Ryzhov Victor, Lam Adrian K Y, O'Hair Richard A J
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois, USA.
J Am Soc Mass Spectrom. 2009 Jun;20(6):985-95. doi: 10.1016/j.jasms.2008.12.026. Epub 2009 Jan 9.
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.
在本工作中，我们描述了两种在气相中生成质子化S-亚硝基半胱氨酸的不同方法。第一种方法涉及质子化半胱氨酸与亚硝酸叔丁酯的气相反应，而第二种方法是利用半胱氨酸与S-亚硝基谷胱甘肽基于溶液的转亚硝基化反应，随后通过电喷雾电离质谱（ESI-MS）将生成的S-亚硝基半胱氨酸转移到气相中。无论其形成方式如何，质子化S-亚硝基半胱氨酸都很容易通过键均裂断裂，形成半胱氨酸的长寿命自由基阳离子（Cys(*+)），该阳离子在碰撞诱导解离（CID）条件下通过以下相对丰度顺序的损失进行断裂：*COOH CH(2)S >> CH(2)SH 约等于 H(2)S。进行了氘标记实验以研究导致这些途径的机制。DFT计算也用于探究质子化S-亚硝基半胱氨酸和半胱氨酸自由基阳离子的断裂方面。发现NO损失是前一种离子的最低能量通道，而最初形成的具有硫自由基位点的双自由基Cys(+)在损失CH(2)S、*COOH、*CH(2)SH和H(2)S之前会发生质子和/或H原子转移反应。
