ROLE OF THE SULFHYDRYL-GROUP ON THE GAS-PHASE FRAGMENTATION REACTIONSOF PROTONATED CYSTEINE AND CYSTEINE-CONTAINING PEPTIDES

The gas phase fragmentation reactions of protonated cysteine and cyste ine-containing peptides have been studied using a combination of colli sional activation in a tandem mass spectrometer and ab initio calculat ions [at the MP2(FC)/6-31G//HF/6-31G* level of theory]. There are two major competing dissociation pathways for protonated cysteine involvi ng: (i) loss of ammonia, and (ii) loss of the elements of [CH2O2]. MS/ MS, MS/MS of selected ions formed by collisional activation in the ele ctrospray ionization source as well as ab initio calculations have bee n carried out to determine the mechanisms of these reactions. The ab i nitio results reveal that the most stable [M + H - NH3](+) isomer is a n episulfonium ion (A), whereas the most stable [M + H - CH2O2](+) iso mer is an immonium ion (B). The effect of the position of the cysteine residue on the fragmentation reactions of the [M + H](+) ions of all the possible simple dipeptide and tripeptide methyl eaters containing one cysteine (where all other residues are glycine) has also been inve stigated. When cysteine is at the N-terminal position, NH, loss is obs erved, although the relative abundance of the resultant [M + H - NH3]( +) ion decreases with increasing peptide size. in contrast, when cyste ine is at any other position, water loss is observed. The proposed mec hanism for loss of H2O is in competition with those channels leading t o the formation of structurally relevant sequence ions. (C) 1998 Ameri can Society for Mass Spectrometry.