Ring Opening of the Cyclobutane in a Thymine Dimer Radical Anion

The reactions of hydrated electrons (e aq − ) with thymine dimer 2 and thymidine have been investigated by radiolytic methods coupled with product studies, and addressed computationally by means of BB1K‐HMDFT calculations. Pulse radiolysis revealed that one‐electron reduction of the thymine dimer 2 affords the radical anion of thymidine ( 5 ) with t 1/2 <35 ns. Indeed, the theoretical study suggests that radical anion 3 , in which the spin density and charge distribution are located in both thymine rings, undergoes a fast partially ionic splitting of the cyclobutane with a half‐life of a few ps. This model fits with the in vivo observation of thymine dimer repair in DNA by photolyase. γ‐Radiolysis of thymine dimer 2 demonstrates that the one‐electron reduction and the subsequent cleavage of the cyclobutane ring does not proceed by means of a radical chain mechanism, that is, in this model reaction the T −. is unable to transfer an electron to the thymine dimer 2 .