Explanation for the disparity among absorption and action spectra of eumelanin

The optical properties of eumelanin (from Sepia officinalis) are found to v ary with particle size. The absorption spectrum for small eumelanin particl es agrees quantitatively with the reported action spectra for photoinduced oxygen consumption and free radical generation by eumelanin. These small pa rticles, unlike the large ones, generate long-lived reactive intermediates upon absorption of UV light. The data presented suggest that the small eume lanin particles may be involved in UV-A-induced photochemical processes bel ieved to lead to DNA damage in skin cells, whereas the large particles effi ciently dispose of UV-A energy through rapid nonradiative decay processes. These results provide new insight into the dichotomy that eumelanin is both photoprotective and photosensitizing. This size-dependent photoreactivity may be one of the contributing factors to the observed variations in skin c ancer rates among different skin types.