Pulse Radiolysis Studies of Electron-Transfer Reaction in Molecules of Biological Interest: II. The Reduction of Cu(II)-Peptide Complexes

The reduction mechanism of two Cu(II)-peptide complexes, <tex-math>${\rm Cu}({\rm II})-(\text{gly})_{3}$</tex-math> and Cu(II)-glutathione, was investigated using the pulse radiolysis method. The hydrated electron, used as a reducing agent, was found to react with the two complexes by a bimolecular process. In the <tex-math>${\rm Cu}({\rm II})-(\text{gly})_{3}$</tex-math> complex the reaction of the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> is both with the Cu(II) ion and with the carbonyl group of the peptide bond to form a carbonyl "electron adduct" radical. This adduct transfers the electron in a first-order process to the Cu(II). In the Cu(II)-glutathione complex the decay of the electron was followed by an appearance of a transient absorption band centered at 410 nm. It was followed by a slower, small increase of absorption at the same wavelength <tex-math>$(\lambda _{{\rm max}}=410)$</tex-math>. This band is assigned to the adduct of an electron with the disulfide bond of the glutathione (<tex-math>${\rm RSSR}^{-}$</tex-math> radical ion). The radical ion is formed either by the direct reaction of <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> with the disulfide bond and the transfer of an electron from the carbonyl adduct formed by the <tex-math>$e{}_{{\rm aq}}{}^{-}$</tex-math> or entirely via the electron-transfer reaction from the adduct to the S-S bridge. The <tex-math>${\rm RSSR}^{-}$</tex-math> decays via a first-order process at the same rate at which the Cu(II) is reduced. CO₂^- reduces the Cu(II) ion in the complexes via a bimolecular reaction. In the Cu(II)-glutathione complex a small part (10%) of the reduction of the Cu(II) proceeds via the disulfide bridge. The pathway of the reduction of the Cu(II) ion in peptide complexes by the hydrated electron may therefore be delineated: It proceeds from the bimolecular encounter step, in which the electron is attached to residues of the peptide (carbonyl and disulfide groups), followed by an intramolecular process to the Cu (II) ion.