Photochemical studies of porphyrin–melanin interactions

Abstract

Laser flash photolysis and electron spin resonance as well as conventional absorption and spectrofluorimetry have been used to study the interaction of anionic, neutral and cationic dyes with two synthetic melanins obtained by polymerising dihydroxyphenylalanine (dopa) and 5-S-cysteinyldopa. Cationic dyes were shown to complex strongly to both melanins via ionictype interactions and this binding was monitored as a broadening of the absorption band, fluorescence quenching, triplet-state quenching and, using e.s.r. oximetry, via oxygen consumption rates which are reduced on adding melanin. These triplet-state results imply that ca. 5–10 monomer units of synthetic melanins are involved in binding a positively charged porphyrin. For the anionic dyes there is no binding and consequently melanin has little or no effect on the absorption, fluorescence or triplet states of the dyes. For these dyes there is a linear relationship between the oxygen consumption rate and the added melanin concentration. Singlet oxygen was shown to be involved using D₂O and, as quencher, azide ion. The results are discussed in terms of the possible use of such porphyrins for the photochemotherapy of pigmented tumours.