Radiolytic Oxidation of Two Inverse Dipeptides, Methionine-Valine and Valine-Methionine: A Joint Experimental and Computational Study.

The two inverse peptides methionine-valine (Met-Val) and valine-methionine (Val-Met) are investigated in an oxidative radiolysis process in water. The OH radical yields products with very different absorption spectra and concentration effects: Met-Val yields one main product with a band at about 400 nm and other products at higher energies; there is no concentration effect. Val-Met yields at least three products, with a striking concentration effect. Molecular simulations are performed with a combination of the Monte Carlo, density functional theory, and reaction field methods. The simulation of the possible transients enables an interpretation of the radiolysis: (1) Met-Val undergoes an H atom uptake leaving mainly a neutral radical with a 2-center-3-electron (2c-3e) SN bond, which cannot dimerize. Other radicals are present at higher energies. (2) Val-Met undergoes mainly an electron uptake leaving a cation monomer with a (2c-3e) SO bond and a cation dimer with a (2c-3e) SS bond. At higher energies, neutral radicals are possible. This cation monomer can transfer a proton toward a neutral peptide, leaving a neutral radical.