![]() Melanins vary their properties with the degree of hydration.They also undergo alteration with physical agents such as heat and light, as well as with aging, even if left dry on a shelf. They may undergo more or less profound structural degradation on chemical treatment with acids (decarboxylation), alkali (oxidative ring fission), oxygen (oxidation of catechol units), and hydrogen peroxide, even during their (bio)synthesis. The following set of practical definitions is thus proposed and recommended. Accordingly, it may be convenient to maintain a wider, general purpose classification, rooted in the tradition but revisited in light of recent progress, that includes (i) sensu stricto melanins, (ii) the dark phenolic pigments from lower organisms, and (iii) synthetic pigments produced either chemically or enzymatically from natural precursors. To limit confusion, Prota proposed a more restrictive usage of the term ‘melanin’ to include only those pigments that are formed intracellularly by the oxidation of tyrosine and related metabolites (Prota, 1995), thus taking both the biogenetic origin from tyrosine and the metabolic activity of melanocytes (and, occasionally, related cell systems) as stringent requisites.Īlthough in the authors’ opinion this restrictive classification of melanins remains valid, it is nonetheless difficult to find alternative definitions for the broad variety of dark phenolic pigments from plants and microorganisms or for the diverse synthetic pigments of biomedical and technological interest, which are driving much of current progress in the field. Plants also produce dark phenolic pigments that have sometimes been referred to as catechol melanins, although the term conveys no information about the broad diversity and complexity of the polyphenolic precursors. Aspergillus fumigatus can also synthesize melanin-type pigments from 1,8-dihydroxynaphthalene, while Serratia marcescens (Trias et al., 1989) or the pathological fungus Cryptococcus neoformans (Casadevall et al., 2000) can produce similar pigments from alternate pathways. Pseudomonas and Aspergillus fumigatus can produce in the presence of tyrosine a eumelanin-like pigment termed pyomelanin via homogentisic acid (Schmaler-Ripcke et al., 2009). ![]() Nicolaus ( 1969) suggested a classification of melanins into three main groups, eumelanins, pheomelanins, and allomelanins, the former two groups comprising animal pigments and the latter encompassing the broad variety of dark non-nitrogenous pigments of plant, fungal, and bacterial origin. Since then, it has been widely used to indicate any black or dark brown organic pigment occurring throughout the phylogenetic scale without any specific structural, biogenetic, or functional implication. The term ‘melanin’ was first coined by Berzelius in 1840 to refer to black animal pigments. Provided herein as integral part of the paper is also the Appendix S1 section, which contains the first systematic collection of reference data and experimental protocols to be recommended as state-of-the-art for future research in the field. Starting from a careful review of current methods and standards, the paper provides a selection of guidelines and procedures that have been verified and optimized, when necessary, through an ad hoc experimental revision. The aim of this paper was to provide a critical assessment of methodological and practical issues in melanin research concerning (i) isolation and purification of natural melanins (ii) preparation of synthetic model pigments (iii) physical, spectral, and chemical characterization (iv) use of melanins and melanogenic enzymes for biological studies (v) preparation and assessment of standard compounds for melanin research, including commercially available pigments and enzymes. Crucial gaps stem from the lack of standardized procedures and methodologies, failure to take in due account melanin properties and the consequences of harsh purification procedures, a widespread tendency to compare materials obtained under different conditions, to extrapolate data referring to natural pigments from studies on synthetic pigments, or to draw conclusions and implications from observations made on unsuitable models. A variety of definitions and models are found in the literature, which reflect, however, an arbitrary use of terminology as well as several assumptions and speculations that have never been proven on experimental grounds. ![]() Unlike the vast majority of natural pigments, the melanins cannot be described in terms of a single well-defined structure and, as a result, there still remains today a lack of general consensus what actually melanin is. The melanins can be still regarded as the most enigmatic pigments/biopolymers found in nature (Ito et al., 2011a). ![]()
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