| Abstract
| - As a first step toward modeling the photoinduced repairof DNA, electronic structure calculations on thecleavage reaction of various pyrimidine dimers (uracil, thymine, andcytosine) as well as of their anion and cationradicals have been carried out using the AM1 UHF method. Twodifferent paths of the splitting reaction have beenstudied by locating all stationary points. Along the first path,the opening of the cyclobutane ring is initiated bybreaking the C5−C5‘ bond which leads to the formation of anintermediate, followed by the cleavage of the C6−C6‘bond; along the second path, the two C−C‘ bonds are broken in reverseorder. The results for the dimer anionradical favor a cleavage reaction along the first path while the secondpath is preferred for the cation radicals. Electrontransfer to the dimers does not appreciably influence the enthalpy ofthe reaction for cycloreversion in the uracil andthymine dimers; however, it causes a dramatic reduction of theactivation barrier for the cleavage reaction. Incontrast,the reactivity of the cytosine dimer is only weakly affected by thiselectron uptake. Differences in the variousreaction profiles are rationalized by invoking an energeticstabilization associated with the charge delocalizationbetween fragments in the corresponding transition states. Thecalculated solvent effects evaluated by a dielectriccontinuum model show that the splitting reaction is sensitive to apolar environment. The reaction barriers of thesplitting reaction are found to increase with the polarity of themedium, rationalizing the experimentally observedsolvent effects on the dimer cleavage.
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