelanin.

Eumelanin (EM) and pheomelanin (PM) are ubiquitous in mammalian skin and hair--protecting against harmful radiation from the sun. Their primary roles are to absorb solar radiation and efficiently dissipate the excess excited state energy in the form of heat without detriment to the polymeric structure. EU and PM exist as polymeric chains consisting of exotic arrangements of functionalised heteroaromatic molecules. Here we have used state-of-the-art electronic structure calculations and on-the-fly surface hopping molecular dynamics simulations to study the intrinsic deactivation paths of various building blocks of EU and PM. Ultrafast excited state decay, via electron-driven proton transfer (in EU and PM) and proton-transfer coupled ring-opening (in PM) reactions, have been identified to proceed along hitherto unknown charge-separated states in EU and PM oligomers. These results shed light on the possible relaxation pathways that dominate the photochemistry of natural skin melanins. Extrapolation of such findings could provide a gateway into engineering more effective molecular constituents in commercial sunscreens--with reduced phototoxicity.

Bio-Inspired Structural Colors Produced via Self-Assembly of Synthetic Melanin Nanoparticles.

Structural colors arising from interactions of light with submicron scale periodic structures have been found in many species across all taxa, serving multiple biological functions including sexual signaling, camouflage, and aposematism. Directly inspired by the extensive use of self-assembled melanosomes to produce colors in avian feathers, we set out to synthesize and assemble polydopamine-based synthetic melanin nanoparticles in an effort to fabricate colored films. We have quantitatively demonstrated that synthetic melanin nanoparticles have a high refractive index and broad absorption spanning across the UV-visible range, similar to natural melanins. Utilizing a thin-film interference model, we demonstrated the coloration mechanism of deposited films and showed that the unique optical properties of synthetic melanin nanoparticles provide advantages for structural colors over other polymeric nanoparticles (i.e., polystyrene colloidal particles).

Dispersive Raman spectroscopy allows the identification and quantification of melanin types.

Melanins are the most prevalent pigments in animals and are involved in visual communication by producing colored traits that often evolve as intraspecific signals of quality. Identifying and quantifying melanins are therefore essential to understand the function and evolution of melanin-based signals. However, the analysis of melanins is difficult due to their insolubility and the lack of simple methods that allow the identification of their chemical forms. We recently proposed the use of Raman spectroscopy as a simple, noninvasive technique that can be used to identify and quantify melanins in feathers and hairs. Contrarily, other authors later stated that melanins are characterized by a lack of defined Raman signals. Here, we use confocal Raman microscopy to confirm previous analyses showing that the two main chemical forms of melanins (eumelanin and pheomelanin) exhibit distinct Raman signal and compare different excitation wavelengths to analyze synthetic pheomelanin and natural melanins in feathers of different species of birds. Our analyses indicate that only laser excitation wavelengths below 1064 nm are useful for the analysis of melanins by Raman spectroscopy, and only 780-nm laser in the case of melanins in feathers. These findings show that the capacity of Raman spectroscopy to distinguish different chemical forms of melanins depends on laser power and integration time. As a consequence, Raman spectroscopy should be applied after preliminar analyses using a range of these parameters, especially in fragile biological tissues such as feathers.

Bacterial melanin in rat models of Parkinson's disease: a potential neuroprotective strategy.

Melanins are widely used in medicine, pharmacology and cosmetics. Different technologies have been used to obtain melanin including: chemical synthesis based on oxidation of tyrosine and its derivatives; extraction from animal materials; alkaline extraction from plant material; and microbiological synthesis. A few number of works have been published that were focused on purification of water insoluble 3,4-dihydroxy-phenylalanine-melanins (Kukulianskaia et al., 2002). The majority of synthetic and natural melanins are insoluble in water that significantly complicates preparation of pharmacological and cosmetic preparations. Obtaining of low-cost soluble biotechnological melanin can speed up application of melanin in medicine and other fields. For the first time, melanin-synthesizing strain with high level of pigment synthesis – Bacillus thuringiensis was obtained. The ecologically safe technology of biosynthesis, isolation and purification of the bacterial melanin has been elaborated.

Biologically derived melanin electrodes in aqueous sodium-ion energy storage devices

Biodegradable electronics represents an attractive and emerging paradigm in medical devices by harnessing simultaneous advantages afforded by electronically active systems and obviating issues with chronic implants. Integrating practical energy sources that are compatible with the envisioned operation of transient devices is an unmet challenge for biodegradable electronics. Although high-performance energy storage systems offer a feasible solution, toxic materials and electrolytes present regulatory hurdles for use in temporary medical devices. Aqueous sodium-ion charge storage devices combined with biocompatible electrodes are ideal components to power next-generation biodegradable electronics. Here, we report the use of biologically derived organic electrodes composed of melanin pigments for use in energy storage devices. Melanins of natural (derived from Sepia officinalis) and synthetic origin are evaluated as anode materials in aqueous sodium-ion storage devices. Na(+)-loaded melanin anodes exhibit specific capacities of 30.4 ± 1.6 mAhg(-1). Full cells composed of natural melanin anodes and λ-MnO2 cathodes exhibit an initial potential of 1.03 ± 0.06 V with a maximum specific capacity of 16.1 ± 0.8 mAhg(-1). Natural melanin anodes exhibit higher specific capacities compared with synthetic melanins due to a combination of beneficial chemical, electrical, and physical properties exhibited by the former. Taken together, these results suggest that melanin pigments may serve as a naturally occurring biologically derived charge storage material to power certain types of medical devices.

Enhanced thermal stability of poly(vinyl alcohol) in presence of melanin

A novel method was developed to enhance the thermal stability of PVA by using natural and synthetic melanins from oxidation of dopamine. Thermogravimetric (TG) curves indicated that the synthetic melanin changed the thermal degradation behaviors of PVA and largely improved the decomposed temperature by 80–110 °C in nitrogen when incorporation of synthetic melanin with low content (0.5–2 mass%). The thermal degradation kinetics suggested the activation energies of PVA/synthetic melanin blends were much higher than these of pure PVA. Isothermal TG curves conformed that the PVA/synthetic melanin blends exhibited more thermal stability than pure PVA. Moreover, the chemical structure changes of macromolecular after degradation were characterized by using fourier transform infrared and the results suggested that elimination reaction on the first degradation step did not took place for the PVA/synthetic melanin blends at 270 °C.

Isolation, Fractionation and Characterization of Melanin‐like Pigments from Chestnut (Castanea mollissima) Shells

Melanins are known as versatile biopolymers, but the utilizations are restricted by their poor solubilities. Therefore, well soluble ones or their analogs are much desired. In this article, a new procedure was developed for fractionation of the pigments isolated from chestnut (Castanea mollissima) shells, and 3 fractions (Fr. 1, Fr. 2, and Fr. 3) were obtained. The solubilities of all the fractions in waters of different pH and in common organic solvents were studied. The physicochemical properties of the fractions were characterized for the first time on the basis of combined chemical analyses and spectroscopic methods including ultraviolet-visible (UV-Vis), Fourier transform infrared (FT-IR), electron spin resonance (ESR), and solid-state ¹³C nuclear magnetic resonance (¹³C-NMR). All the fractions could be bleached by NaOCl and H₂O₂ and give a positive reaction for polyphenols, which are usually used as typical tests for allomelanins. Their UV-Vis, FT-IR, and ESR spectra resembled those of synthetic and some natural melanins. Elemental data and quantitative analyses of ¹³C-NMR spectra revealed that pigment-bound proteins and polysaccharides were the most abundant in Fr. 1, while Fr. 2 was presented with the highest aromaticity. We provided a new, simple, and inexpensive method to fractionate the melanin-like pigments from chestnut shells. This technique can be used to produce natural melanin-like food colorants with different solubilities from chestnut shells.

Studies on Synthetic and Natural Melanin and Its Affinity for Fe(III) Ion

In this work, we measured the metal-binding sites of natural and synthetic dihydroxyindole (DHI) melanins and their respective interactions with Fe(III) ions. Besides the two acid groups detected for the DHI system: catechol (Cat) and quinone-imine (QI), acetate groups were detected in the natural oligomer by potentiometric titrations. At acidic pH values, Fe(III) complexation with synthetic melanin was detected in an Fe(OH)(CatH2Cat) interaction. With an increase of pH, three new interactions occurred: dihydroxide diprotonated catechol, Fe(OH)2(CatH2Cat)−, dihydroxide monoprotonated catechol, [Fe(OH)2(CatHCat)]2−, and an interaction resulting from the association of one quinone-imine and a catechol group, [Fe(OH)2(Qi−)(CatHCat)]3−. In the natural melanin system, we detected the same interactions involving catechol and quinone-imine groups but also the metal interacts with acetate group at pH values lower than 4.0. Furthermore, interactions in the synthetic system were also characterized by infrared spectroscopy by using the characteristic vibrations of catechol and quinone-imine groups. Finally, scanning electronic microscopy (SEM) and energy-dispersive X-ray (EDS) analysis were used to examine the differences in morphology of these two systems in the absence and presence of Fe(III) ions. The mole ratio of metal and donor atoms was obtained by the EDS analysis.

Thermooxidative Stabilization of Polymers Using Natural and Synthetic Melanins

Melanin is a biopolymer well-known for its intriguing chemical structure and physiological functions including photoprotection, radical scavenging, and metal-ion chelation. Although it has a suite of properties not common to many known organic materials, efforts to exploit those properties in technologically relevant materials have been few compared to other biopolymers such as cellulose, chitin, or collagen. Besides its natural presence in many animals including humans, melanin is also commonly consumed by humans in soups, sauces, and pastas and is widely available in large quantities from a variety of natural sources, suggesting it could serve as a nontoxic additive for enhancing the properties of common polymers. To this end, we report for the first time the potential of natural and synthetic melanins as thermal stabilizers for common polymers by evaluating the addition of melanin to several model polymers with well-known degradation pathways. When added to poly(methyl methacrylate) (PMMA) in very low ...