文摘
Neutral sugar radicals formed in DNA sugar鈥損hosphate backbone are well-established as precursors of biologically important damage such as DNA strand scission and cross-linking. In this work, we present electron spin resonance (ESR) evidence showing that the sugar radical at C5鈥?(C5鈥?sup>鈥?/sup>) is one of the most abundant (ca. 30%) sugar radicals formed by 纬- and Ar ion-beam irradiated hydrated DNA samples. Taking dimethyl phosphate as a model of sugar鈥損hosphate backbone, ESR and theoretical (DFT) studies of 纬-irradiated dimethyl phosphate were carried out. CH3OP(O2鈥?/sup>)OCH2鈥?/sup> is formed via deprotonation from the methyl group of directly ionized dimethyl phosphate at 77 K. The formation of CH3OP(O2鈥?/sup>)OCH2鈥?/sup> is independent of dimethyl phosphate concentration (neat or in aqueous solution) or pH. ESR spectra of C5鈥?sup>鈥?/sup> found in DNA and of CH3OP(O2鈥?/sup>)OCH2鈥?/sup> do not show an observable 尾-phosphorus hyperfine coupling (HFC). Furthermore, C5鈥?sup>鈥?/sup> found in DNA does not show a significant C4鈥?H 尾-proton HFC. Applying the DFT/B3LYP/6-31G(d) method, a study of conformational dependence of the phosphorus HFC in CH3OP(O2鈥?/sup>)OCH2鈥?/sup> shows that in its minimum energy conformation, CH3OP(O2鈥?/sup>)OCH2鈥?/sup>, has a negligible 尾-phosphorus HFC. On the basis of these results, the formation of radiation-induced C5鈥?sup>鈥?/sup> is proposed to occur via a very rapid deprotonation from the directly ionized sugar鈥損hosphate backbone, and the rate of this deprotonation must be faster than that of energetically downhill transfer of the unpaired spin (hole) from ionized sugar鈥損hosphate backbone to the DNA bases. Moreover, C5鈥?sup>鈥?/sup> in irradiated DNA is found to be in a conformation that does not exhibit 尾-proton or 尾-phosphorus HFCs.