A novel salt bridge mechanism highlights the need for nonmobile proton conditions to promote disulfide bond cleavage in protonated peptides under low-energy collisional activation
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The gas-phase fragmentation mechanisms of small models for peptides containing intermolecular disulfide links have been studied using a combination of tandem mass spectrometry experiments, isotopic labeling, structural labeling, accurate mass measurements of product ions, and theoretical calculations (at the MP2/6-311 + G(2d,p)//B3LYP/3-21G(d) level of theory). Cystine and its C-terminal derivatives were observed to fragment via a range of pathways, including loss of neutral molecules, amide bond cleavage, and S—S and C—S bond cleavages. Various mechanisms were considered to rationalize S—S and C—S bond cleavage processes, including charge directed neighboring group processes and nonmobile proton salt bridge mechanism. Three low-energy fragmentation pathways were identified from theoretical calculations on cystine N-methyl amide: (1) S—S bond cleavage dominated by a neighboring group process involving the C-terminal amide N to form either a protonated cysteine derivative or protonated sulfenyl amide product ion (44.3 kcal mol−1); (2) C—S bond cleavage via a salt bridge mechanism, involving abstraction of the α-hydrogen by the N-terminal amino group to form a protonated thiocysteine derivative (35.0 kcal mol−1); and (3) C—S bond cleavage via a Grob-like fragmentation process in which the nucleophilic N-terminal amino group forms a protonated dithiazolidine (57.9 kcal mol−1). Interestingly, C—S bond cleavage by neighboring group processes have high activation barriers (63.1 kcal mol−1) and are thus not expected to be accessible during low-energy CID experiments. In comparison to the energetics of simple amide bond cleavage, these S—S and C—S bond cleavage reactions are higher in energy, which helps rationalize why bond cleavage processes involving the disulfide bond are rarely observed for low-energy CID of peptides with mobile proton(s) containing intermolecular disulfide bonds. On the other hand, the absence of a mobile proton appears to “switch on” disulfide bond cleavage reactions, which can be rationalized by the salt bridge mechanism. This potentially has important ramifications in explaining the prevalence of disulfide bond cleavage in singly protonated peptides under MALDI conditions.