FTIR-based characterization of photodegradation in synthetic eumelanin and pheomelanin models supported by molecular electrostatic potential maps.

Melanins are pigments that protect tissues against the damaging effects of solar radiation. Photodegradation of these pigments adversely alters their key physicochemical properties, diminishing redox buffering while increasing photochemical reactivity and driving supramolecular reorganization manifested as a decrease in the size of pigment-forming aggregates. In this study, UV-Vis, EPR, and FTIR were combined to directly correlate photodegradation-induced changes in optical and paramagnetic properties, with particular emphasis on alterations in the chemical groups present in the pigments. In addition, molecular electrostatic potential maps were computed for selected eumelanin and pheomelanin models spanning distinct redox states and levels of structural complexity. Upon aerobic photolysis, both pigments gradually bleached, their free radical content decreased and in pheomelanin, marked changes in radical character occurred. FTIR analysis revealed progressive loss of hydroxyl and amino groups, increased oxidation of carbonyl and aromatic functionalities, and alterations in aliphatic side chains and heterocycles, with distinct patterns for eumelanin and pheomelanin. The FTIR changes observed in photodegraded pheomelanin indicate a shift in ring character consistent with a benzothiazine-to-benzothiazole transformation, accompanied by a substantial reduction in bands associated with intermolecular interactions between structural motifs within the polymer network. In eumelanin, the spectral changes indicate a shift toward more conjugated structures. Analysis of molecular electrostatic potential maps indicated an increasing potential for hydrogen-bonded interactions upon eumelanin oxidation, as well as a greater propensity of eumelanin for stacking compared to pheomelanin, which has more H-bond donor and acceptor sites.