Comparison of the ultrafast absorption dynamics of eumelanin and pheomelanin

An enhanced photoreactivity of the red melanin, pheomelanin compared to the black melanin, eumelanin is commonly invoked to explain why fair-skinned individuals are more susceptible to skin cancers. Ultrafast pump-probe spectroscopy is used to examine the primary photodynamics of eumelanin from Sepia officinalis and synthetic pheomelanin following excitation at 303 nm. Both melanins reveal a transient absorption throughout the visible spectrum that rises within the instrument response. On the picosecond time scale, the transient absorption signal decays to a constant nonzero value, and the dynamics to nonexponential. Thus, both melanins do not exhibit complete ground-state recovery, suggestive of the formation of long-lived intermediates. Comparison to the initial transient absorption indicates the yield of such species is identical for the two pigments. The shape of the transient spectrum for the two melanins differ; however, global fits to the 13 different probe wavelengths used to construct each spectrum reveal both pheomelanin and eumelanin exhibit similar dynamics. The decay associated spectrum for each time constant is examined. Comparison of the transient absorption dynamics to data obtained from time-resolved emission experiments of melanins suggest that different molecular species are responsible for the data recorded in the two experiments.