EFFICIENT PYRIMIDINE DIMER RADICAL-ANION SPLITTING IN LOW POLARITY SOLVENTS

Photosensitized pyrimidine dimer splitting by a covalently linked methoxybenzene exhibited a strong solvent dependence. Fluorescence of the chromophore was quenched by the attached dimer, which was indicative of electron transfer from excited chromophore to dimer. The quantum efficiency of splitting of the dimer radical anion in the linked dimer.--chromophore.+ was calculated from the observed quantum yields of splitting and the degree of fluorescence quenching. The quantum efficiency of dimer radical anion splitting was remarkably dependent on solvent polarity, ranging from 0.05 in water to approximately 0.5 in low polarity solvent mixtures (e.g., heptane/1,4-dioxane, 95:5). The results were rationalized in terms of competition of splitting and back electron transfer within the charge-separated species. The latter pathway may be slowed due to its exergonicity in low polarity media, in accord with Marcus inverted behavior. Photolyases may be effective for splitting dimers by providing catalytic groups and a medium in which both dimer radical anion formation and splitting are efficient.