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.