One-Electron Oxidation of Methionine-Containing Dipeptides of Reverse Sequence: Sulfur versus Sulfoxide Characterized by IRMPD Spectroscopy and Static and Dynamics DFT Simulations

Gas-phase structural modifications induced by the oxidation of methionine of the two peptides of reverse sequence, methionine–valine ( Met-Val ) and valine–methionine ( Val-Met ), have been studied by mass-selected IR multiple photon dissociation (IRMPD) spectroscopy in the 800–2000 cm –1 fingerprint range at the Centre Laser Infrarouge d’Orsay free-electron laser facility. The oxidation has been achieved by • OH radicals generated by γ radiolysis. IRMPD spectra were interpreted by static and harmonic DFT calculations and Born–Oppenheimer molecular dynamics simulations, which are employed to take into account all anharmonic and finite-temperature effects. The diagnostic signature of the sulfoxide group in the final products of Met-Val and Val-Met oxidations, which is missing in the spectra of native peptides, has been recorded. Evidence has also been gathered that a mixture of R and S isomers of close energies is formed. An interconversion between different isomers has been unveiled in the case of the oxidized Met-Val dipeptide.