Pulse Radiolysis Studies of Electron Transfer Reaction in Molecules of Biological Interest: I. The Reduction of a Disulfide Bridge by Peptide Radicals

The reduction of the disulfide bridge in the lipoate anion by peptide radicals and the formate radical ion <tex-math>$({\rm CO}{}_{2}{}^{-})$</tex-math> has been studied using the pulse radiolysis method. Peptide radicals were produced by the reaction of the hydrated electrons with the peptide. It was found that <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> reacts with diglycine, chloro-acetyl glycine and chloro-acetyl diglycine to form a deaminated peptide radical <tex-math>$(\cdot {\rm CH}_{2}{\rm CONHCH}_{2}{\rm COR})$</tex-math>. The reaction of <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> with triglycine at nuetral pH forms two radicals: (a) the deaminated radical which is formed when the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> reacts with the carbonyl group in an α position to the amino group, and (b) an "electron adduct" on the carbonyl group of the peptide bond <tex-math>$ \smallmatrix \quad \quad \quad \quad \ {\rm O}^{-} \\ \quad \quad \quad \| \\ ({\rm H}{}_{3}{}^{+}{\rm NCH}_{2}{\rm CONHCH}_{2}{\rm C}\ {\rm NHCH}_{2}{\rm COO}^{-}) \\ \quad \quad \quad \ | \endsmallmatrix $</tex-math>, which is formed when the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> peptide reaction occurs at other positions. The "electron adduct" on the carbonyl radical can be formed by the reaction of the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> with acetyl amide. The rate constant for reduction of the lipoate ion disulfide bridge by the deaminated radical was evaluated to be <tex-math>$5\times 10^{6}\ M^{-1}\ {\rm sec}^{-1}$</tex-math>, while that for "electron adduct" radical and the formate radical is <tex-math>$\sim 5\times 10^{8}\ M^{-1}\ {\rm sec}^{-1}$</tex-math>. In view of these results a multistep mechanism is suggested for the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> reduction of S-S bridge proteins. CO₂^- reacts directly with the disulfide bridge.