THE INTERACTION OF MELANIN WITH 8-METHOXYPSORALEN - EFFECT ON RADIATIVE AND NONRADIATIVE-TRANSITIONS - A FLUORESCENCE AND PULSED PHOTOACOUSTIC STUDY

The interaction of 8-methoxypsoralen (8-MOP) with synthetic eumelanin was investigated using static and time-resolved fluorescence and pulse d photoacoustic calorimetry. Due to the strong overlap of the absorpti on bands of melanin and 8-MOP, a method is presented to account for th e systematic errors introduced by the optical filter effect exerted by each absorbing species in the fluorescence and the photoacoustic meas urements. As a preliminary step to the understanding of the nonradiati ve behavior of the psoralen-melanin complexes, the photoacoustic param eters of 8-MOP in various solvents were determined. Spectroscopic data indicate the absence of interaction at the ground-state level, wherea s the singlet excited state of 8-MOP is quenched by the pigment; the a verage fluorescence lifetimes are independent of the melanin concentra tion, thus indicating a static quenching mechanism. The photoacoustic data show that the quenching process involves an increased intersystem crossing probability, which is almost unaffected by the presence of o xygen, as expected for a molecule essentially acting as a type I photo sensitizing, agent.