The Reaction of the Hydrated Electron with Amino Acids, Peptides, and Proteins in Aqueous Solution: II. Formation of Radicals and Electron Transfer Reactions

The steps in the reaction of the hydrated electron with several simple peptides have been studied. As already suggested the initial step is an attachment of the$e_{{\rm aq}}{}^{-}$ to a carbonyl group in the peptide chain. This reaction can be followed by a further intermolecular electron migration (depending on the nature of the peptide), resulting in a deaminated radical. Thus the reaction of$e_{{\rm aq}}{}^{-}$ with peptides yields two types of radicals: the carbonyl electron adduct$ \smallmatrix {\rm O}^{-} \\ | \\ -{\rm C}- \\ \cdot\endsmallmatrix $ and the deaminated radical$ \smallmatrix \quad \quad \quad \ {\rm O} \\ \quad \quad \quad \parallel \\ \cdot {\rm CH}_{2}-{\rm C}- \endsmallmatrix $. These two radicals were found to differ much in their spectral and chemical properties. The absorption spectra of radicals produced from peptides of alanine and glycine were recorded and examined with respect to their pH, ionic strength and peptide con-concentration dependence. In agreement with previous results, at all concentrations, (>90%) is shown to be the deamination result of the reaction of$e_{{\rm aq}}{}^{-}$ with dipeptide having a protonated terminal amino group. However, different results were obtained with higher peptides (tri, tetra, etc.). Here the percentage of deamination, at low peptide concentration, decreased with increasing the number of peptide bonds [∼60% for 10^-3 M$(\text{gly})_{3}$]. This yield could be increased by increasing the peptide concentration [∼95% for 10^-1 M$(\text{gly})_{3}$]. These findings led to the suggestion that aqueous solutions of peptides at neutral pH exists at least as dimers formed by hydrogen bonds. It was further suggested that these bonds allow the migration of the electron between the carbonyl groups of the peptides this migration results in a higher yield of the deaminated radical.