Wavelength-dependent photoacoustic calorimetry study of melanin.

Photoacoustic calorimetry is used to examine the energy dissipation in melanin under physiological conditions (pH 7.2) following irradiation by UV and visible (VIS) light. Four different excitation wavelengths were examined: 264 nm, representative of UVC radiation, 351 nm and 400 nm (UVA-I radiation) and 527 nm, representative of VIS radiation. Following absorption at 527 nm, essentially all of the photon energy is released nonradiatively on a sub-nanosecond of excitation. Similar results are observed at 400 nm. At 351 nm, most of the energy was released as heat; a small amount of energy was retained (5 +/- 5%). When melanin is excited at 264 nm, 29 +/- 7% of the photon energy is retained by the molecule for a time period longer than a few hundred nanoseconds. These results show that a long-lived excited state or reactive intermediate is generated upon UV irradiation, whereas all of the excitation energy is dissipated nonradiatively in the visible portion of the spectrum. These results establish that the photochemistry of melanin is wavelength dependent.