Infrared and water sorption studies of the hydration structure and mechanism in natural and synthetic melanin.

Although water is a structural and functional determinant in melanins, a direct study of the interaction between water and melanin is still lacking and is the subject of the present work. Melanin forms in cells and organisms' colloidal particles deriving from the hierarchical aggregation of smaller subunits such as protomolecules, stacking units, and small aggregates: its functions must be interpreted in terms of solid-state and surface properties. They are strictly connected to the porosity of the particles when they interact with water and other chemicals. The structural characteristics of water sorbed in the pores are investigated by means of FTIR in the OH stretching vibration region (4000-3000 cm(-1)) on Sepia and synthetic l-Dopa melanins at different and controlled hydration degrees (a(w) = 0.06 to 0.92). Three distinct component bands are recognized in the main OH stretching band, corresponding to three distinct water populations, showing differing behavior for the two kinds of melanin and thus accounting for different molecular structures. On this basis, the historical, albeit current, model of hydration structure of melanin granules, a "strongly" and a "weakly" bound fraction, is reassessed and rediscussed.

An XAS study of the sulfur environment in human neuromelanin and its synthetic analogs.

Neuromelanin is a complex molecule accumulating in the catecholaminergic neurons that undergo a degenerative process in Parkinson's disease. It has been shown to play either a protective or a toxic role depending on whether it is present in the intraneuronal or extraneuronal milieu. Understanding its structure and synthesis mechanisms is mandatory to clarify the reason for this remarkable dual behavior. In the present study, X-ray absorption spectroscopy is employed to investigate the sulfur binding mode in natural human neuromelanin, synthetic neuromelanins, and in certain structurally known model compounds, namely cysteine and decarboxytrichochrome C. Based on comparative fits of human and synthetic neuromelanin spectra in terms of those of model compounds, the occurrence of both cysteine- and trichochrome-like sulfur coordination modes is recognized, and the relative abundance of these two types of structural arrangement is determined. Data on the amount of cysteine- and trichochrome-like sulfur measured in this way indicate that among the synthetic neuromelanins those produced by enzymatic oxidation are the most similar ones to natural neuromelanin. The interest of the method described here lies in the fact that it allows the identification of different sulfur coordination environments in a physically nondestructive way.

New melanic pigments in the human brain that accumulate in aging and block environmental toxic metals.

Neuronal pigments of melanic type were identified in the putamen, cortex, cerebellum, and other major regions of human brain. These pigments consist of granules 30 nm in size, contained in organelles together with lipid droplets, and they accumulate in aging, reaching concentrations as high as 1.5-2.6 microg/mg tissue in major brain regions. These pigments, which we term neuromelanins, contain melanic, lipid, and peptide components. The melanic component is aromatic in structure, contains a stable free radical, and is synthesized from the precursor molecule cysteinyl-3,4-dihydroxyphenylalanine. This contrasts with neuromelanin of the substantia nigra, where the melanic precursor is cysteinyl-dopamine. These neuronal pigments have some structural similarities to the melanin found in skin. The precursors of lipid components of the neuromelanins are the polyunsaturated lipids present in the surrounding organelles. The synthesis of neuromelanins in the various regions of the human brain is an important protective process because the melanic component is generated through the removal of reactive/toxic quinones that would otherwise cause neurotoxicity. Furthermore, the resulting melanic component serves an additional protective role through its ability to chelate and accumulate metals, including environmentally toxic metals such as mercury and lead.

Photophysical properties of pyrene in interaction with the surface of melanin particles.

Melanins perform their biological activity (photoprotection and light enhanced chemical reactivity) under the form of porous aggregates on which ions and neutral molecules can be adsorbed. For this reason, the photochemistry of natural and synthetic melanins must be investigated in the framework of the physico-chemical theory of the heterogeneous reactions and a detailed knowledge of the surface properties, is therefore, necessary. In this work, some surface characteristics of melanin particles have been investigated taking advantage of the photophysical behaviour of pyrene, a dye widely used in studies of the interface properties of micelles and colloidal semiconductors. Our fluorescence study has allowed to obtain valuable informations regarding the micro-environmental polarity of the melanin surface (that influences the vibronic structure of the emission spectra), the excimer formation, the lifetimes of the emissions and the kinetics of quenching by Cu2+.