Factors Affecting the Rate of Hydrated Electron Attack on Polynucleotides

The rate constants for the attack of e_ aq⁻ on adenine, cytosine, and several derivatives were measured in the pH range 6-14. The rate constants at neutral pH are 0.9× 10¹⁰ M⁻¹s⁻¹ and 1.3× 10¹⁰ M⁻¹s⁻¹ for adenine and cytosine, respectively. The rate constants, per base, for the attack of e_ aq⁻ on polynucleotides are much lower. At pH 7 they are 7.5× 10⁸ M⁻¹s⁻¹, 2.5× 10⁸ M⁻¹s⁻¹, and 1.3× 10⁸ M⁻¹s⁻¹ for poly U, poly A, and poly (A + U), respectively. It is shown that the low overall reactivity of polynucleotides results from a lowered collision frequency with e_ aq⁻. Secondary structure is an important factor, responsible for the observed differences in the values of the rate constants for the polynucleotides studied. Poly (A + U), which has a double helical structure, is least reactive, and poly U, an unstacked random coil, is most reactive. The inherent reactivity of ...