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

Abstract

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 γ-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• or 6-D-U-5-S•) and CN− via the initial π-anion radical (SCNdU•−) intermediate in which the excess electron is on the uracil base. HPLC and LC-MS/MS studies of γ-irradiated N2-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•− and dU-5-S• formation are preferred for the production of the σ-type uracilyl radical (dU•) 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• and CN− formation was thermodynamically favored by over 15 kcal/mol (ΔG) compared to dU• and SCN− production. The activation barriers for C5-S and S-CN bond cleavage in SCNdU•− amount to 8.7 and 4.0 kcal/mol, respectively, favoring dU-5-S• 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• 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.

Uracil-5-yl O-Sulfamate: An Illusive Radiosensitizer. Pitfalls in Modeling the Radiosensitizing Derivatives of Nucleobases

“… c Calculated for 5-thiocyanato-2′-deoxyuridine (SCNdU). 46 d Calculated for 1-methyl-5-thiocyanatouracil (MetSCNU). e Calculated for 5-bromo-1-methyluracil (MetBrU).…”

Electrophilic 5‐Substituted Uracils as Potential Radiosensitizers: A Density Functional Theory Study

“…Similar studies from our group were published recently, and the current work can be considered as an extension of our previous project to investigate uracil derivatives in which dissociative electron attachment (DEA) leads not only to the uracil‐5‐yl radical (U ⋅ ) but also to the UX ⋅ radical. Whereas U ⋅ occurring in a DNA strand may cause strand breaks, the UX ⋅ radical might also induce the production of inter‐ or intrastrand crosslinks, as we recently suggested for the U−S ⋅ species . Crosslinks are among the most cytotoxic forms of DNA damage; thus, the incorporation of such derivatives into DNA may lead to extremely efficient radiosensitization.…”

“…Whereas U · occurring in aD NA strand may cause strand breaks, the UX · radical might also inducet he productiono fi nter-or intrastrand crosslinks, as we recently suggested for the UÀS · species. [29] Crosslinks are among the most cytotoxic forms of DNA damage; [14] thus, the incorporation of such derivatives into DNA may lead to extremelye fficient radiosensitization.…”

Molecular features of thymidine analogues governing the activity of human thymidine kinase

“…Recently, we have undertaken attempts for rational designing of 5-substituted uracils prone to the dissociative electron attachment (DEA) process [18,19]. As a result, we proposed a series of 5-modified uracils with the DEA characteristics much better than those of BrdU [18,19] and the superior characteristics of some of them were confirmed in the photoelectron spectroscopy [18] and radiolytic studies [20,21].…”