Ultrafast Radical Photogeneration Pathways in Eumelanin

AbstractEumelanin is a ubiquitous biological pigment that rapidly and efficiently deactivates excited states created by UV or visible radiation. Paradoxically, photoirradiation of eumelanin also generates radicals and harmful reactive oxygen species, but the relationship between these pathways and excited state deactivation is uncertain. Here, greatly expanding the excitation tuning range (225 –620 nm) and probing window (400–1500 nm) in femtosecond transient absorption spectroscopy of the synthetic eumelanin, DOPA melanin, enables the detection of photogenerated radials with ultrafast time resolution for the first time. Despite its heterogeneous nature, the transient absorption signa ls can be modeled by two spectral components assigned to solvated electrons and photogenerated radicals. Radical absorbance measured several nanoseconds after excitation increases exponentially with increasing photon energy, matching the trend in radical yields measured in electron paramagnetic reso nance spectroscopy experiments. Spectral modeling of the transient signals reveals two radical generation mechanisms: (1) photoionization by UV light and (2) photoinduced charge transfer among eumelanin chromophores by UVA and visible wavelengths capable of reaching the pigment in skin. Concurrent u ltrafast relaxation and radical generation underlie the ability of eumelanin to be both photoprotective and photodamaging, and the branching between these pathways likely depends on the wavelength of ...

Source: Photochemistry and Photobiology - September 30, 2022 Category: Science Authors: Christopher Grieco, Forrest R. Kohl, Bern Kohler Tags: Special Issue Research Article Source Type: research