Solvent dependence of pyrimidine dimer splitting in a covalently linked dimer-indole system.

Cyclobutadipyrimidines (pyrimidine dimers) undergo splitting that is photosensitized by indole derivatives. We have prepared a compound in which a two-carbon linker connects a dimer to an indolyl group. Indolyl fluorescence quenching indicated that the two portions of the molecule interact in the excited state. Intramolecular photosensitization of dimer splitting was remarkably solvent dependent, ranging from phi spl = 0.06 in water to a high value of phi spl = 0.41 in the least polar solvent mixture examined, 1,4-dioxane-isopentane(5 : 95). A derivative with a 5-methoxy substituent on the indolyl ring behaved similarly. These results have been interpreted in terms of electron transfer from the excited indolyl group to the dimer, which would produce a charge-separated species. The dimer anion within such a species could split or undergo back electron transfer. The possibility that back electron transfer is in the Marcus inverted region can be used to rationalize the observed solvent dependence of splitting. In the inverted region, the high driving force of a charge recombination exceeds the reorganization energy of the solvent, which is less for solvents of low polarity than those of high polarity. If this theory is applicable to the hypothetical charge-separated species, a slower back electron transfer, and consequently higher splitting efficiencies, would be expected in solvents of lower polarity. Photolyases may have evolved in which a low polarity active site retards back transfer of an electron and thereby contributes to the efficiency of the enzymatic dimer splitting.