Gas-phase fragmentation of long-lived cysteine radical cations formed via NO loss from protonated S-nitrosocysteine.
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 approximately = 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.
Ryzhov
Victor
V
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, USA.
Lam
Adrian K Y
AK
O'Hair
Richard A J
RA
D002412:Cations / Q000737:chemistry; D003545:Cysteine / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry*; D003903:Deuterium; D005740:Gases; D007536:Isomerism; D008956:Models, Chemical; D009573:Nitrites / Q000737:chemistry; D011522:Protons; D026422:S-Nitrosoglutathione / Q000737:chemistry; D026403:S-Nitrosothiols / Q000138:chemical synthesis / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013816:Thermodynamics
D002412:Cations; D005740:Gases; D009573:Nitrites; D011522:Protons; D026403:S-Nitrosothiols; D026422:S-Nitrosoglutathione; C021315:S-nitrosocysteine; D003903:Deuterium; D003545:Cysteine