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 rem ains unclear. We studied the interaction of a synthetic dopa melanin w ith positively and negatively charged porphyrins using an array of spe ctroscopic techniques. Thus absorption, fluorescence, time-resolved de layed luminescence in the microsecond time range, and photoacoustic sp ectra 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 signifi cantly modifies the optical properties of the dye molecule. Although n o such complex can be detected for the anionic porphyrin, some interac tion 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 mu s, which is substantially reduced by melanin. Our ph otoacoustic measurements indicate that upon photoexcitation, melanin e fficiently liberates heat. Our data also suggest that the thermal prop erties of melanin can be modified by porphyrins, particularly by the c ationic form of the dye. The ionic complexes of dopa melanin with the positively charged porphyrin exhibit properties typical for a supermol ecular system that liberates heat as a whole. (C) 1997 Elsevier Scienc e S.A.