REDUCTION OF SELECTED OLIGOPEPTIDES CONTAINING METHIONINE INDUCED BY HYDRATED ELECTRONS

Bimolecular rate constants for the reaction of the hydrated electron w ith zwitterionic forms of several linear oligopeptides containing meth ionine were determined using the pulse radiolysis technique. The rate constants were found to vary in the range of (0.2-1.2) X 10(9) M-1 s(- 1). The reactivity of the peptides toward e(aq)(-) is governed by the pK(a) of the N-terminal amino group and the number of peptide bonds. A t a fixed number of peptide bonds, the reactivity increases with the p K(a), and for a given N-terminal amino acid residue, it increases with the number of peptide bonds. For the linear peptides the observed tra nsient spectra are assigned to deaminated oligopeptide radicals,. CHRC ONH similar to, obtained due to rapid deamination of the corresponding electron adducts formed initially. The radicals derived from oligopep tides containing methionine at the N-terminus absorb at similar to 400 nm with extinction coefficients of similar to 1300 +/- 150 M-1 cm(-1) . Their absorption maxima are shifted hypsochromically by similar to 3 0 mn with respect to those derived from oligopeptides with glycine at the N-terminus. The e(aq)(-) reacts with cyclic Met-Met, k = 2.0 X 109 M-1 s(-1), probably by addition to the carbonyl bond, forming an addu ct absorbing below 250 nm with epsilon(250) = 2300 +/- 250 M-1 cm(-1). (C) 1998 by Radiation Research Society.