Melanin-porphyrin interaction monitored by delayed luminescence and photoacoustics

Even though melanin is commonly viewed as a photoprotective agent, the molecular mechanism of the melanin-related photoprotective action remains unclear. We studied the interaction of a synthetic dopa melanin with positively and negatively charged porphyrins using an array of spectroscopic techniques. Thus absorption, fluorescence, time-resolved delayed luminescence in the microsecond time range, and photoacoustic spectra at different modulation frequencies of both porphyrins and their mixtures with dopa melanin were measured in buffered aqueous solution at room temperature. It has been confirmed that dopa melanin forms a complex with the cationic porphyrin in its ground state, which significantly modifies the optical properties of the dye molecule. Although no such complex can be detected for the anionic porphyrin, some interaction between melanin and the porphyrin molecule can be detected in its excited state. Both porphyrins show delayed luminescence with a decay time of 35-38 microseconds, which is substantially reduced by melanin. Our photoacoustic measurements indicate that upon photoexcitation, melanin efficiently liberates heat. Our data also suggest that the thermal properties of melanin can be modified by porphyrins, particularly by the cationic form of the dye. The ionic complexes of dopa melanin with the positively charged porphyrin exhibit properties typical for a supermolecular system that liberates heat as a whole.