Infrared and Water Sorption Studies of the Hydration Structure and Mechanism in Natural and Synthetic Melanin

Abstract

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.

Novel insights on the physicochemical properties of eumelanins and their DMSO derivatives

“…The temperature at which the first minimum of the derivative (water loss) takes place is slightly higher than for Sigma melanin, 171 ± 5 °C, in good agreement with the literature . The second maximum could tentatively be ascribed to the loss of strongly bound water; however, in the literature it has been reported at higher temperatures . The second decomposition step can be tentatively ascribed to the decarboxylation of DHICA building blocks as well as to the rupture of the supramolecular structure and substructure of natural melanin and to the non‐covalent bonds between layers in oligomer planes .…”

“…44 The second maximum could tentatively be ascribed to the loss of strongly bound water; however, in the literature it has been reported at higher temperatures. 47,48 The second decomposition step can be tentatively ascribed to the decarboxylation of DHICA building blocks 41 as well as to the rupture of the supramolecular structure and substructure of natural melanin and to the non-covalent bonds between layers in oligomer planes. 47 The further three steps are associated with breaking of the bonds between the indole units and to the opening of pyrrole and indole rings.…”

Evidence of Porphyrin‐Like Structures in Natural Melanin Pigments Using Electrochemical Fingerprinting

“…These data suggest that NatMel cathodes present discrete well‐defined binding sites for sodium ion insertion. Conversely, discharge curves for both NTP/SynMel full cells and SynMel half‐cells suggest that indolequinone monomers adopt a broad range of configurations that generate a distribution of reduction potentials for sodium ion insertion and catechol formation …”

UV–vis spectra as an alternative to the Lowry method for quantify hair damage induced by surfactants

“…Although the chemical structure of hair melanin is still unknown [4,5], it is known that these granules possess a hierarchical structure, composed by the self-assembly of several minor constituents [6]. Many studies show that this assembled structure has a bandgap in its nanoaggregates [4,[7][8][9], producing hair color [10].…”

“…Many studies show that this assembled structure has a bandgap in its nanoaggregates [4,[7][8][9], producing hair color [10]. Its disaggregation and structure modification by adsorbed water modulates different color shades [5,11].…”