The interactions of various pyrimidines (1,3-dimethylthymine, DMT, 1,3-bis(N 4 ,N 4 -dimethylcytosin-1yl)propane, DMC) and their corresponding cis-syn cyclobutane dimers (DMTD and DMCD) with a series of excitedstate electron donors were examined with the goal of understanding the energetics and mechanism of UV repair by DNA photolyase. For each substrate there is a good correlation between the excited state oxidation potential (E ox *) and the quenching rate constant (k q ). The value for k q increases as E ox * becomes more negative, asymptotically approaching a value that is at or below the solvent diffusion limit. These data all showed good fits to the Rehm-Weller equation. Reduction potentials for each of the substrates could be extracted from this analysis: -2.20 V (vs SCE) for DMTD; -2.14 V for DMT; -2.17 V for DMCD; and -2.16 for DMC. These values show that the initial electron transfer step in the photolyase mechanism is exergonic by ca. 10-15 kcal/mol. Thus these data support the reductive electron transfer mechanism for DNA photolyases proposed by Jorns et al.

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.

The enzyme DNA photolyase mediates the repair of pyrimidine dimers. This repair step, a net retro [2 + 2] reaction, proceeds through either the cation or anion radical of the pyrimidine dimer. In order to understand how electron transfer makes the repair process possible, its energetics have been examined by photothermal beam deflection calorimetry, fluorescence quenching and quantum yield studies. The enthalpy for the cleavage reaction of cis-syn 1,3-dimethylthymine dimer itself was found to be -19 kcal/mol. In addition, from the redox potentials, the enthalpies for the cleavage reactions of the dimer cation radical and the anion radical were determined to be -19 kcal/mol and -28 kcal/mol, respectively.