A photochemical mechanism for single-strand cleavage of DNA is proposed in which a photoexcited intercalator transfers an electron to an externally bound cosensitizer. Once formed, the oxidized intercalator oxidizes an adjacent base, creating a charge-separated complex from which reactions leading to cleavage of the sugar-phosphate backbone occur in competition with back electron transfer. Using ethidium bromide (EB) as the intercalator and methyl viologen (MV) as the externally bound cosensitizer, a 10-fold enhancement in the rate of single-strand break formation was found in pBR322 DNA over that for EB alone using 488-nm excitation. The rate of cleavage correlated with the amount of MV bound to DNA. In accord with the expected redox properties of the one-electron-oxidized EB and the DNA bases, cleavage occurs selectively at guanines. Although the reaction proceeds in nitrogen-purged solutions, the rate of cleavage in air-saturated solutions was enhanced 2-fold. Treatment of irradiated samples with alkali leads to a 2-fold increase in the yield of single-strand breaks. These results support a mechanism in which cleavage occurs by selective oxidation of guanines in DNA, initiated by photochemical cosensitized electron transfer from intercalated EB to externally bound MV, and may provide a basis for the development of light-activated base-selective DNA cleaving agents.

The effect of three purine nucleotides on the fluorescence of methylene blue in aqueous buffer has been investigated. Guanosine-5'-monophosphate (GMP) and xanthosine-5'-monophosphate cause fluorescence quenching while adenosine-5'-monophosphate causes a red shift in the fluorescence maximum. All three nucleotides form ground state complexes with the nucleotides as indicated by absorption spectroscopy. The fluorescence changes at nucleotide concentrations less than 30 mM are best described by a static mechanism involving the formation of non-fluorescent binary and ternary complexes in competition with dimerization of the dye. Quenching of the fluorescence decay (tau = 368 ps) at high GMP concentrations (10-100 mM) occurs at the rate of diffusion. The mechanism of fluorescence quenching may involve electron transfer within the singlet excited dye-nucleotide complex although published values of the oxidation potentials of various purine derivatives would suggest that all three nucleotides should cause quenching. Evidence for electron transfer was obtained from flash photolysis experiments in which 100 mM GMP was found to cause the appearance of a long lived transient species absorbing in the region expected for semimethylene blue.