Recent experimental results about the oxidation of methionine enkephalin by center dot OH radicals indicated an intramolcciilar 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(center dot+)/Met couple is higher than that of the TyrOH(center dot+)/TyrOH one (tyrosine is denoted as TyrOH) for all conformations: The intramolecular electron transfer between both residues (from TyrOH to Met(center dot+)) 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.