An ab initio mechanism for efficient population of triplet states in cytotoxic sulfur substituted DNA bases: the case of 6-thioguanine

The deactivation mechanism of the cytotoxic 6-thioguanine, the 6-sulfur-substituted analogue of the canonical DNA base, is unveiled by ab initio calculations. Oxygen-by-sulfur substitution leads to efficient population of triplet states-the first step for generating singlet oxygen-which is responsible for its cytotoxicity.