The influence of protonation or alkylation of the phosphate group on the e.s.r. spectra and on the rate of phosphate elimination from 2-methoxyethyl phosphate 2-yl radicals

The e.s.r. spectra of l-yl, 2-yl and 3'-yl methoxethyl phosphate radicals derived from CH3OCH2CH2-OPO3H2 by hydrogen abstraction have been measured in aqueous solutions and the hyperfine constants determined. The coupling constants vary strongly with protonation or alkylation of the phosphate group. The 2-yl radicals eliminate phosphate. The rate-constants for the elimination (ksub(e)) have been estimated by e.s.r. measurements and by product studies as a function of pH using 60Co γ-radiolysis. The ksub(e) values vary from approximately 0.3 s-1 for the CH3OCHCH2OPO3-- radical and approximately 103s-1 for CH3OCHCH2OPO3H-, to approximately 3 x 106s-1 for CH3OCHCH2OPO3H2. Alkylation of the phosphate group increased the elimination rate-constant to a similar extent as protonation. The results support a recent mechanism which described the OH-radical-induced single-strand breaks of DNA in aqueous solution starting from the C-4' radical of the sugar moiety. It is further concluded the C-4' radical of DNA eliminates the 3'-phosphate group faster than the 5'-phosphate group. (author)