Photochemistry of Pheomelanin Building Blocks and Model Chromophores: Excited-State Intra- and Intermolecular Proton Transfer

Pheomelanins, the epidermal pigments of red-haired people responsible for their enhanced UV susceptibility, contain 1,4-benzothiazines and 1,3-benzothiazole as main structural components. Despite the major role played in pheomelanin phototoxicity, the photoreactivity of these species has so far remained unexplored. Static and time-resolved fluorescence spectroscopy was used to identify excited-state reactions of the two main pheomelanin benzothiazole building blocks, namely, the 6-(2-amino-2-carboxyethyl)-4-hydroxy-1,3-benzothiazole (BT) and the 2-carboxy derivative (BTCA) together with model chromophores lacking some of the ionizable functions. The results show that in aqueous buffer solution the OH at 4-position and the benzothiazole nitrogen atom control the photochemistry of both BT and BTCA via excited-state proton transfer to solvent (ESPT) and excited-state intramolecular proton transfer (ESIPT), while the amino acidic groups of the alanyl chain have a minor influence on the photochemistry. The ESPT and ESIPT produce several different excited-state ionic species with lifetimes ranging from ∼100 ps to ∼3 ns