Observation of an unusually facile fragmentation pathway of gas-phase peptide ions: a study on the gas-phase fragmentation mechanism and energetics of tryptic peptides modified with 4-sulfophenyl isothiocyanate (SPITC) and 4-chlorosulfophenyl isocyanate (SPC) and their 18-crown-6 complexes.
Various peptide modifications have been explored recently to facilitate the acquisition of sequence information. N-terminal sulfonation is an interesting modification because it allows unambiguous de novo sequencing of peptides, especially in conjunction with MALDI-PSD-TOF analysis; such modified peptide ions undergo fragmentation at energies lower than those required conventionally for unmodified peptide ions. In this study, we systematically investigated the fragmentation mechanisms of N-terminal sulfonated peptide ions prepared using two different N-terminal sulfonation reagents: 4-sulfophenyl isothiocyanate (SPITC) and 4-chlorosulfophenyl isocyanate (SPC). Collision-induced dissociation (CID) of the SPC-modified peptide ions produced a set of y-series ions that were more evenly distributed relative to those observed for the SPITC-modified peptides; y(n-1) ion peaks were consistently and significantly larger than the signals of the other y-ions. We experimentally investigated the differences between the dissociation energies of the SPITC- and SPC-modified peptide ions by comparing the MS/MS spectra of the complexes formed between the crown ether 18-crown-6 (CE) and the modified peptides. Upon CID, the complexes formed between 18-crown-6 ether and the protonated amino groups of C-terminal lysine residues underwent either peptide backbone fragmentation or complex dissociation. Although the crown ether complexes of the unmodified ([M + CE + 2H]2+) and SPC-modified ([M* + CE + 2H]2+) peptides underwent predominantly noncovalent complex dissociation upon CID, the low-energy dissociations of the crown ether complexes of the SPITC-modified peptides ([M' + CE + 2H]2+) unexpectedly resulted in peptide backbone fragmentations, along with a degree of complex dissociation. We performed quantum mechanical calculations to address the energetics of fragmentations observed for the modified peptides.