The DNA (6-4) photoproduct photolyases are proteins that bind to UV-damaged DNA, specifically to sites that contain a (6-4) pyrimidine-pyrimidone lesion. Upon absorption of UV-A and visible light they catalyze the reversal of these lesions back to normal bases. It has been proposed that the photorepair occurs via an oxetane intermediate, which is formed from a ring-closing isomerization of the (6-4) photoproduct. Four model compounds for the oxetane intermediate have been prepared through photocycloaddition of carbonyl compounds (benzophenone, benzaldehyde, tolualdehyde, and anisaldehyde) with the 5,6 CdC of 1,3dimethylthymine. The behavior of these compounds under sensitized photolysis conditions has been examined. On the basis of laser flash photolysis, fluorescence quenching, and product analysis experiments, it is demonstrated that these oxetane intermediates undergo a cycloreversion reaction upon photosensitized reductive electron-transfer reactions. The cycloreversion process yields the anion radicals of the carbonyl compounds. A lower limit on the rate constant of this anion radical splitting reaction is estimated to be >5 × 10 7 s -1 . These results support the proposed mechanism for DNA (6-4) photoproduct photolyase.
The dynamics of flourescence quenching of excited state electron donor sensitizers by various pyrimidine and 5,6-dihydropyrimidine substrates was examined. For all of the substrates studied the rate constant of fluorescence quenching (k q) increases as the excited state oxidation potential (E ox * ) becomes more negative. The dependence of k q on E ox * in each case is well described by the Rehm−Weller relationship. Fits of the data to this relationship allow for the estimation of the reduction potentials of the substrates (E red). The pyrimidines 1,3-dimethylthymine, 1,3-dimethyluracil, and 1,3,6-trimethyluracil give E red values (in CH3CN) ranging from −2.06 (vs SCE) to −2.14 V. Their dihydro derivatives, 1,3-dimethyl-5,6-dihydrothymine, 1,3-dimethyl-5,6-dihydrouracil, and 1,3,6-trimethyl-5,6-dihydrouracil gave E red values ranging from −1.90 to −2.07 V. The higher E red values for the dihydropyrimidines compared with their unsaturated derivatives is attributed to aromatic stabilization in the pyrimidines, which is not present in the dihydro derivatives. In addition, the E red for both the trans- syn and cis-syn diastereomers of the dimethylthymine cyclobutane dimer was examined using the same method. The trans-syn dimer gives an E red of −1.73 V and the cis-syn dimer gives an E red of −2.20 V. This remarkable difference is attributed to a stereoelectronic effect. The cis-syn dimer anion radical suffers from an unfavorable charge−dipole interaction between the added electron and the O4 carbonyl group in the remaining pyrimidine ring. In contrast, the trans- syn dimer anion radical shows mainly a stabilizing inductive electron-withdrawing effect of the remaining O4 carbonyl group. Solvent effects on E red were also examined. It is shown that the protic solvent, CH3OH, significantly stabilizes the anion radicals, raising E red by ca. 400 mV over the value in CH3CN.
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