Factors Affecting the Rate of Hydrated Electron Attack on Polynucleotides

Abstract

The rate constants for the attack of ${\rm e}_{{\rm aq}}^{-}$ on adenine, cytosine, and several derivatives were measured in the pH range 6-14. The rate constants at neutral pH are $0.9\times 10^{10} M^{-1}s^{-1}$ and $1.3\times 10^{10} M^{-1}s^{-1}$ for adenine and cytosine, respectively. The rate constants, per base, for the attack of ${\rm e}_{{\rm aq}}^{-}$ on polynucleotides are much lower. At pH 7 they are $7.5\times 10^{8} M^{-1}s^{-1}$, $2.5\times 10^{8} M^{-1}s^{-1}$, and $1.3\times 10^{8} M^{-1}s^{-1}$ 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 ${\rm e}_{{\rm aq}}^{-}$. 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 ...