Electron transfer reactions of methionine peptides with photochemically generated ruthenium(III)–polypyridyl complexes

• Ru3+ is generated photochemically from Ru2+.
• Ru3+ oxidizes peptide containing methionines to the corresponding sulfoxides.
• The oxidation proceeds through electron transfer mechanism.
• The observation of methionine sulfide cation radical as the transient from the laser flash photolysis study supports the operation electron transfer mechanism.
• The kinetic data are in accordance with the ΔG° values of the reaction.

The dynamics of the oxidation of five methionine carrying peptides with six Ru(III)–polypyridyl complexes in aqueous acetonitrile medium have been followed by spectrophotometric technique. The electron transfer (ET) reaction of [Ru(NN)3]3+ complex (NN = polypyridyl ligand) with peptide containing methionine is sensitive to the change in the structure of ligand of Ru(III) complex and N-terminal component of the peptide Met-Gly Met-Ala Met-Ser Met-Val and Met-Leu. The reaction follows the overall second-order kinetics, first order each in the oxidant and the substrate. Based on the substituent and solvent effects, an outer-sphere electron transfer from the sulfur center of the peptide to Ru(III) has been proposed as the rate controlling step of the reaction. The ET nature of the reaction is confirmed from the recorded transient absorption spectrum of peptide containing methionine sulfur radical cation by laser flash photolysis technique. Marcus theory is successfully applied to the ET reaction.

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