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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.