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

Summary

The e.s.r. spectra of 1-yl, 2-yl and 3′-yl methoxyethyl 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 (k_e) have been estimated by e.s.r. measurements and by product studies as a function of pH using ⁶⁰Co γ-radiolysis. The k_e values vary from ∼0·3 s⁻¹ for the CH₃OĊHCH₂OPO^—₃ radical and ∼10³ s⁻¹ for CH₃OĊHCH₂OPO₃H⁻, to ∼3 × 10⁶ s⁻¹ for CH₃OĊHCH₂OPO₃H₂. Alkylation of the phosphate group increases 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.