5-Thiocyanato-2 '-deoxyuridine as a possible radiosensitizer: electron-induced formation of uracil-C5-thiyl radical and its dimerization

In this work, we have synthesized 5-thiocyanato-2'-deoxyuridine (SCNdU) along with the C6-deuterated nucleobase 5-thiocyanatouracil (6-D-SCNU) and studied their reactions with radiation-produced electrons. ESR spectra in gamma-irradiated nitrogen-saturated frozen homogeneous solutions (7.5 M LiCl in H2O or D2O) of these compounds show that electron-induced S-CN bond cleavage occurs to form a thiyl radical (dU-5-S-center dot or 6-D-U-5-S-center dot) and CN- via the initial pi-anion radical (SCNdU center dot-) intermediate in which the excess electron is on the uracil base. HPLC and LC-MS/MS studies of gamma-irradiated N-2-saturated aqueous solutions of SCNdU in the presence of sodium formate as a OH-radical scavenger at ambient temperature show the formation of the dU-5S-5S-dU dimer in preference to dU by about 10 to 1 ratio. This shows that both possible routes of electron-induced bond cleavage (dUC5-SCN and S-CN) in SCNdU center dot- and dU-5-S-center dot formation are preferred for the production of the sigma-type uracilyl radical (dU(center dot)) by 10 fold. DFT/M06-2x/6-31++G(d,p) calculations employing the polarizable continuum model (PCM) for aqueous solutions show that dU-5-S-center dot and CN- formation was thermodynamically favored by over 15 kcal mol(-1) (DG) compared to dU(center dot) and SCN- production. The activation barriers for C5-S and S-CN bond cleavage in SCNdU center dot- amount to 8.7 and 4.0 kcal mol(-1), respectively, favoring dU-5-S-center dot and CN- formation. These results support the experimental observation of S-CN bond cleavage by electron addition to SCNdU that results in the formation of dU-5-S-center dot and the subsequent dU-5S-5S-dU dimer. This establishes SCNdU as a potential radiosensitizer that could cause intra- and inter-strand crosslinking as well as DNA-protein crosslinking via S-S dimer formation.