title: Radical Cations of the Monomer and van der Waals Dimer of a Methionine Residue as Prototypes of (2 Center-3 Electron) SN and SS Bonds. Molecular Simulations of Their Absorption Spectra in Water.
abstract: Oxidation of peptides or proteins by the OH(•) radicals produced by pulse radiolysis yields species identified by their absorption spectra in the UV-visible domain. However, the case of methionine (Met) in peptides is complex because its oxidation can lead to various free radicals with 2 center-3 electron (2c-3e) bonds. We have performed Monte Carlo/density functional theory molecular simulations of the radical cation of the methylated methionine aminoacid, Met(•+), taken as a model of the methonine residue of peptides, and of the radical cation of its van der Waals dimer, Met2(•+). The cation of the methionine residue displays a 2c-3e SN bond. The cation of dimer Met2(•+) displays three quasidegenerate conformers, one stabilized by a 2c-3e SS bond and the other two stabilized by ion-molecule interactions and made up of a neutral and a cationic unit. These conformers are characterized by their charge and spin density localization and their UV-visible absorption spectra. These spectra enable a discussion of the absorption spectra of the literature; in particular, we emphasize the role of dimers before and after the oxidation process.
authors: Laboratoire de Chimie Physique, CNRS, UMR 8000, Université Paris-Sud , F91405 Orsay, France.
authors: Pierre
authors: P
authors: Archirel
authors: Laboratoire de Chimie Théorique, UPMC Univ Paris 06, CNRS, UMR 7616, Sorbonne Universités , CC 137-4, place Jussieu, F75252 Paris Cedex 05, France.
authors: Jacqueline
authors: J
authors: Bergès
authors: Laboratoire de Chimie Physique, CNRS, UMR 8000, Université Paris-Sud , F91405 Orsay, France.
authors: Chantal
authors: C
authors: Houée-Lévin