TRIPLET EXCITED STATE OF FUROCOUMARINS: REACTION WITH NUCLEIC ACID BASES AND AMINO ACIDS

Abstract

Abstract. The triplet‐triplet absorption spectra of psoralen, xanthotoxin, angelicin and bergapten have been determined using the technique of pulse radiolysis and laser flash photolysis in benzene and water. The extinction coefficients of the triplet transitions have been measured and used to determine the singlet → triplet intersystem crossing quantum yields for 353 nm excitation. These yields vary considerably in passing from benzene to water as solvent. The highest yields were obtained for psoralen and angelicin in water, being 0.45 and 0.33, respectively. Reaction rate constants of the psoralen and angelicin triplets with various pyrimidine and purine nucleic acid bases and amino acids have been measured. The high reactivities support the suggestion that the photosensitising properties of furocou‐marins are mediated by the corresponding furocoumarin triplet states. The similarly high S → T quantum yields, and triplet reactivities with pyrimidines of psoralen and angelicin, also support the suggestion that the observed differences in photosensitising properties of these two psoralens may be due to their differing geometries, which allow psoralen itself to intercalate DNA more easily than angelicin. The efficient reactions also found for psoralen and angelicin triplets with amino acids suggest that such triplets in vivo may also react with these components. Significant damage may thus be caused to protein as well as nucleic acid components of the cell.