Toward an Understanding of the Oxidation Process of Methionine Enkephalin: A Combined Electrochemistry, Quantum Chemistry and Quantum Chemical Topology Analysis
2015
Bergès, Jacqueline | Kamar, Amanda | Oliveira, Pedro de | Pilmé, Julien | Luppi, Eleonora | Houée-Levin, Chantal
Recent experimental results about the oxidation of methionine enkephalin by ·OH radicals indicated an intramolecular electron transfer between the C-terminal methionine radical cation and the tyrosine N-terminus too fast to be observed. We have investigated the thermodynamic possibility of this intramolecular electron transfer by calculating the one-electron redox potentials of both residues for several conformations of the peptide, extracted from the experimental data of the Protein Data Bank (1PLW). Using a QM/MM approach, we show that the redox potential of the Met•⁺/Met couple is higher than that of the TyrOH•⁺/TyrOH one (tyrosine is denoted as TyrOH) for all conformations. The intramolecular electron transfer between both residues (from TyrOH to Met•⁺) is thus always thermodynamically allowed. Previously, we had performed topological studies on the intramolecular electron transfer which predicted this charge transfer. A study by cyclic voltammetry pointed out that the wave belonging to methionine is not present when methionine enkephalin is oxidized and only the direct involvement of the tyrosine residue is observed.
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