title: The interaction of melanin with 8-methoxypsoralen: effect on radiative and nonradiative transitions. A fluorescence and pulsed photoacoustic study.
abstract: The interaction of 8-methoxypsoralen (8-MOP) with synthetic eumelanin was investigated using static and time-resolved fluorescence and pulsed photoacoustic calorimetry. Due to the strong overlap of the absorption bands of melanin and 8-MOP, a method is presented to account for the systematic errors introduced by the optical filter effect exerted by each absorbing species in the fluorescence and the photoacoustic measurements. As a preliminary step to the understanding of the nonradiative behavior of the psoralen-melanin complexes, the photoacoustic parameters of 8-MOP in various solvents were determined. Spectroscopic data indicate the absence of interaction at the ground-state level, whereas the singlet excited state of 8-MOP is quenched by the pigment; the average fluorescence lifetimes are independent of the melanin concentration, 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 oxygen, as expected for a molecule essentially acting as a type I photosensitizing agent.
authors.affiliation: Department of Physics, University of Parma, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Losi
authors.forename: C
authors.initials: C
authors.lastname: Viappiani
authors.forename: P R
authors.initials: PR
authors.lastname: Crippa
mesh_terms: D000162:Acoustics; D002151:Calorimetry; D006801:Humans; D066298:In Vitro Techniques; D008543:Melanins / Q000737:chemistry* / Q000378:metabolism; D008730:Methoxsalen / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology; D008956:Models, Chemical; D010777:Photochemistry; D013050:Spectrometry, Fluorescence
chemical_list: D008543:Melanins; C041877:eumelanin; D008730:Methoxsalen