The Mechanism of UVA Degradation of Synthetic Eumelanin

Eumelanin is a ubiquitous natural pigment that has a broad absorption across ultraviolet (UV, 100-400 nm) and visible wavelengths (400-700 nm), and can protect against radiation. Synthetic eumelanin with properties similar to natural eumelanin has been made using dopamine or dihydroxyindole (DHI). Here, we use solid-state nuclear magnetic resonance spectroscopy (ssNMR) and Fourier transform infrared spectroscopy (FTIR) to elucidate the chemical structure of synthetic eumelanins (made from dopamine and L-DOPA (L-3,4-dihydroxyphenylalanine) precursors) and investigate how their structures change after intensive UVA (315-400 nm) exposure. We first confirm that polydopamine has indole units. Upon UV exposure, the pyrrole ring in this indole unit remains intact, and a fraction of the six-membered benzyl ring is broken and the indole potentially converted to furo[3,4-b]pyrrole. This change in chemical structure is accompanied by a release of carbon dioxide. In addition, the sepia (natural) eumelanin used for co...