The action spectrum for generation of the primary intermediate revealed by ultrafast absorption spectroscopy studies of pheomelanin.

The observation that fair-skinned individuals are more susceptible to skin cancers is commonly explained by invoking an enhanced photoreactivity of the red melanin, pheomelanin compared with the black melanin, eumelanin. For the wavelength range from 500 to 1000 nm, pump-probe spectroscopic measurements reveal the photoexcitation of pheomelanin by UVA light that generates an immediate (< 100 fs) transient absorption centered at 780 nm. Using a tunable femtosecond excitation source, the action spectrum between 300 and 390 nm for generation of the primary intermediate was measured. Similar action spectra are found for the sample with molecular weight (MW) between 1000 and 10 000 and the one with MW > 10 000 fractions of pheomelanin, indicating that the reactive chromophore has a low MW but is present and its photophysics is similar in the aggregated pigment. The shape of the action spectrum differs from the absorption spectrum of bulk melanin and mass-selected fractions but resembles reported absorption spectrum of benzothiazines, oxidation products of 5-S-cysteinyl-dopa, which are formed along the biosynthetic pathway of pheomelanin.