Transient Intermediates in Intramolecularly Photosensitized Pyrimidine Dimer Splitting by Indole Derivatives

Intramolecularly photosensitized pyrimidine dimer splitting can serve as a model for some aspects of the monomerization of dimers in the enzyme‐substrate complex composed of a photolyase and UV‐damaged DNA. We studied compounds in which a pyrimidine dimer was covalently linked either to indole or to 5‐methoxyindole. Laser flash photolysis studies revealed that the normally observed photoejection of electrons from the indole or the 5‐methoxyindole to solvent was diminished by an order of magnitude for indoles with dimer attached (dimer‐indole and dimer‐methoxyindole). The fluorescence lifetime of dimer‐indole in aqueous methanol was 0.85 ns, whereas that of the corresponding indole without attached dimer (tryptophol) was 9.7 ns. Similar results were obtained for the dimer‐methoxyindole (0.53 ns) and 5‐methoxytryptophol (4.6 ns). The quantum yield of dimer splitting for the dimer‐methoxyindole (φ287K7 = 0.08) was only slightly greater than the value found earlier for the dimer bearing the unsubstituted indole (4>2K7= 0.04). Transient absorption spectroscopy also revealed lower yields of indole radical cations following laser flash photolysis of dimer‐indole compared to the indole without attached dimer. Dimer‐methoxyindole behaved similarly. These results are interpreted in terms of an enhanced rate of radiationless relaxation of the indole and methoxyindole excited singlet states in dimer‐indoles. The possible quenching of the indole and methoxyindole excited states via electron abstraction by the covalently linked dimer is discussed.