Reduction of Selected Oligopeptides Containing Methionine Induced by Hydrated Electrons

Bimolecular rate constants for the reaction of the hydrated electron with zwitterionic forms of several linear oligopeptides containing methionine were determined using the pulse radiolysis technique. The rate constants were found to vary in the range of $(0.2-1.2)\times 10^{9}\ M^{-1}\ {\rm s}^{-1}$. The reactivity of the peptides toward ${\rm e}{}_{{\rm aq}}{}^{-}$ is governed by the $pK_{{\rm a}}$ of the N-terminal amino group and the number of peptide bonds. At a fixed number of peptide bonds, the reactivity increases with the $pK_{{\rm a}}$, and for a given N-terminal amino acid residue, it increases with the number of peptide bonds. For the linear peptides the observed transient spectra are assigned to deaminated oligopeptide radicals, ^{\bullet}{\rm CHRCONH}\sim $, obtained due to rapid deamination of the corresponding electron adducts formed initially. The radicals derived from oligopeptides containing methionine at the N-terminus absorb at ∼400 nm with extinction coefficients of $\sim 1300\pm 150\ M^{-1}\ {\rm cm}^{-1}$. Their absorption maxima are shifted hypsochromically by ∼30 nm with respect to those derived from oligopeptides with glycine at the N-terminus. The ${\rm e}{}_{{\rm aq}}{}^{-}$ reacts with cyclic Met-Met, $k=2.0\times 10^{9}\ M^{-1}\ {\rm s}^{-1}$, probably by addition to the carbonyl bond, forming an adduct absorbing below 250 nm with $\epsilon _{250}=2300\pm 250\ M^{-1}\ {\rm cm}^{-1}$.