Substituent effects on the gas‐phase fragmentation reactions of sulfonium ion containing peptides

Abstract

The multistage mass spectrometric (MS/MS and MS 3 ) gas-phase fragmentation reactions of methionine side-chain sulfonium ion containing peptides formed by reaction with a series of parasubstituted phenacyl bromide (XBr where X ¼ CH 2 COC 6 H 4 R, and R ¼ -COOH, -COOCH 3 , -H, -CH 3 and -CH 2 CH 3 ) alkylating reagents have been examined in a linear quadrupole ion trap mass spectrometer. MS/MS of the singly (M R ) and multiply) charged precursor ions results in exclusive dissociation at the fixed charge containing side chain, independently of the amino acid composition and precursor ion charge state (i.e., proton mobility). However, loss of the methylphenacyl sulfide side-chain fragment as a neutral versus charged (protonated) species was observed to be highly dependent on the proton mobility of the precursor ion, and the identity of the phenacyl group para-substituent. Molecular orbital calculations were performed at the B3LYP/ 6-31RGÃÃ level of theory to calculate the theoretical proton affinities of the neutral side-chain fragments. The log of the ratio of neutral versus protonated side-chain fragment losses from the derivatized side chain were found to exhibit a linear dependence on the proton affinity of the side-chain fragmentation product, as well as the proton affinities of the peptide product ions. R2H]3R precursor ions, respectively, from the peptide GAILM(X)GAILK revealed significant differences in the abundances of the resultant product ions. These results suggest that the protonated peptide product ions formed by gas-phase fragmentation of sulfonium ion containing precursors in an ion trap mass spectrometer do not necessarily undergo intramolecular proton 'scrambling' prior to their further dissociation, in contrast to that previously demonstrated for peptide ions introduced by external ionization sources.

‘Fixed charge’ chemical derivatization and data dependant multistage tandem mass spectrometry for mapping protein surface residue accessibility

“…The results obtained following CID-MS/MS and -MS 3 of the modified forms of peptide 18-42, (LLKVL-GVNVMLRKIAVAAASKPAVE) containing three lysine residues (resulting in the potential formation of three singly modified peptides (18-42 20 20,30,38 ), further demonstrates the utility of the sulfonium ion derivatization approach for the 'targeted' identification and characterization of modified peptide ions. As shown in Figure 1, two singly modified forms of the 18-42 peptide were observed.…”

“…As expected, CID-MS/MS of the triply charged [18-42 38 group (data not shown). By analyzing the MS 3 spectra obtained by dissociation of the [18-42 38 ϩ2H-S(CH 3 ) 2 ] 3ϩ and [18-42 30 ϩ2H-S(CH 3 ) 2 ] 3ϩ product ions, the modification sites were located on the K 38 residue for the 18-42 38 form of the peptide (Figure 4a), and on the K 30 residue for the 18-42 30 form of the peptide (Figure 4b). No evidence was found for a form of the 18-42 peptide that was singly modified at the K 20 residue, suggesting that this residue was less reactive to the DMBNHS reagent (i.e., less accessible) compared with the adjacent K 30 and K 38 residues (see the section below for further discussion).…”

“…CID-MS/MS of the quadruply charged precursor ions of the three doubly modified 18-42 peptide ions (the 18-42 20,38 peak was separately resolved while the 18-42 20,30 and 18-42 30,38 peaks were closely eluting) each resulted in the exclusive loss of up to two S(CH 3 ) 2 groups, similar to that described above for the 1-13 peptide, indicating the presence of two modifications within the peptides (data not shown). Once again, the loss of two S(CH 3 ) 2 groups was the dominant fragmentation pathway.…”

“…Once again, the loss of two S(CH 3 ) 2 groups was the dominant fragmentation pathway. The MS 3 spectra shown in Figure 5 were used to assign the sites of modification to the K 20 and K 38 residues for the 18-42 20,38 peptide (Figure 5a ) were also observed in the spectrum from the 18-42 30,38 peptide.…”

“…Finally, Supplemental Figure S2 shows the CID-MS/MS and -MS 3 spectra of the quadruply charged precursor ion of the triply modified 18-42 20,30,38 peptide. MS/MS resulted in the characteristic neutral loss of up to three S(CH 3 ) 2 groups, indicating the presence of three modifications within the peptide, while MS 3 confirmed the identity of the peptide sequence and localization of the modifications on each of the three lysine residues, K 20 , K 30 , and K 38 .…”