Gas-phase fragmentation of long-lived cysteine radical cations formed via no loss from protonated S-nitrosocysteine
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- Ryzhov, V., Lam, A.K.Y. & O’Hair, R.A.J. J Am Soc Mass Spectrom (2009) 20: 985. doi:10.1016/j.jasms.2008.12.026
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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 ≫ CH2S > •CH2SH-H2S. 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 CH2S, •COOH, •CH2SH, and H2S.