TRIPLET EXCITED STATE OF THE 4'5' PHOTOADDUCT OF PSORALEN AND THYMINE

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Abstract
Abstract— The triplet‐triplet absorption spectrum of the 4'5' psoralen‐thymine mono‐adduct has been determined in water and methanol using the technique of laser flash photolysis. The extinction coefficient of the triplet was measured by the energy‐transfer method with retinol triplet as standard, and used to determine the singlet → triplet intersystem crossing quantum yield for 353 nm excitation. Reaction rate constants for mono‐adduct triplet with thymine and tryptophan were measured in water. Long‐lived transient absorptions detected after quenching the mono‐adduct triplet with thymine and tryptophan are assigned mainly to the corresponding mono‐adduct radical anion, whose spectrum was established in separate pulse radiolysis studies of the mono‐adduct in aqueous formate.The significant singlet → triplet quantum yields found for the mono‐adduct might be consistent with the involvement of triplet excited mono‐adduct in DNA cross‐link formation, as also may be the high reactivity obtained for the triplet with thymine. The initial quenching products observed resulted from a charge‐transfer reaction.

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