The One-Electron Reduction Potential of Methionine-Containing Peptides Depends on the Sequence

The protein residue methionine (Met) is one of the main targets of oxidizing free radicals produced in oxidative stress. Despite its biological importance, the mechanism of the oxidation of this residue is still partly unknown. In particular the one-electron redox potentials of the couple Met(center dot+)/Met have not been measured. In this work, two approaches, experimental as well as theoretical, were applied for three dipeptides L-Met L-Gly, L-Gly L-Met and L-Met L-Met. Measurements by electrochemistry indicated differences in the ease of oxidation. Two DFT methods (BH&HLYP and PBE0) with two basis sets (6-31G(d) and 6-311+G(2d,2p)) were used to determine the redox potentials of Met in these peptides present in different conformations. In agreement with experimental results, we show that they vary with the sequence and the spatial structure of the peptide, most of the values being higher than 1 V (up to 2 V) vs NHE.