Sigma radicals in gamma-irradiated single crystals of 2-thiothymine

Abstract

Thioanalogs of the nucleic-acid bases are known photosensitive probes and good traps of radiation energy. Radiation-induced sulfur-centered free radicals stabilized on the base thioanalogs molecules are of the cationic origin. Their electronic configuration is of the π type, unless an electron-donating group, like Cl⁻, is added to the sulfur atom, forming the σσ* “ three-electron” S∴Cl bond. The σ radical observed in irradiated 2-thiothymine is formed simply by deprotonation of the pristine cation radical. The 2σ character of the radical is determined from the nature of the g tensor and the A(¹⁴N) coupling tensor and their relation to the radical skeleton. The deprotonation of the cation radical takes place at N(3) rather than at N(1), generally observed in the irradiated pyrimidine natural bases and their other thioanalogs.