Interaction of Triplet Photosensitizers with Nucleotides and DNA in Aqueous Solution at Room Temperature

Abstract

The study of triplet excited state behavior of nucleic acids and component mononucleotides is hampered by the very small yields produced by direct photolysis. We have used high energy triplet sensitizers to generate these species in high yield, thus facilitating the study of their photophysical and photochemical behavior. Acetone-sensitized triplet formation of all triplet state nucleotides allowed nucleotide triplet−triplet absorption spectra to be measured. Triplet−triplet absorption coefficients were determined using comparative actinometry. Self-quenching of the nucleotide triplet states was found to occur efficiently with rate constants, ksq > 107 M-1 s-1. The interaction of a variety of ketone triplet sensitizers with mononucleotides has been studied as a function of the relative energies of the sensitizer−nucleotide pair. In all cases, the triplet states of the sensitizers were efficiently quenched by the nucleotides, although different reaction mechanisms were observed depending on the reaction pair under study. Acetone, the sensitizer with the highest triplet energy, sensitized all triplet state nucleotides. Sensitizers with triplet energies, ET > 74 kcal mol-1, sensitized TMP and those with ET < 74 kcal mol-1 did not exhibit any triplet sensitization, although an efficient quenching reaction (kq > 108 M-1 s-1) was observed. Where energy transfer did not take place, sensitizers were quenched by electron transfer from the purines. The quantum yield for this process was determined as 0.31 for GMP and 0.09 for AMP. In DNA, triplet energy transfer from the same sensitizers was probed by determining the relative efficiency of pyrimidine dimer formation in pBR322, an exclusively triplet-mediated reaction under sensitized conditions. Our results allow some conclusions to be drawn on triplet properties and intramolecular energy transfer in DNA. Base triplet energy levels appear to be lower in DNA than in the isolated mononucleotides. In any system where ketone triplet states are generated, electron transfer from a purine should be considered as a significant reaction pathway.