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Open AccessArticle

Gas-Phase Fragmentation Reactions of Protonated Cystine using High-Resolution Tandem Mass Spectrometry

1
Pilot Laboratory, Institute of Translational Medicine, Faculty of Health Sciences, University of Macau, Macau, China
2
Department of Chemistry, Hong Kong University of Science and Technology, Hong Kong, China
3
Proteomics Core, Faculty of Health Sciences, University of Macau, Macau, China
*
Author to whom correspondence should be addressed.
Molecules 2019, 24(4), 747; https://doi.org/10.3390/molecules24040747
Received: 19 January 2019 / Revised: 13 February 2019 / Accepted: 14 February 2019 / Published: 19 February 2019
(This article belongs to the Section Analytical Chemistry)
Cystine is an important biomolecule in living systems. Although collision-induced dissociation (CID)-based tandem mass spectrometry (MS/MS) is commonly applied for identification and quantification of cystine in both biomedical and nutritional studies, gas-phase fragmentation reactions of cystine in CID has remained unclear. This may lead to improper assay design, which may in turn result in inaccurate test results. In the present study, gas-phase fragmentation reactions of protonated cystine in CID were characterized using high-resolution MS/MS and pseudo MS3. Fragmentations started from cleavages of disulfide bond (S–S) and carbon–sulfur bond (C–S). When cleaving at the S–S, protonated cysteine was generated as one of the predominant fragmentation products. Minor fragmentations started from the loss of H2O + CO and the loss of NH3. Our results reveal that the m/z 74 fragment ion, which is commonly used as a product ion of the transition (precursor/product ion pair) in selected reaction monitoring (SRM) assay for quantifying cystine, comprises two isobaric fragments originating from different parts of cystine. This indicates the need for careful selection of a stable isotope-labeled cystine molecule as an internal standard for SRM assays. Here, we provide a clear picture of the fragmentation reactions of protonated cystine in CID. It can serve as a useful guidance for designing MS/MS-based assays for cystine testing. View Full-Text
Keywords: cystine; cysteine; gas-phase fragmentation reaction; high-resolution MS/MS; pseudo MS3; isobaric fragment cystine; cysteine; gas-phase fragmentation reaction; high-resolution MS/MS; pseudo MS3; isobaric fragment
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Zhang, P.; Chan, W.; Ang, I.L.; Wei, R.; Lam, M.M.T.; Lei, K.M.K.; Poon, T.C.W. Gas-Phase Fragmentation Reactions of Protonated Cystine using High-Resolution Tandem Mass Spectrometry. Molecules 2019, 24, 747.

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