Cu2+ probe of metal-ion binding sites in melanin using electron paramagnetic resonance spectroscopy. I. Synthetic melanins

Multifunctional Natural and Synthetic Melanin for Bioelectronic Applications: A Review

Emerging material interest in bioelectronic applications has highlighted natural melanin and its derivatives as promising alternatives to conventional synthetic conductors. These materials, traditionally noted for their adhesive, antioxidant, biocompatible, and biodegradable properties, have barely been used as conductors due to their extremely low electrical activities. However, recent studies have demonstrated good conductive properties in melanin materials that promote electronic-ionic hybrid charge transfer, attributed to the formation of an extended conjugated backbone. This review examines the multifunctional properties of melanin materials, focusing on their chemical and electrochemical synthesis and their resulting structure-property-function relationship. The wide range of bioelectronic applications will also be presented to highlight their importance and potential to expand into new design concepts for high-performance electronic functional materials. The review concludes by addressing the current challenges in utilizing melanin for biodegradable bioelectronics, providing a perspective on future developments.

A Comparative Study of Cr(VI) Sorption by Aureobasidium pullulans AKW Biomass and Its Extracellular Melanin: Complementary Modeling with Equilibrium Isotherms, Kinetic Studies, and Decision Tree Modeling

Melanin as a natural polymer is found in all living organisms, and plays an important role in protecting the body from harmful UV rays from the sun. The efficiency of fungal biomass (Aureobasidium pullulans) and its extracellular melanin as Cr(VI) biosorbents was comparatively considered. The efficiency of Cr(VI) biosorption by the two sorbents used was augmented up to 240 min. The maximum sorption capacities were 485.747 (fungus biomass) and 595.974 (melanin) mg/g. The practical data were merely fitted to both Langmuir and Freundlich isotherms. The kinetics of the biosorption process obeyed the pseudo-first-order. Melanin was superior in Cr(VI) sorption than fungal biomass. Furthermore, four independent variables (contact time, initial concentration of Cr(VI), biosorbent dosage, and pH,) were modeled by the two decision trees (DTs). Conversely, to equilibrium isotherms and kinetic studies, DT of fungal biomass had lower errors compared to DT of melanin. Lately, the DTs improved the efficacy of the Cr(VI) removal process, thus introducing complementary and alternative solutions to equilibrium isotherms and kinetic studies. The Cr(VI) biosorption onto the biosorbents was confirmed and elucidated through FTIR, SEM, and EDX investigations. Conclusively, this is the first report study attaining the biosorption of Cr(VI) by biomass of A. pullulans and its extracellular melanin among equilibrium isotherms, kinetic study, and algorithmic decision tree modeling.

Genome sequencing of Cladophialophora exuberans, a novel candidate for bioremediation of hydrocarbon and heavy metal polluted habitats

Cladophialophora exuberans is a filamentous fungus related to black yeasts in the order Chaetothyriales. These melanized fungi are known for their 'dual ecology', often occurring in toxic environments and also being frequently involved in human infection. Particularly Cladophialophora exuberans, C. immunda, C. psammophila, and Exophiala mesophila have been described with a pronounced ability to degrade aromatic compounds and xenobiotic volatiles, such as benzene, toluene, ethyl-benzene, and xylene, and are candidates for bioremediation applications. The objective of the present study is the sequencing, assembly, and description of the whole genome of C. exuberans focusing on genes and pathways related to carbon and toxin management, assessing the tolerance and bioremediation of lead and copper, and verifying the presence of genes for metal homeostasis. Genomic evaluations were carried out through a comparison with sibling species including clinical and environmental strains. Tolerance of metals was evaluated via a microdilution method establishing minimum inhibitory (MIC) and fungicidal concentrations (MFC), and agar diffusion assays. Heavy metal bioremediation was evaluated via graphite furnace atomic absorption spectroscopy (GFAAS). The final assembly of C. exuberans comprised 661 contigs, with genome size of 38.10 Mb, coverage of 89.9X and a GC content of 50.8%. In addition, inhibition of growth was shown at concentrations of 1250 ppm for copper and at 625 ppm for lead, using the MIC method. In the agar tests, the strain grew at 2500 ppm of copper and lead. In GFAAS tests, uptake capacities were observed of 89.2% and 95.7% for copper and lead, respectively, after 21 experimental days. This study enabled the annotation of genes involved in heavy metal homeostasis and also contributed to a better understanding of the mechanisms used in tolerance of and adaptation to extreme conditions.

MoS2/PDA@Cu composite as a peroxidase-mimicking enzyme with high-effect antibacterial and anticancer activity

Since nanozymes were proposed, their applications have become more and more extensive. As a research hotspot in recent years, MoS₂ also shows many enzyme-like properties. However, as a novel peroxidase, MoS₂ has the disadvantage of a low maximum reaction rate. In this study, the MoS₂/PDA@Cu nanozyme was synthesized by a wet chemical method. The modification of PDA on the surface of MoS₂ achieved the uniform growth of small-sized Cu Nps. The obtained MoS₂/PDA@Cu nanozyme displayed excellent peroxidase-like activity and antibacterial properties. The minimum inhibitory concentration (MIC) of the MoS₂/PDA@Cu nanozyme against S. aureus reached 25 μg mL^-1. Furthermore, it showed a more pronounced inhibitory effect on bacterial growth with the addition of H₂O₂. The maximum reaction rate (V_max) of the MoS₂/PDA@Cu nanozyme is 29.33 × 10^-8 M s^-1, which is significantly higher as compared to that of HRP. It also exhibited excellent biocompatibility, hemocompatibility and potential anticancer properties. When the concentration of the nanozyme was 160 μg mL^-1, the viabilities of 4T1 cells and Hep G2 cells were 45.07% and 32.35%, respectively. This work indicates that surface regulation and electronic transmission control are good strategies for improving peroxidase-like activity.

Graphite-like Charge Storage Mechanism in a 2D π-d Conjugated Metal-Organic Framework Revealed by Stepwise Magnetic Monitoring

Quasi-two-dimensional (2D) fully π-d conjugated metal-organic frameworks (MOFs) have been widely employed as active materials of secondary batteries; however, the origin of their high charge storage capacity is still unknown. Some reports have proposed a mechanism by assuming the formation of multiple radicals on one organic ligand, although there is no firm evidence for such a mechanism, which would run counter to the resonance theory. In this work, we utilized various magnetometric techniques to monitor the formation and concentration of paramagnetic species during the electrochemical process of 2D π-d conjugated Cu-THQ MOF (THQ = tetrahydroxy-1,4-benzoquinone). The spin concentration of the fully reduced (discharged 1.5 V) electrode was estimated to be around only 0.1 spin-1/2 per CuO₄ unit, which is much lower than that of the expected "diradical" form. More interestingly, a significant elevation of the temperature-independent paramagnetic term was simultaneously observed, which indicates the presence of delocalized π electrons in this discharged state. Such results were corroborated by first-principles density functional theory calculations and the electrochemically active density of states, which reveal the microscopic mechanism of the charge storage in the Cu-THQ MOF. Hence, a graphite-like charge storage mechanism, where the π-electron band accepts/donates electrons during the charge/discharge process, was suggested to explain the excessive charge storage of Cu-THQ. This graphite-like charge storage mechanism revealed by magnetic studies can be readily generalized to other π-d conjugated MOFs.

Melanoma, Melanin, and Melanogenesis: The Yin and Yang Relationship

Melanin pigment plays a critical role in the protection against the harmful effects of ultraviolet radiation and other environmental stressors. It is produced by the enzymatic transformation of L-tyrosine to dopaquinone and subsequent chemical and biochemical reactions resulting in the formation of various 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihydroxyindole (DHI) oligomers-main constituents of eumelanin, and benzothiazine and benzothiazole units of pheomelanin. The biosynthesis of melanin is regulated by sun exposure and by many hormonal factors at the tissue, cellular, and subcellular levels. While the presence of melanin protects against the development of skin cancers including cutaneous melanoma, its presence may be necessary for the malignant transformation of melanocytes. This shows a complex role of melanogenesis in melanoma development defined by chemical properties of melanin and the nature of generating pathways such as eu- and pheomelanogenesis. While eumelanin is believed to provide radioprotection and photoprotection by acting as an efficient antioxidant and sunscreen, pheomelanin, being less photostable, can generate mutagenic environment after exposure to the short-wavelength UVR. Melanogenesis by itself and its highly reactive intermediates show cytotoxic, genotoxic, and mutagenic activities, and it can stimulate glycolysis and hypoxia-inducible factor 1-alpha (HIF-1α) activation, which, combined with their immunosuppressive effects, can lead to melanoma progression and resistance to immunotherapy. On the other hand, melanogenesis-related proteins can be a target for immunotherapy. Interestingly, clinicopathological analyses on advanced melanomas have shown a negative correlation between tumor pigmentation and diseases outcome as defined by overall survival and disease-free time. This indicates a "Yin and Yang" role for melanin and active melanogenesis in melanoma development, progression, and therapy. Furthermore, based on the clinical, experimental data and diverse effects of melanogenesis, we propose that inhibition of melanogenesis in advanced melanotic melanoma represents a realistic adjuvant strategy to enhance immuno-, radio-, and chemotherapy.

Fast and Reliable Synthesis of Melanin Nanoparticles with Fine-Tuned Metal Adsorption Capacities for Studying Heavy Metal Ions Uptake

Purpose

Adsorption and uptake of heavy metals by polymeric nanoparticles is driven by a variety of physicochemical processes. In this work, we examined heavy metal uptake by synthetic melanin nanoparticles and analyzed physicochemical properties that affect the extent of metal uptake by the nanoparticles.

Methods

Eumelanin nanoparticles were synthesized in a one-pot fast process from a 5,6-diacetoxy indole precursor that is hydrolyzed in situ into dihydroxy indole (DHI). The method allows the possibility of changing the level of sodium ions that ends up in the nanoparticles. Two variants of synthetic DHI–melanin (low-sodium and high sodium variants) were evaluated and demonstrated different relative adsorption efficiencies for heavy metal cations.

Results and Discussion

For the low-sodium DHI–melanin and in terms of percentages of metal ion removal, the relative order of extraction from 50 ppm solutions was Zn²⁺ > Cd²⁺ > Ni²⁺ > Co²⁺ > Cu²⁺ > Pb²⁺, with the extraction percentages ranging from 90% down to 76%, for a 30-minute adsorption time before equilibrium. The lower-sodium DHI–melanin consistently removed more Zn²⁺ than the higher-sodium variant. Electron microscopy (SEM) showed an increase in melanin particle size after metal ions uptake. In addition, X-ray photoelectron spectroscopy (XPS) of DHI–melanin particles with depth profiling after Zn ions uptake supported particle swelling and ion transport within the particles.

Conclusion

These initial studies showed the potential of this straightforward synthesis to obtain synthetic DHI–melanin nanoparticles similar to those from biological sources with the possibility to fine-tune their metal adsorption capacity. These synthetic nanoparticles can be used either for the removal of a variety of metal ions or to mimic and study mechanisms of metal uptake by melanin deriving from biological sources, with the potential to understand, for instance, differential heavy metal uptake by various melanic pigments.

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.

Polydopamine-Modified Copper-Doped Titanium Dioxide Nanotube Arrays for Copper-Catalyzed Controlled Endogenous Nitric Oxide Release and Improved Re-Endothelialization

The controllable release is necessary for ideal drug delivery technologies. Because of their high specific surface area and high porosity, titanium dioxide nanotubes (TNTs) have been widely used as drug carriers in medical devices. By loading copper as the catalyst, nitric oxide (NO) generation was facilitated by catalyzing the decomposition of renewable endogenous NO donors in vivo. Herein, the long-term controllable release profile of NO is highlighted owing to the multilayer polydopamine (PDA) cap structure. Different layers of PDA are used to adjust the NO release behavior, and the results show that three layers of PDA can not only effectively prevent the burst release of NO but also maintain long-term stable release of copper ion and NO. The bioactivity of the NO generated from three-layer PDA-modified copper-loaded TNTs (PDA-3L-NTCu2) and unmodified copper-loaded TNTs (NTCu2) are verified by our work, indicating effective inhibition of platelet activation, thrombosis, inflammation, and intimal hyperplasia. Importantly, the PDA-3L-NTCu2 show selectively promote the growth of endothelial cells in vitro and outstanding re-endothelialization for 4 weeks in vivo, as compared to NTCu2, TNTs, and 316L stain steel. This study suggests that copper-loaded with PDA modification helps us achieve controlled long-term stable local NO release with well-retained bioactivity and enhanced re-endothelialization.

Engineering proton conductivity in melanin using metal doping

The proton conductivity in the model bioelectronic material melanin, is increased via a unique doping strategy utilising the chelation of the transition metal ion copper II. We also propose a potential mechanism for future such ionic studies.

Autobiography of James S. Hyde

The papers, book chapters, reviews, and patents by James S. Hyde in the bibliography of this document have been separated into EPR and MRI sections, and within each section by topics. Within each topic, publications are listed chronologically. A brief summary is provided for each patent listed. A few publications and patents that do not fit this schema have been omitted. This list of publications is preceded by a scientific autobiography that focuses on selected topics that are judged to have been of most scientific importance. References to many of the publications and patents in the bibliography are made in the autobiography.

Surface-rough Fe-N/C composite wrapped on carbon nanotubes as efficient electrocatalyst for oxygen reduction reaction

Fe-N/C composites are considered one of the most promising non-precious-metal electrocatalysts for oxygen reduction reaction (ORR). In this paper, we fabricate a novel and efficient carbon nanotube (CNT)-supported Fe-N/C composite catalyst, via the surface-self-polymerization of polydopamine and then the incorporation with Fe species on CNTs, followed by the pyrolysis process. The obtained catalyst demonstrates excellent electrocatalytic performance towards ORR in alkaline media. The modification of Fe-incorporated nitrogen-rich-carbons (Fe-CN_x) on CNTs lowers the ORR half-wave-potential by ∼190 mV, giving this catalyst with an onset ORR potential of 0.95 V (versus reversible hydrogen electrode (RHE)), a half-wave potential of 0.82 V (versus RHE), and the limiting current density of 5.39 mA cm^-2 in 0.1 M KOH. The performance of the as-prepared catalyst is comparatively better than the commercially available Pt/C in terms of positive half-wave potential and larger limiting current, superior durability, and higher tolerance to the methanol.

Redox Active Transition Metal ions Make Melanin Susceptible to Chemical Degradation Induced by Organic Peroxide

With aging, retinal pigment epithelium melanosomes, by fusion with the age pigment lipofuscin, form complex granules called melanolipofuscin. Lipofuscin granules may contain oxidized proteins and lipid hydroperoxides, which in melanolipofuscin could chemically modify melanin polymer, while transition metal ions present in melanin can accelerate such oxidative modifications. The aim of this research was to examine the effect of selected transition metal ions on melanin susceptibility to chemical modification induced by the water-soluble tert-butyl hydroperoxide used as an oxidizing agent. Synthetic melanin obtained by DOPA autooxidation and melanosomes isolated from bovine retinal pigment epithelium were analyzed. To monitor tert-butyl hydroperoxide-induced oxidative changes of DMa and BMs, electron paramagnetic resonance spectroscopy, UV-vis absorption spectroscopy, dynamic light scattering, atomic force microscopy and electron paramagnetic resonance oximetry were employed. These measurements revealed that both copper and iron ions accelerated chemical degradation induced by tert-butyl hydroperoxide, while zinc ions had no effect. Strong prooxidant action was detected only in the case of melanosomes and melanin degraded in the presence of iron. It can be postulated that similar chemical processes, if they occur in situ in melanolipofuscin granules of the human retinal pigment epithelium, would modify antioxidant properties of melanin and its reactivity.

Melanins and melanogenesis: from pigment cells to human health and technological applications

During the past decade, melanins and melanogenesis have attracted growing interest for a broad range of biomedical and technological applications. The burst of polydopamine-based multifunctional coatings in materials science is just one example, and the list may be expanded to include melanin thin films for organic electronics and bioelectronics, drug delivery systems, functional nanoparticles and biointerfaces, sunscreens, environmental remediation devices. Despite considerable advances, applied research on melanins and melanogenesis is still far from being mature. A closer intersectoral interaction between research centers is essential to raise the interests and increase the awareness of the biomedical, biomaterials science and hi-tech sectors of the manifold opportunities offered by pigment cells and related metabolic pathways. Starting from a survey of biological roles and functions, the present review aims at providing an interdisciplinary perspective of melanin pigments and related pathway with a view to showing how it is possible to translate current knowledge about physical and chemical properties and control mechanisms into new bioinspired solutions for biomedical, dermocosmetic, and technological applications.

Integration of inorganic nanostructures with polydopamine-derived carbon: tunable morphologies and versatile applications

Polydopamine (PDA), a mussel adhesive-inspired biomimetic polymer, has attracted tremendous attention owing to its extremely versatile adhesion properties, facile aqueous coating process, capability of self-assembly to form nanostructures, and abundant surface functional groups for secondary modification. PDA is also a fantastic carbon source because it gives nitrogen (N)-doped graphite-like carbon in high yield, and the carbonized PDA (C-PDA) thin coatings have similar properties to those of N-doped multilayered graphene, i.e., they exhibit high electrical conductivity, and good electrochemical and mechanical properties. In comparison with other carbon sources, an outstanding feature of PDA lies in its ease of integration with inorganic nanostructures and capability for easy tailoring the structure and morphology of the resultant composite nanostructures. In this article, different routes for the preparation of C-PDA-based composite nanostructures, such as carbon/metal oxide and carbon/Si hollow, mesoporous, core-shell, yolk-shell nanostructures, are introduced with typical examples. The structures, morphologies and properties of the C-PDA-based composite nanostructures are also reviewed, and their potential applications in various engineering fields, such as energy storage, solar water splitting, flexible electronics, catalysis, sensing and environmental engineering, are highlighted. Finally a future outlook for this fascinating composite-nanostructure enabler is also presented.

Electronic structure calculations of ESR parameters of melanin units

Melanins represent an important class of natural pigments present in plants and animals that are currently considered to be promising materials for applications in optic and electronic devices. Despite their interesting properties, some of the basic features of melanins are not satisfactorily understood, including the origin of their intrinsic paramagnetism. A number of experiments have been performed to investigate the electron spin resonance (ESR) response of melanin derivatives, but until now, there has been no consensus regarding the real structure of the paramagnetic centers involved. In this work, we have employed electronic structure calculations to evaluate the ESR parameters of distinct melanin monomers and dimers in order to identify the possible structures associated with unpaired spins in this biopolymer. The g-factors and hyperfine constants of the cationic, anionic and radicalar structures were investigated. The results confirm the existence of at least two distinct paramagnetic centers in melanin structure, identifying the chemical species associated with them and their roles in electrical conductivity.

EPR Methods for Biological Cu(II): L-Band CW and NARS

Copper has many roles in biology that involve the change of coordination sphere and/or oxidation state of the copper ion. Consequently, the study of copper in heterogeneous environments is an important area in biophysics. EPR is a primary technique for the investigation of paramagnetic copper, which is usually the isolated Cu(II) ion, but sometimes as Cu(II) in different oxidation states of multitransition ion clusters. The gross geometry of the coordination environment of Cu(II) can often be determined from a simple inspection of the EPR spectrum, recorded in the traditional X-band frequency range (9-10 GHz). Identification and quantitation of the coordinating ligand atoms, however, is not so straightforward. In particular, analysis of the superhyperfine structure on the EPR spectrum, to determine the number of coordinated nitrogen atoms, is fraught with difficulty at X-band, despite the observation that the overwhelming number of EPR studies of Cu(II) in the literature have been carried out at X-band. Greater reliability has been demonstrated at S-band (3-4 GHz), using the low-field parallel (gz) features. However, analysis relies on clear identification of the outermost superhyperfine line, which has the lowest intensity of all the spectral features. Computer simulations have subsequently indicated that the much more intense perpendicular region of the spectrum can be reliably interpreted at L-band (2 GHz). The present work describes the development of L-band EPR of Cu(II) into a routine method that is applicable to biological samples.

Hydration-controlled X-band EPR spectroscopy: a tool for unravelling the complexities of the solid-state free radical in eumelanin

Melanin, the human skin pigment, is found everywhere in nature. Recently it has gained significant attention for its potential bioelectronic properties. However, there remain significant obstacles in realizing its electronic potential, in particular, the identity of the solid-state free radical in eumelanin, which has been implicated in charge transport. We have therefore undertaken a hydration-controlled continuous-wave electron paramagnetic resonance study on solid-state eumelanin. Herein we show that the EPR signal from solid-state eumelanin arises predominantly from a carbon-centered radical but with an additional semiquinone free radical component. Furthermore, the spin densities of both of these radicals can be manipulated using water and pH. In the case of the semiquinone radical, the comproportionation reaction governs the pH- and hydration-dependent behavior. In contrast, the mechanism underlying the carbon-centered radical's pH- and hydration-dependent behavior is not clear; consequently, we have proposed a new destacking model in which the intermolecular structure of melanin is disordered due to π-π destacking, brought about by the addition of water or increased pH, which increases the proportion of semiquinone radicals via the comproportionation reaction.

Role of semiconductivity and ion transport in the electrical conduction of melanin

Melanins are pigmentary macromolecules found throughout the biosphere that, in the 1970s, were discovered to conduct electricity and display bistable switching. Since then, it has been widely believed that melanins are naturally occurring amorphous organic semiconductors. Here, we report electrical conductivity, muon spin relaxation, and electron paramagnetic resonance measurements of melanin as the environmental humidity is varied. We show that hydration of melanin shifts the comproportionation equilibrium so as to dope electrons and protons into the system. This equilibrium defines the relative proportions of hydroxyquinone, semiquinone, and quinone species in the macromolecule. As such, the mechanism explains why melanin at neutral pH only conducts when "wet" and suggests that both carriers play a role in the conductivity. Understanding that melanin is an electronic-ionic hybrid conductor rather than an amorphous organic semiconductor opens exciting possibilities for bioelectronic applications such as ion-to-electron transduction given its biocompatibility.

Characterization of human hair melanin and its degradation products by means of magnetic resonance techniques

Melanin granules (MGs) have been extracted from human Chinese black hairs by either acid hydrolysis (CH-type MGs) or enzymatic digestion (CP-type MGs), and their chemical structure investigated at the solid state by means of (13)C cross polarization magic angle spinning (CPMAS NMR) and EPR spectroscopy. Both types of MGs contain a large amount of protein that is tightly bound to the true melanin polymer, with CP-type MGs having a larger protein content than CH-type ones. Moreover, MGs may also contain variable amounts of lipid-like material. A high amount of paramagnetic metals is detected by EPR in CP-type MGs, in particular Fe(III). Iron can be bound in two chemical forms: as isolated high spin Fe(III) ions with rhombic symmetry and as small oxy-hydroxy Fe(III) aggregates. Iron is poorly available to chelators. CH-type MGs contain much fewer metals. CP-type MGs have then been subjected to partial bleaching by hydrogen peroxide in ammonia, yielding a residual solid, called residual oxidized melanin (ROM) and a soluble but still pigmented fraction called melanin free acid (MFA). MFA can be isolated by precipitation at acidic pH. The (13)C-CPMAS NMR and EPR spectra of these derivatives indicated that ROM has a structure very similar to that of parent MGs, whereas MFA shows a decrease of the protein content with respect to the melanin and a decreased amount of bound iron. Thus, the oxidative degradation of CP-type MGs is a process not involving the bulk of MGs, but rather it proceeds from the solvent-exposed outer parts to the interior.

Current understanding of the binding sites, capacity, affinity, and biological significance of metals in melanin

Metal chelation is often invoked as one of the main biological functions of melanin. In order to understand the interaction between metals and melanin, extensive studies have been carried out to determine the nature of the metal binding sites, binding capacity, and affinity. These data are central to efforts aimed at elucidating the role metal binding plays in determining the physical, structural, biological, and photochemical properties of melanin. This article examines the current state of understanding of this field.

The physical and chemical properties of eumelanin

In this article, we review the current state of knowledge concerning the physical and chemical properties of the eumelanin pigment. We examine properties related to its photoprotective functionality, and draw the crucial link between fundamental molecular structure and observable macroscopic behaviour. Where necessary, we also briefly review certain aspects of the pheomelanin literature to draw relevant comparison. A full understanding of melanin function, and indeed its role in retarding or promoting the disease state, can only be obtained through a full mapping of key structure-property relationships in the main pigment types. We are engaged in such an endeavor for the case of eumelanin.

Insight into the Binding of Divalent Cations to Sepia Eumelanin from IR Absorption Spectroscopy

IR absorption spectroscopy is used to examine the binding of the divalent cations Mg(II), Ca(II), Zn(II) and Cu(II) to melanin granules isolated from the ink sacs of Sepia officinalis. The functional groups of the melanin granules interacting with the bound metal ions are deduced by examining the effect of metal concentration on transition frequencies associated with the COOH, NH and OH moieties of the pigment. The coordinating groups vary with metal ion and with concentration. For the experimental conditions used (initial solution pH of 4, ionic strength of 100 mM and a melanin concentration of 1 mg mL(-1)) Mg(II), Ca(II) and Zn(II) bind to carboxylate groups and Cu(II) binds predominantly to phenolic (catechol) groups However, at a concentration of 10 mM Cu(II) also shows evidence of binding to carboxylate and amine groups, reflecting a secondary binding site that becomes populated as the catechol sites are depleted.

Aluminum ions accelerated the oxidative stress of copper-mediated melanin formation

A comparison between the effects of aluminum and cupric ions on the dopachrome (DC) conversion and the cooperation effect of the both ions in the DOPA oxidation to melanin pathway has been studied by UV-Vis spectrophotometric method. Both aluminum and cupric ions catalyze the DC conversion reaction, which is an important step in the melanin synthesis pathway. However, cupric ions catalyze the conversion of DC to yield 5,6-dihydroxyindole-2-carboxylic acid (DHICA) but the product of DC conversion catalyzed by aluminum is 5,6-dihydroxyindole (DHI). DOPA oxidation catalyzed by aluminum and cupric ions is studied in the presence of hydrogen peroxide. The results from our experiments provide evidence that aluminum can markedly increase the oxidative stress of copper-mediated the melanin formation and influence the properties of the melanin by means of changing the ratio of DHICA/DHI in the acidic environment (pH 5.5).

Metal binding by melanins: studies of colloidal dihydroxyindole-melanin, and its complexation by Cu(II) and Zn(II) ions

Melanins are colloidal pigments known to have a high affinity for metal ions. In this work, the nature of the metal-binding sites are determined and the binding affinities are quantified. Initial potentiometric titrations have been performed on synthetic dihydroxyindole (DHI) melanin solutions to determine the chemical speciation of quinole/quinone subunits. Two types of acidic functionalities are assignable: catechol groups, with pK(a) between 9 and 13, and quinone imines (QI), with pK(a) of 6.3. The presence of the quinone-imine tautomer has, to our knowledge, never been assessed in polymeric melanins. Melanin solutions obtained from N-methylated DHI lack the pK(a) 6.3 buffer, consistent with its inability to form the quinone-imine tautomer. EPR spectroscopy of the DHI-melanin samples demonstrates that the semiquinone radical is in too low a concentration to contribute to the bulk binding of metals. Changes in the titration curves after addition of Cu(II) and Zn(II) ions were analyzed to obtain the binding constants and stoichiometry of the metal-melanin complexes, using the BEST7 program. UV-Vis spectra at neutral and high pH are used to identify absorbances due to Cu-bound quinone imine and catechol groups. The derived binding constants were used to determine speciation of the Cu(II) and Zn(II) ions coordinated to the quinone imine and catechol groups at various pH. The mixed complexes, Zn(QI)(Cat)(-) and Cu(QI)(Cat)(-) are shown to dominate at physiological pH.

On the structure of human hair melanins from an infrared spectroscopy analysis of their interactions with Cu2+ ions

Melanins were isolated from dark and red human hair and complexed with copper ions at various pH values in a complexing medium. IR spectra of melanins and their Cu2+-complexes for pellets with KBr were obtained. The IR spectra indicate that Cu2+ ions bound to melanins are fixed by different carboxyl and hydroxyl (phenolic and/or alcoholic) groups in the macromolecule. From these results it is concluded that, generally, melanin carboxyl groups are responsible for interactions of metal ions with the melanin molecule. Complexes of melanins isolated from dark and red human hair show structural differences when analysed by IR spectroscopy. Conclusions from these investigations assist in the differentiation of structures of analysed hair melanins. IR spectral analysis of melanin samples and their complexes suggest that melanin samples obtained from red hair may contain eumelanin.

The influence of aging and low zinc nutrition on the choroid in the pig: II. The melanosome

The effects of low zinc nutrition and aging on central choroidal melanosomes were examined in the pig. Melanosomes of central choroidal melanocytes were morphologically and morphometrically examined in three populations of pigs (young, pregnant and aged), that were maintained on either control (C) or low zinc (LZ) diets. In C groups, the typical melanosomes decreased in size with age, although a subpopulation of larger melanosomes occurred in the oldest group. In contrast, the melanosomes of the animals on LZ diets increased in size significantly in the adult groups. The melanosomes in the pregnant and aged groups were 65% and 30-40% greater than those of the age-matched C groups. Extremely large melanosomes were frequently encountered in adult LZ choroidal melanocytes. Melanogenesis of these large bodies included the formation of one or more outer shells. Fusion of adjacent large melanosomes was also observed. Melanolysosomal-like bodies were observed, particularly among the pigmented cells in the large blood vessel region of C and LZ adults. Melanin dynamics, i.e. its production and breakdown, occurred within choroidal melanocytes throughout much of the lifespan of the pig. This dynamic was greatly influenced by low zinc nutrition, resulting in unusual and aberrant melanin activity.

Melanin-porphyrin interaction monitored by delayed luminescence and photoacoustics

Even though melanin is commonly viewed as a photoprotective agent, the molecular mechanism of the melanin-related photoprotective action remains unclear. We studied the interaction of a synthetic dopa melanin with positively and negatively charged porphyrins using an array of spectroscopic techniques. Thus absorption, fluorescence, time-resolved delayed luminescence in the microsecond time range, and photoacoustic spectra at different modulation frequencies of both porphyrins and their mixtures with dopa melanin were measured in buffered aqueous solution at room temperature. It has been confirmed that dopa melanin forms a complex with the cationic porphyrin in its ground state, which significantly modifies the optical properties of the dye molecule. Although no such complex can be detected for the anionic porphyrin, some interaction between melanin and the porphyrin molecule can be detected in its excited state. Both porphyrins show delayed luminescence with a decay time of 35-38 microseconds, which is substantially reduced by melanin. Our photoacoustic measurements indicate that upon photoexcitation, melanin efficiently liberates heat. Our data also suggest that the thermal properties of melanin can be modified by porphyrins, particularly by the cationic form of the dye. The ionic complexes of dopa melanin with the positively charged porphyrin exhibit properties typical for a supermolecular system that liberates heat as a whole.

Fungal melanins and their interactions with metals

Fungal melanins are dark brown or black pigments located in cell walls. They also exist as extracellular polymers. Melanized fungi possess increased virulence and resistance to microbial attack as well as enhanced survival while under environmental stress. Melanins contain various functional groups which provide an array of multiple nonequivalent binding sites for metal ions. Pigmented Cladosporium cladosporoides was shown to biosorb 2.5- to four-fold more Ni, Cu, Zn, Cd, and Pb than albino Penicillium digitatum and at four- to six-fold higher rates. Metal desorption was significantly lower for extracellular melanin than from pigmented or albino biomass which indicated the strength of the melanin-metal bond. At equilibrium, tributyltin chloride (TBTC) concentrations of 2.5 mM, pigmented and albino Aureobasidium pullulans absorbed approximately 0.9 and 0.7 mumol TBTC mg -1 dry wt, respectively, whereas purified extracellular melanin exhibited uptake levels of approximately 22 mumol TBTC mg-1 dry wt at an equilibrium concentration of only 0.4 mM. Addition of melanin to the growth medium reduced the toxic effect of CuSO4 and TBTC due to melanin metal binding and sequestration.

Effect of Cu(2+)-ions on semiconductor properties of synthetic DOPA melanin polymer

The purpose of the present study was to examine semiconductor properties of synthetic DOPA melanin, which are basic for future biological applications. DC conductivity, electron spin resonance (ESR), and atomic absorption spectroscopy (AAS) measurements have been performed to investigate the effect of Cu(2+)-ions on the semiconductor properties of melanin polymer synthesized from DOPA (3,4-dihydroxyphenylalanine). DOPA melanin - Cu2+ complexes examined show the decrease of both thermal activation energy delta Ea and pre-exponential factor sigma o values upon doping. At the same time no substantial changes in conductivity at 293 K have been observed. Formation of bipolaron states due to chelation of copper ions by melanin orthosemiquinones has been postulated. The Meyer-Neldel rule with a characteristic temperature T0 equal to 298 K and possible physiological implication of this fact are discussed. These data suggest, that DOPA melanin polymer could be useful as a type of culture substratum.

The roles of neuromelanin, binding of metal ions, and oxidative cytotoxicity in the pathogenesis of Parkinson's disease: a hypothesis

A characteristic feature of both Parkinson's disease (idiopathic paralysis agitans) and normal aging is loss of pigmented neurons in the substantia nigra. This has been found to correlate with the accumulation of neuromelanin and with oxidative stress in this brain region, but a clear association between these factors has not been established. Based on our recent demonstration that neuromelanin is a true melanin, containing bound metal ions in situ, we present a general model for its accumulation in vivo and the hypotheses (1) that it has a cytoprotective function in the sequestration of redox-active metal ions under normal conditions but (2) that it has a cytotoxic role in the pathogenesis of Parkinson's disease. Thus, neuromelanin accumulates normally through the autooxidation of catecholamines and serves tightly to bind redox-active metal ions, processes which would accelerate under conditions of intracellular or extracellular oxidative stress. Based on the known properties of melanin, however, neuromelanin also has the potential for exacerbating oxidative stress, eg by generating H2O2 when it is intact or by releasing redox-active metal ions if it loses its integrity; these reactions also would modulate the reactivity of the neuromelanin. By overwhelming intracellular antioxidative defense mechanisms, such a positive-feedback cycle could turn a condition of chronic or repeated oxidative stress in vulnerable neurons into an acute crisis, leading to cellular death. If the cumulative stress in duration and/or degree is severe enough, neuronal depletion could be sufficient to cause Parkinson's disease during life. One possible trigger for this cascade is suggested by the increased nigral iron contents in postmortem parkinsonian brains and the correlation of this disease with urban living where exposure to heavy metal ions is high: the saturation of neuromelanin with redox-active metal ions. Parkinson's disease therefore may be a form of accelerated aging in the substantia nigra associated with environmental toxins in which neuromelanin has a central, active role.

Purified human neuromelanin, synthetic dopamine melanin as a potential model pigment, and the normal human substantia nigra: characterization by electron paramagnetic resonance spectroscopy

Neuromelanin is a poorly understood pigment that accumulates in catecholaminergic neurons during normal aging. Electron paramagnetic resonance spectroscopy, an especially effective technique for investigating melanins, is used in the present study to show unambiguously that neuromelanin is a melanin; however, it is not well modeled by synthetic dopamine melanin and thus is an atypical melanin. Some of the unusual features of neuromelanin can be explained by postulating two distinct sources for its free radicals, the dominant one possibly derived from a precursor containing sulfur. Examination of human substantia nigra by electron paramagnetic resonance spectroscopy during the purification of neuromelanin also demonstrates, contrary to some other studies, that a portion of the paramagnetic metal ions in this tissue are bound to the pigment in situ. Combined with previous histochemical data, these observations have implications for the mechanism through which neuromelanin accumulates in vivo and are consistent with its having a cytoprotective function under normal conditions, but a cytotoxic role at advanced ages and in patients with Parkinson's disease. Other results of this study show that homogenizing tissues during the purification of any natural pigment may cause contamination of the pigment by extraneous metal ions and that subsequent incubation in hot acid, though most effective in removing metal ions and hydrolyzing proteins, leads to degradation of melanin. A purification procedure using incubation in acid at room temperature, however, is well suited for identifying and characterizing unknown natural pigments by electron paramagnetic resonance spectroscopy.

Properties and function of the ocular melanin--a photobiophysical view

This paper reviews the biosynthesis and physicochemical properties of the ocular melanin. Age-related changes of melanin granules and the corresponding formation of lipofuscin pigments in the retinal pigment epithelium (RPE) are also described. Adverse photoreactions of the eye and, in particular, light-induced damage to the RPE-retina are reviewed in relation to the ocular pigmentation. A hypothesis on the photoprotective role of the RPE melanin is presented that is based on the ability of the cellular melanin to bind redoxactive metal ions. Since bound-to-melanin metal ions are expected to be less damaging to the pigment cells, it is proposed that sequestration of heavy metal ions by the RPE melanin is an efficient detoxifying mechanism. It is postulated that oxidative degradation of RPE melanin may lower its metal-binding capability and decrease its anti-oxidant efficiency. Cellular and environmental factors that may contribute to possible oxidative damage of the RPE melanin are discussed in connection with the etiology of age-related macular degeneration.

Dialysis of plasma soluble lipofuscins in patients with end-stage renal failure

Fluorescence spectrophotometry demonstrates that the levels of plasma soluble lipofuscins (SL) in patients with end-stage renal failure, undergoing continuous ambulatory peritoneal dialysis (CAPD) or haemodialysis (HD), remain significantly higher than in normal subjects. Plasma samples from these patients show the presence of SL generated from 3-hydroxy-anthranilic acid [excitation (ex) at approximately 324 nm and emission (em) at approximately 413 nm] and of other SL generated from dopa, catecholamines, 3-hydroxykynurenine and from structurally related precursors (ex at approximately 345 nm, em at approximately 445 nm). These precursors form the melanin components, which are approximately 3 wt % of SL. The fluorescence of SL appears to originate mainly from the melanin components. Peaks and shoulders at these wavelengths are found in the spectra of all dialysates. Based on intensity measurements at 413 nm and 445 nm, the weekly clearance rates with HD are in general greater than those with CAPD. The saponified cellulose ester membrane used in HD passes only lower-molecular-weight SL and/or components of SL. After HD, the greatest reductions in plasma intensities are found at approximately 324 nm and approximately 413 nm. The clearance rates (l/week) are always greater at 413 nm [means HD: 18.47 +/- 4.44 standard deviation (SD), n = 8; CAPD: 12.50 +/- 2.47, n = 4] than at 445 nm (HD: 10.94 +/- 3.86; CAPD: 7.95 +/- 1.75) both with HD and CAPD. In CAPD, the membrane also permits the passage of large amounts of albumin and other high-molecular-weight substances.(ABSTRACT TRUNCATED AT 250 WORDS)