Superior Photoprotective Motifs and Mechanisms in Eumelanins Uncovered(full pdf)
Alice Corani , Annemarie Huijser , Thomas Gustavsson , Dimitra Markovitsi , Per-Ake Malmqvist , Alessandro Pezzella , Marco d'Ischia, and Villy Sundstrom
Journal of the American Chemical Society
vol. 136 issue 33 p 11626-11635 july 31, 2014
Human pigmentation is a complex phenomenon commonly believed to serve a photoprotective function through the generation and strategic localization of black insoluble eumelanin biopolymers in sun exposed areas of the body. Despite compelling biomedical relevance to skin cancer and melanoma, eumelanin photoprotection is still an enigma: What makes this pigment so efficient in dissipating harmful UV-light energy as heat? Why has Nature selected 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as the major pigment building block instead of the decarboxylated derivative (DHI)? By using pico- and femtosecond fluorescence spectroscopy we demonstrate herein that DHICA oligomers can induce excited state deactivation thousand times more efficiently than DHI-oligomers by virtue of their specific coupling patterns enabling multiple pathways of inter-unit proton transfer. The discovery that DHICA-based scaffolds specifically confer uniquely robust photoprotective properties to natural eumelanins settles a fundamental gap in the biology of human pigmentation and opens the doorway to attractive advances and applications.