Zinc-catalyzed oxidation of 5-S-cysteinyldopa to 2,2'-bi(2H-1,4-benzothiazine): tracking the biosynthetic pathway of trichochromes, the characteristic pigments of red hair

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.

First Evidence of Pheomelanin-UVA-Driven Synthesis of Pummerer's Ketones by Peroxidase-Mediated Oxidative Coupling of Substituted Phenols

Photoexcitation of pheomelanin produces high-energy singlet oxygen and the superoxide anion, which are reactive species in damage of cellular targets. In principle, these species can be involved in processes of synthetic utility when adequate experimental conditions are defined. Here, we describe that pheomelanin performs as a selective UVA antenna for the horseradish peroxidase oxidative coupling of substituted phenols to biologically active Pummerer's ketones under 2-methyltetrahydrofuran/buffer biphasic conditions. In this system, singlet oxygen is scavenged by conversion of 2-methyltetrahydrofuran into the corresponding organic hydroperoxide, while the superoxide anion is dismutated into hydrogen peroxide. Both these intermediates are able to oxidize the active site of horseradish peroxidase triggering the oxidative coupling reaction. Trimer derivatives, produced by addition of phenoxy radicals on preformed Pummerer's ketones were also isolated, suggesting the possibility to further improve the structural complexity of the reaction products.

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.

Melanins as Sustainable Resources for Advanced Biotechnological Applications

Melanins are a class of biopolymers that are widespread in nature and have diverse origins, chemical compositions, and functions. Their chemical, electrical, optical, and paramagnetic properties offer opportunities for applications in materials science, particularly for medical and technical uses. This review focuses on the application of analytical techniques to study melanins in multidisciplinary contexts with a view to their use as sustainable resources for advanced biotechnological applications, and how these may facilitate the achievement of the United Nations Sustainable Development Goals.

Proton-Sensitive Free-Radical Dimer Evolution Is a Critical Control Point for the Synthesis of Δ2,2'-Bibenzothiazines

The mechanism of the acid-dependent interring dehydrogenation in the conversion of the single-bonded 3-phenyl-2H-1,4-benzothiazine dimer 2 to the Δ^2,2′-bi(2H-1,4-benzothiazine) scaffold of red hair pigments is disclosed herein. Integrated chemical oxidation and oxygen consumption experiments, coupled with electron paramagnetic resonance (EPR) analyses and DFT calculations, allowed the identification of a key diprotonated free-radical intermediate, which was implicated in a remarkable oxygen-dependent chain process via peroxyl radical formation and evolution to give the Δ^2,2′-bi(2H-1,4-benzothiazine) dimer 3 by interring dehydrogenation. The critical requirement for strongly acidic conditions was rationalized for the first time by the differential evolution channels of isomeric peroxyl radical intermediates at the 2- versus 3-positions. These results offer for the first time a rationale to expand the synthetic scope of the double interring dehydrogenation pathway for the preparation of novel symmetric double-bond bridged captodative heterocycles.

Bioinspired Heterocyclic Partnership in a Cyanine-Type Acidichromic Chromophore

A new red hair-inspired 1,4-benzothiazine-based scaffold is disclosed herein, built upon a modular D-π-A architecture via condensation of the easily accessible 3-phenyl-2H-1,4-benzothiazine with indole-3-carboxaldehyde. The compound was obtained in around 50% yields and was characterized by complete spectroscopic analysis. The new benzothiazine-based cyanine displayed a characteristic reversible acidichromic behavior with a marked bathochromic shift upon acidification. The chromophore resisted at least fifteen hydrochloric acid/sodium hydroxide cycles without appreciable alterations. The expedient and scalable synthetic procedure together with the pH sensitive chromophoric properties would make the new compound an attractive prototype for novel modular chromophore for pH-sensing and other applications.

The Pro-Oxidant Activity of Pheomelanin is Significantly Enhanced by UVA Irradiation: Benzothiazole Moieties Are More Reactive than Benzothiazine Moieties

It is generally considered that eumelanin (EM) is photoprotective while pheomelanin (PM) is phototoxic. A recent study using a mouse model demonstrated that PM produces reactive oxygen species (ROS) that cause DNA damage and eventually lead to melanomagenesis. A biochemical study showed that PM possesses a pro-oxidant activity. PM consists of benzothiazine (BT) and benzothiazole (BZ) moieties, BT moieties being transformed to BZ moieties by heat or light. In this study, we compared the effects of ultraviolet A (UVA) irradiation using synthetic PMs with different BT to BZ ratios and using various coat color mouse hairs. We found that UVA irradiation of BZ-PM increased glutathione (GSH) depletion and generated more H₂O₂ than UVA irradiation of BT-PM. Non-irradiated controls did not exhibit strong pro-oxidant activities. Upon UVA irradiation, yellow mouse hairs oxidized GSH and produced H₂O₂ faster than black or albino mouse hairs. Next, to examine the mechanism of the pro-oxidant activity of BT-PM and BZ-PM, we examined the pro-oxidant activities of 7-(2-amino-2-carboxyethyl)-dihydro-1,4-benzothiazine-3-carboxylic acid (DHBTCA) and 6-(2-amino-2-carboxyethyl)-4-hydroxybenzothiazole (BZ-AA) as BT and BZ monomers, respectively. Their pro-oxidant activities were similar, but a large difference was seen in the effects of ROS scavengers, which suggests that the redox reactions may proceed via singlet oxygen in BZ-AA and via superoxide anions in DHBTCA. These results show that UVA enhances the pro-oxidant activity of PM, in particular BZ-PM.

"Fifty Shades" of Black and Red or How Carboxyl Groups Fine Tune Eumelanin and Pheomelanin Properties

Recent advances in the chemistry of melanins have begun to disclose a number of important structure-property-function relationships of crucial relevance to the biological role of human pigments, including skin (photo) protection and UV-susceptibility. Even slight variations in the monomer composition of black eumelanins and red pheomelanins have been shown to determine significant differences in light absorption, antioxidant, paramagnetic and redox behavior, particle morphology, surface properties, metal chelation and resistance to photo-oxidative wear-and-tear. These variations are primarily governed by the extent of decarboxylation at critical branching points of the eumelanin and pheomelanin pathways, namely the rearrangement of dopachrome to 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA), and the rearrangement of 5-S-cysteinyldopa o-quinoneimine to 1,4-benzothiazine (BTZ) and its 3-carboxylic acid (BTZCA). In eumelanins, the DHICA-to-DHI ratio markedly affects the overall antioxidant and paramagnetic properties of the resulting pigments. In particular, a higher content in DHICA decreases visible light absorption and paramagnetic response relative to DHI-based melanins, but markedly enhances antioxidant properties. In pheomelanins, likewise, BTZCA-related units, prevalently formed in the presence of zinc ions, appear to confer pronounced visible and ultraviolet A (UVA) absorption features, accounting for light-dependent reactive oxygen species (ROS) production, whereas non-carboxylated benzothiazine intermediates seem to be more effective in inducing ROS production by redox cycling mechanisms in the dark. The possible biological and functional significance of carboxyl retention in the eumelanin and pheomelanin pathways is discussed.

Bird Integumentary Melanins: Biosynthesis, Forms, Function and Evolution

Melanins are the ubiquitous pigments distributed in nature. They are one of the main pigments responsible for colors in living cells. Birds are among the most diverse animals regarding melanin-based coloration, especially in the plumage, although they also pigment bare parts of the integument. This review is devoted to the main characteristics of bird melanins, including updated views of the formation and nature of melanin granules, whose interest has been raised in the last years for inferring the color of extinct birds and non-avian theropod dinosaurs using resistant fossil feathers. The molecular structure of the two main types of melanin, eumelanin and pheomelanin, and the environmental and genetic factors that regulate avian melanogenesis are also presented, establishing the main relationship between them. Finally, the special functions of melanin in bird feathers are also discussed, emphasizing the aspects more closely related to these animals, such as honest signaling, and the factors that may drive the evolution of pheomelanin and pheomelanin-based color traits, an issue for which birds have been pioneer study models.

Trichocyanines: a Red-Hair-Inspired Modular Platform for Dye-Based One-Time-Pad Molecular Cryptography

Current molecular cryptography (MoCryp) systems are almost exclusively based on DNA chemistry and reports of cryptography technologies based on other less complex chemical systems are lacking. We describe herein, as proof of concept, the prototype of the first asymmetric MoCryp system, based on an 8-compound set of a novel bioinspired class of cyanine-type dyes called trichocyanines. These novel acidichromic cyanine-type dyes inspired by red hair pigments were synthesized and characterized with the aid of density functional theory (DFT) calculations. Trichocyanines consist of a modular scaffold easily accessible via an expedient condensation of 3-phenyl- or 3-methyl-2H-1,4-benzothiazines with N-dimethyl- or o-methoxyhydroxy-substituted benzaldehyde or cinnamaldehyde derivatives. The eight representative members synthesized herein can be classified as belonging to two three-state systems tunable through four different control points. This versatile dye platform can generate an expandable palette of colors and appears to be specifically suited to implement an unprecedented single-use asymmetric molecular cryptography system. With this system, we intend to pioneer the translation of digital public-key cryptography into a chemical-coding one-time-pad-like system.

Support and challenges to the melanosomal casing model based on nanoscale distribution of metals within iris melanosomes detected by X-ray fluorescence analysis

Melanin within melanosomes exists as eumelanin or pheomelanin. Distributions of these melanins have been studied extensively within tissues, but less often within individual melanosomes. Here, we apply X-ray fluorescence analysis with synchrotron radiation to survey the nanoscale distribution of metals within purified melanosomes of mice. The study allows a discovery-based characterization of melanosomal metals, and, because Cu is specifically associated with eumelanin, a hypothesis-based test of the 'casing model' predicting that melanosomes contain a pheomelanin core surrounded by a eumelanin shell. Analysis of Cu, Ca, and Zn shows variable concentrations and distributions, with Ca/Zn highly correlated, and at least three discrete patterns for the distribution of Cu vs. Ca/Zn in different melanosomes - including one with a Cu-rich shell surrounding a Ca/Zn-rich core. Thus, the results support predictions of the casing model, but also suggest that in at least some tissues and genetic contexts, other arrangements of melanin may co-exist.

Pheomelanin-induced oxidative stress: bright and dark chemistry bridging red hair phenotype and melanoma

The complex interplay of genetic and epigenetic factors linking sun exposure to melanoma in the red hair phenotype hinges on the peculiar physical and chemical properties of pheomelanins and the underlying biosynthetic pathway, which is switched on by the effects of inactivating polymorphisms in the melanocortin 1 receptor gene. In addition to the long recognized UV-dependent pathways of toxicity and cell damage, a UV-independent pro-oxidant state induced by pheomelanin within the genetically determined background of the red hair phenotype has recently been disclosed. This review provides a detailed discussion of the possible UV-dependent and UV-independent chemical mechanisms underlying pheomelanin-mediated oxidative stress, with special reference to the oxygen-dependent depletion of glutathione and other cell antioxidants. The new concept of pheomelanin as a 'living' polymer and biocatalyst that may grow by exposure to monomer building blocks and may trigger autooxidative processes is also discussed. As a corollary, treatment of inflammatory skin diseases in RHP patients is briefly commented. Finally, possible concerted strategies for melanoma prevention in the red hair phenotype are proposed.

Vibrational characterization of pheomelanin and trichochrome F by Raman spectroscopy

We characterize for the first time the vibrational state of natural pheomelanin using Raman spectroscopy and model pigment synthesized from 5-S-cysteinyldopa. The shape of the Raman spectrum was very different from that of eumelanin. Four Raman bands were visible in the 500-2000 cm(-1) wavenumber region about 500, 1150, 1490 and 2000 cm(-1), which we assigned to the out-of-plane deformation and the stretching vibration of the phenyl rings, to the stretching vibration of C-N bonds or the stretching and wagging vibration of CH2, and to overtone or combination bands. Interestingly, we also show that the Raman spectrum of synthetic trichochrome F, a pigment that may be produced along with pheomelanin during pheomelanogenesis, is different from that of pheomelanin and similar to the spectrum of eumelanin. We could detect Raman signal of both eumelanin and pheomelanin in feathers and hairs where both pigments simultaneously occur without the need of isolating the pigment. This indicates that Raman spectroscopy represents a non-invasive method to detect pheomelanin and distinguish it from other pigments. This may be especially relevant to detect pheomelanin in animal skin including humans, where it has been associated with animal appearance and classification, human phototypes, prevention of skin diseases and cancer risk.

Red hair benzothiazines and benzothiazoles: mutation-inspired chemistry in the quest for functionality

Nature provides a primary source of leads for the design of π-conjugated organic chromophores and other functional molecular systems useful for molecular recognition, light harvesting, photoconversion, and other technological applications. In this Account, we draw attention to a unique group of naturally occurring heterocyclic compounds, the 2H-1,4-benzothiazines and related benzothiazole derivatives. Derived from tyrosine and cysteine, these molecules arise from a mutation-induced deviation of the melanin pathway to provide the core structure of the red human hair pigments pheomelanins. Since the elucidation of the biosynthetic pathway of pheomelanins in the 1960s, researchers have focused on 1,4-benzothiazines and red hair pigments. Not only do these molecules have interesting photochemical and molecular recognition properties, they also have compelling biomedical significance. Numerous studies have linked higher levels of pheomelanins and mutations in the pathways that produce these pigments in individuals with red hair and fair skin with an increased sensitivity to UV light and a higher susceptibility to melanoma and other skin cancers. Prompted by new data about the structure and photochemistry of the bibenzothiazine system, this Account highlights the chemistry of benzothiazines in red-haired individuals as a novel source of inspiration in the quest for innovative scaffolds and biomimetic functional systems. Model studies have gradually shed light on a number of remarkable physical and chemical properties of benzothiazine-based systems. Bibenzothiazine is a robust visible chromophore that combines photochromism and acidichromism. Benzothiazine-based polymers (synthetic pheomelanins) show remarkable photochemical, paramagnetic, and redox cycling properties. Biomimetic or synthetic manipulations of the benzothiazine systems, through decarboxylation pathways controlled by metal ions or unusually facile ring-contraction processes, can produce a diverse set of molecular scaffolds.

Mushroom tyrosinase oxidizes tyrosine-rich sequences to allow selective protein functionalization

We show that mushroom tyrosinase catalyzes the formation of reactive o-quinones on unstructured, tyrosine-rich sequences such as hemagglutinin (HA) tags (YPYDVPDYA). In the absence of exogenous nucleophiles and at low protein concentrations, the o-quinone decomposes with fragmentation of the HA tag. At higher protein concentrations (>5 mg mL⁻¹), crosslinking is observed. Besthorn's reagent intercepts the o-quinone to give a characteristic pink complex that can be observed directly on a denaturing SDS-PAGE gel. Similar labeled species can be formed by using other nucleophiles such as Cy5-hydrazide. These reactions are selective for proteins bearing HA and other unstructured poly-tyrosine-containing tags and can be performed in lysates to create specifically tagged proteins.

Uncovering the structure of human red hair pheomelanin: benzothiazolylthiazinodihydroisoquinolines as key building blocks

Biomimetic oxidation of the pheomelanin precursor 5-S-cysteinyldopa in the presence of Zn(2+) ions led to the isolation of two isomeric products, one of which could be identified as the benzothiazolylthiazinodihydroisoquinoline 5, while the other proved too unstable for a complete characterization. Both these products were converted into more stable oxidized forms, which after ethylchloroformate derivatization were characterized as the ethyl ester/ethoxycarbonyl isoquinolines 8 and 9. Compound 5 exhibited absorption characteristics similar to those of red hair pheomelanin, including a main band around 400 nm in acids. Similarly to red hair pheomelanin and synthetic pigments, 5 afforded on chemical degradation a thiazolylpyridinecarboxylic acid fragment. Model chemical studies allowed the proposal of a formation mechanism for the benzothiazole and dihydroisoquinoline systems in compound 5.

Current challenges in understanding melanogenesis: bridging chemistry, biological control, morphology, and function

Melanin is a natural pigment produced within organelles, melanosomes, located in melanocytes. Biological functions of melanosomes are often attributed to the unique chemical properties of the melanins they contain; however, the molecular structure of melanins, the mechanism by which the pigment is produced, and how the pigment is organized within the melanosome remains to be fully understood. In this review, we examine the current understanding of the initial chemical steps in the melanogenesis. Most natural melanins are mixtures of eumelanin and pheomelanin, and so after presenting the current understanding of the individual pigments, we focus on the mixed melanin systems, with a critical eye towards understanding how studies on individual melanin do and do not provide insight in the molecular aspects of their structures. We conclude the review with a discussion of important issues that must be addressed in future research efforts to more fully understand the relationship between molecular and functional properties of this important class of natural pigments.

Chemical analysis of late stages of pheomelanogenesis: conversion of dihydrobenzothiazine to a benzothiazole structure

Pheomelanogenesis is a complex pathway that starts with the oxidation of tyrosine (or DOPA, 3,4-dihydroxyphenylalanine) by tyrosinase in the presence of cysteine, which results in the production of 5-S-cysteinyldopa and its isomers. Beyond that step, relatively little has been clarified except for a possible intermediate produced, dihydro-1,4-benzothiazine-3-carboxylic acid (DHBTCA). We therefore carried out a detailed study on the course of pheomelanogenesis using DOPA and cysteine and the physiological enzyme tyrosinase. To elucidate the later stages of pheomelanogenesis, chemical degradative methods of reductive hydrolysis with hydroiodic acid and alkaline peroxide oxidation were applied. The results show that: (1) DHBTCA accumulates after the disappearance of the cysteinyldopa isomers, (2) DHBTCA is then oxidized by a redox exchange with dopaquinone to form ortho-quinonimine, which leads to the production of pheomelanin with a benzothiazine moiety, and (3) the benzothiazine moiety gradually degrades to form a benzothiazole moiety. This latter process is consistent with the much higher ratio of benzothiazole-derived units in human red hair than in mouse yellow hair. These findings may be relevant to the (photo)toxic effects of pheomelanin.

Chemistry of mixed melanogenesis--pivotal roles of dopaquinone

Melanins can be classified into two major groups-insoluble brown to black pigments termed eumelanin and alkali-soluble yellow to reddish-brown pigments termed pheomelanin. Both types of pigment derive from the common precursor dopaquinone (ortho-quinone of 3,4-dihydroxyphenylalanine) which is formed via the oxidation of l-tyrosine by the melanogenic enzyme tyrosinase. Dopaquinone is a highly reactive ortho-quinone that plays pivotal roles in the chemical control of melanogenesis. In the absence of sulfhydryl compounds, dopaquinone undergoes intramolecular cyclization to form cyclodopa, which is then rapidly oxidized by a redox reaction with dopaquinone to give dopachrome (and dopa). Dopachrome then gradually and spontaneously rearranges to form 5,6-dihydroxyindole and to a lesser extent 5,6-dihydroxyindole-2-carboxylic acid, the ratio of which is determined by a distinct melanogenic enzyme termed dopachrome tautomerase (tyrosinase-related protein-2). Oxidation and subsequent polymerization of these dihydroxyindoles leads to the production of eumelanin. However, when cysteine is present, this process gives rise preferentially to the production of cysteinyldopa isomers. Cysteinyldopas are subsequently oxidized through redox reaction with dopaquinone to form cysteinyldopaquinones that eventually lead to the production of pheomelanin. Pulse radiolysis studies of early stages of melanogenesis (involving dopaquinone and cysteine) indicate that mixed melanogenesis proceeds in three distinct stages-the initial production of cysteinyldopas, followed by their oxidation to produce pheomelanin, followed finally by the production of eumelanin. Based on these data, a casing model of mixed melanogenesis is proposed in which a preformed pheomelanic core is covered by a eumelanic surface.

Counterions influence reactivity of metal ions with cysteinyldopa model compounds

Cysteinyldopas are naturally occurring conjugates of cysteine and dopa (3,4-dihydroxy-l-phenylalanine) that are precursors to red pheomelanin pigments. Metal ions are known to influence pheomelanogenesis in vitro and may be regulatory factors in vivo. Cydo (3-[(2-amino-ethyl)sulfanyl]-4,6-di-tert-butylbenzene-1,2-diol) and CarboxyCydo (2-amino-3-(4,6-di-tert-butyl-2,3-dihydroxyphenylsulfanyl)-propionic acid) are model compounds of cysteinyldopa that retain its metal-binding functionalities but cannot polymerize due to the presence of blocking tert-butyl groups. Cydo reacts readily with zinc(II) acetate or nickel(II) acetate to form a cyclized 1,4-benzothiazine (zine) intermediate that undergoes ring contraction to form benzothiazole (zole) unless it is stabilized by coordination to a metal ion. The crystal structure of [Ni(zine)2] is reported. The acetate counteranion is required for the zinc-promoted reactivity, as neither zinc(II) sulfate nor zinc(II) chloride alone promotes the transformation. The counterion is less important for redox-active copper and iron, which both readily promote the oxidation of Cydo to zine and zole species; Cu(II) complexes of both zine and zole have been characterized by X-ray crystallography. In the case of CarboxyCydo, a 3-carboxy-1,4-benzothiazine intermediate decarboxylates to form [Cu(zine)2] under basic conditions, but in the absence of base forms a mixture of products that includes the carboxylated dimer 2,2'-bibenzothiazine (bi-zine). These products are consistent with species implicated in the pheomelanogenesis biosynthetic pathway and emphasize how metal ions, their counteranions, and reaction conditions can alter pheomelanin product distribution.

Sclerotia of the acellular (true) slime mould Fuligo septica as a model to study melanization and anabiosis

Acellular (true) slime moulds (Myxomycetes) are capable of a transition to the stage of sclerotium - a dormant form of plasmodium produced under unfavourable environmental conditions. In this study, sclerotia of Fuligo septica were analyzed by means of electron paramagnetic resonance (EPR) spectroscopy. The moulds were cultivated in vitro on filter paper, fed with oat flour, and kept until the plasmodia began to produce sclerotia. The obtained sclerotia differed in colour from yellow through orange to dark-brown. The EPR spectra revealed a free radical, melanin-like signal correlated with the depth of the colour; it was strongest in the dark sclerotia. Sclerotization only took place when the plasmodia were starved and very slowly dried. Only the yellow sclerotia were able to regenerate into viable plasmodia. This suggests that myxomycete cytoplasm dehydration is an active process regulated metabolically. Plasmodial sclerotization may therefore serve as a convenient model system to study the regulation of cytoplasmatic water balance, and sclerotia as a convenient material for EPR measurements, combining the quality of plasmodia with the technical simplicity of the measurements characteristic of dry spores. Darkening of the sclerotia is most probably a pathological phenomenon connected with the impairment of water balance during sclerotization.

The inhibition kinetics and thermodynamic changes of tyrosinase via the zinc ion

We found that Zn(2+) conspicuously inactivated tyrosinase in a mixed-type inhibition manner: the final level of residual activity was abolished at the equilibrium state with concentration of 0.25 mM Zn(2+). Changes of both K(m) and V(max) by various concentrations of Zn(2+) in Lineweaver-Burk plot were observed. To see whether Zn(2+) also induced conformational change of tyrosinase and how thermodynamical changes by ligand binding were occurred, the intrinsic fluorescence studies as well as calorimetric measurements were conducted. The results showed that the Zn(2+) binding to tyrosinase directly induced conformational change of tyrosinase, and the changes of thermodynamic parameters such as enthalpy (DeltaH), Gibbs free-energy (DeltaG), and entropy (DeltaS) were obtained as 60+/-7.0 kJ/mol, -14.54 kJ/mol and 248.53 J/(K mol), respectively. The inactivating effect of Zn(2+) on tyrosinase was completely prevented by incubation with bovine serum albumin, which has a Zn(2+) binding motif in its structure. We suggested that Zn(2+) ligand-binding affected the substrate's accessibility due to the conformational changes and thus, the complex type of inhibition has occurred with the calorimetric changes.

Synthesis and characterization of copper(II) complexes of cysteinyldopa and benzothiazine model ligands related to pheomelanin

A new cysteinyldopa model ligand Cydo {3-[(2-aminoethyl)sulfanyl]-4,6-di-tert-butylbenzene-1,2-diol} was prepared and its reactivity with Cu(II) explored. Under anaerobic conditions, tetranuclear [Cu4(Cydo)4] is isolated, but in the presence of O2, a benzothiazine intermediate accumulates that is trapped as the Cu(II) complex [Cu(zine)2]. Under slightly different reaction conditions, the benzothiazine further oxidizes to benzothiazole (zole). All three compounds were characterized by X-ray crystallography, and the reactions were monitored spectrophotometrically.

An easy-to-run method for routine analysis of eumelanin and pheomelanin in pigmented tissues

A procedure for analysis of melanin-pigmented tissues based on alkaline hydrogen peroxide degradation coupled with high-performance liquid chromatography (HPLC) ultraviolet determination of pyrrole-2,3,5-tricarboxylic acid (PTCA) for eumelanin and 6-(2-amino-2-carboxyethyl)-2-carboxy-4-hydroxybenzothiazole (BTCA) and 1,3-thiazole-2,4,5-tricarboxylic acid for pheomelanin was recently developed. Despite advantages related to the degradation conditions and sample handling, a decrease of the reproducibility and resolution was observed after several chromatographic runs. We report herein an improved chromatographic methodology for simultaneous determination of PTCA and BTCA as representative markers of eumelanin and pheomelanin, respectively, based on the use of an octadecylsilane column with polar end-capping with 1% formic acid (pH 2.8)/methanol as the eluant. The method requires conventional HPLC equipments and gives very good peak shapes and resolution, without need of ion pair reagents or high salt concentrations in the mobile phase. The intra-assay precision of the analytical runs was satisfactory with CV values < or = 4.0% (n = 5) for the two markers which did not exceed 8% after 50 consecutive injections on the column over 1 week. The peak area ratios at 254 and 280 nm (A(280)/A(254): PTCA = 1.1, BTCA = 0.6) proved a valuable parameter for reliable identification of the structural markers even in the most complex degradation mixtures. The method can be applied to various eumelanin and pheomelanin pigmented tissues, including mammalian hair, skin and irides, and is amenable to be employed in population screening studies.

Oral zinc sulphate causes murine hair hypopigmentation and is a potent inhibitor of eumelanogenesis in vivo

BACKGROUND

C57BL/6 a/a mice have been widely used to study melanogenesis, including in electron paramagnetic resonance (EPR) studies. Zinc cations modulate melanogenesis, but the net effect of Zn2+ in vivo is unclear, as the reported effects of Zn2+ on melanogenesis are ambiguous: zinc inhibits tyrosinase and glutathione reductase in vitro, but also enhances the activity of dopachrome tautomerase (tyrosinase-related protein-2) and has agonistic effects on melanocortin receptor signalling.

OBJECTIVES

To determine in a C57BL/6 a/a murine pilot study whether excess zinc ions inhibit, enhance or in any other way alter hair follicle melanogenesis in vivo, and to test the usefulness of EPR for this study.

METHODS

ZnSO(4).7H2O was continuously administered orally to C57BL/6 a/a mice during spontaneous and depilation-induced hair follicle cycling (20 mg mL-1; in drinking water; mean+/-SD daily dose 1.2+/-0.53 mL), and hair pigmentation was examined macroscopically, by routine histology and by EPR.

RESULTS

Oral zinc cations induced a bright brown lightening of new hair shafts produced during anagen, but without inducing an EPR-detectable switch from eumelanogenesis to phaeomelanogenesis. The total content of melanin in the skin and hair shafts during the subsequent telogen phase, i.e. after completion of a full hair cycle, was significantly reduced in Zn-treated mice (P=0.0005). Compared with controls, melanin granules in precortical hair matrix keratinocytes, hair bulb melanocytes and hair shafts of zinc-treated animals were reduced and poorly pigmented. Over the course of several hair cycles, lasting hair shaft depigmentation was seen during long-term exposure to high-dose oral Zn2+.

CONCLUSIONS

High-dose oral Zn2+ is a potent downregulator of eumelanin content in murine hair shafts in vivo. The C57BL/6 mouse model offers an excellent tool for further dissecting the as yet unclear underlying molecular basis of this phenomenon, while EPR technology is well suited for the rapid, qualitative and quantitative monitoring of hair pigmentation changes.

Spectroscopy and Photoreactivity of Trichochromes: Molecular Components of Pheomelanins†

The trichochromes are a class of small molecules present in pheomelanin (the red melanin) and absent in eumelanin (the black melanin). Herein trichochrome F (TF) and decarboxytrichochrome C (dTC) are examined. Both trichochromes are characterized by a visible absorption band, which is shown to be the result of overlapping transitions of the cis and trans isomers. The temperature dependence of the absorption spectrum of dTC suggests the additional presence of equilibrium between the enol and keto forms of the molecule. These conclusions are supported by ground-state energies of these isomers obtained using a continuum solvation model. Near-infrared emission measurements were not able to detect photoproduction of 1O2, and spin-trapping experiments revealed formation of O2*-. DNA nicking assays also revealed a low level of light-induced aerobic activity of dTC, suggesting a quantum efficiency of at most 5 x 10(-6) for the photogeneration of O2*-. These results are consistent with pump-probe optical experiments, which reveal efficient and nearly complete ground-state recovery within a few picoseconds of excitation. Both trichochromes are efficient quenchers of 1O2, exhibiting a bimolecular rate constant comparable with vitamin C. These results suggest that trichochromes could serve a protective role in pheomelanin pigments.

1,4-benzothiazines as key intermediates in the biosynthesis of red hair pigment pheomelanins

Following the discovery of cysteinyldopas as the early intermediates in the biogenesis of pheomelanins, the typical red hair pigments, the reactivity of the biosynthetic precursors under biomimetic conditions was extensively investigated. As a result, the early stages of pheomelanogenesis were envisaged as involving oxidative cyclization of cysteinyldopas, mainly the 5-S-isomer, to 1,4-benzothiazine (BTZ) intermediates which undergo oxidative polymerization leading eventually to the pigments. In the last decade, several aspects of the chemistry and biosynthesis of pheomelanins were re-examined. In particular, (i) transient BTZ intermediates were identified by pulse radiolytic techniques and NMR analysis; (ii) the effect of reaction conditions and additives on the rearrangement vs. redox exchange reaction paths of such intermediates were investigated in detail; (iii) the mechanism of the oxidative polymerization of BTZs was characterized by the first isolation of oligomer species, and (iv) the pigment eventually resulting from oxidation of 5-S-cysteinyldopa (CD) was directly analyzed by spectroscopic and chemical methodologies in comparison with pheomelanins isolated from human hair. These advances led eventually to an integrated picture of the biogenetic route highlighting the intervention of various chemical and enzymatic factors which affect the kinetics of the different steps and the nature of the key benzothiazine precursors. A likely biogenetic route was also postulated for the delta2,2'-bi(2H-1,4-benzothiazine) pigments, termed trichochromes, whose origin had remained an open issue since their first isolation from red human hair and avian feathers. Finally, a more detailed description of the structure of pheomelanin pigments in terms of the monomer units, their mode of linking, and postsynthetic modifications was gained.

Ultrafast absorption and photothermal studies of decarboxytrichochrome C in solution

The trichrochromes are natural constituents of pheomelanins. Herein, time-resolved spectroscopic techniques are used to quantify the energetics and dynamics of the primary photoprocesses of trichochromes following excitation into the lowest excited singlet state of the molecule. The absorption spectrum of decarboxytrichochrome C (dTC) reveals multiple electronic states are accessible upon visible and UV excitation. Emission is not observed upon excitation into the lowest energy absorption band. Photothermal measurements reveal 91% of the photon energy is promptly released as heat. Femtosecond time-resolved absorption studies reveal a ground-state recovery time of approximately 2.4 ps. Complete recovery of the ground state is not observed; 15% of the initially excited molecules do not recover on the approximately 150 ps timescale. The combination of ultrafast absorption and photoacoustic data suggest photoexcitation produces a long-lived intermediate and the energy of this species is at least 133 kJ mol(-1) above the ground state of the dTC molecule.

Metal ions as potential regulatory factors in the biosynthesis of red hair pigments: a new benzothiazole intermediate in the iron or copper assisted oxidation of 5-S-cysteinyldopa

In the presence of iron or copper ions, the course of the oxidation in air of 5-S-cysteinyldopa (1), the main biosynthetic precursor of pheomelanins and trichochromes, was markedly changed affording two main products. One of these was identified as the oxobenzothiazine 8, previously obtained under nonphysiologically relevant conditions, while the other was characterized as the novel hydroxybenzothiazole 9. Besides 8 and 9, carboxylated and noncarboxylated benzothiazine products were obtained by persulfate oxidation of 1 in the presence of iron or copper ions. The ratio of formation yields of carboxylated/noncarboxylated benzothiazines, determined after reduction of the mixture, was lower than that of the control reaction run in the absence of metal ions, and much lower than that of the oxidation carried out in the presence of zinc ions, in agreement with a recent report. Notably, 8 and 9 were formed in variable yields under different oxidation conditions including tyrosinase/O(2), peroxidase/hydrogen peroxide, and the hydrogen peroxide/or (9E,11Z,13S)-13-hydroperoxyoctadeca-9,11-dienoic acid/Fe(III) systems. Mechanistic routes to 8 and 9 were proposed based on the results of experiments involving in situ generation of labile benzothiazine intermediates. Overall, these results allow to formulate an improved biosynthetic scheme in which metal ions act as critical regulatory factors determining pheomelanin vs. trichochromes formation.