Surprising Radiolytic Stability of 8-Thiomethyladenine in an Aqueous Solution

8-Thiomethyladenine (ASCH₃), a potentially radiosensitizing modified nucleobase, has been synthesized in a reaction between 8-thioadenine and methyl iodide. Despite favorable dissociative electron attachment (DEA) characteristics, the radiolysis of an aqueous solution of ASCH₃ with a dose of X-ray amounting to as much as 300 Gy leads to no effects. Nevertheless, crossed electron-molecule beam experiments in the gas phase on ASCH₃ confirm the theoretical findings regarding the stability of its radical anion, namely, the most abundant reaction channel is related to the dissociation of the S-CH₃ bond in the respective anion. Furthermore, electron-induced degradation of ASCH₃ has been observed in aprotic acetonitrile, which is strong evidence for the involvement of proton transfer (PT) in stabilizing the radical anion in an aqueous solution. These findings demonstrate that PT in water can be the main player in deciding the radiosensitizing properties of modified nucleobases/nucleosides.