Glutathione radical cation in the gas phase; generation, structure and fragmentation

Abstract

Two different chemical methods have been used to form glutathione radical cations: (1) collision-induced dissociations (CIDs) of the ternary complex [Cu^II(tpy)(M)]˙²⁺ (M = GSH, tpy = 2,2′:6′,2′′-terpyridine) and (2) homolysis of the S–NO bond in protonated S-nitrosoglutathione. The radical cations, M˙⁺, were trapped and additional CIDs were performed. They gave virtually identical CID spectra, suggesting a facile interconversion between initial structures prior to fragmentation. DFT calculations at the B3LYP/6–31++G(d,p) level of theory have been used to study interconversion between different isomers of the glutathione radical cation and to examine mechanisms by which these ions fragment. The N-terminal α-carbon-centred radical cation, strongly stabilized by the captodative effect, is at the global minimum, which is 8.5 kcal mol⁻¹ lower in enthalpy than the lowest energy conformer of the S-centred radical cation. The barrier against interconversion is 18.1 kcal mol⁻¹ above the S-centred radical.