Gas-phase fragmentation of long-lived cysteine radical cations formed via no loss from protonated S-nitrosocysteine

Abstract In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with...
Ausführliche Beschreibung

Abstract In this work, we describe two different methods for generating protonated S-nitrosocysteine in the gas phase. The first method involves a gas-phase reaction of protonated cysteine with t-butylnitrite, while the second method uses a solution-based transnitrosylation reaction of cysteine with S-nitrosoglutathione followed by transfer of the resulting S-nitrosocysteine into the gas phase by electrospray ionization mass spectrometry (ESI-MS). Independent of the way it was formed, protonated S-nitrosocysteine readily fragments via bond homolysis to form a long-lived radical cation of cysteine ($ Cys^{•+} $), which fragments under collision-induced dissociation (CID) conditions via losses in the following relative abundance order: •COOH ≫ $ CH_{2} $S > •$ CH_{2} $SH-$ H_{2} $S. Deuterium labeling experiments were performed to study the mechanisms leading to these pathways. DFT calculations were also used to probe aspects of the fragmentation of protonated S-nitrosocysteine and the radical cation of cysteine. NO loss is found to be the lowest energy channel for the former ion, while the initially formed distonic $ Cys^{•+} $ with a sulfur radical site undergoes proton and/or H atom transfer reactions that precede the losses of $ CH_{2} $S, •COOH, •$ CH_{2} $SH, and $ H_{2} $S. Ausführliche Beschreibung