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.
Shin
Joong-Won
JW
Imagene Co., Ltd., #406 Biotechnology Incubating Center, Seoul National University, Shinlim-dong, Gwanak-gu, Seoul 151-742, Republic of Korea.
Lee
Yong Ho
YH
Hwang
Sungu
S
Lee
Sang-Won
SW
D001557:Benzenesulfonates / Q000737:chemistry*; D003198:Computer Simulation; D043844:Crown Ethers / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D007477:Ions; D017953:Isocyanates / Q000737:chemistry*; D017879:Isothiocyanates / Q000737:chemistry*; D008956:Models, Chemical; D008958:Models, Molecular; D010449:Peptide Mapping / Q000379:methods*; D010455:Peptides / Q000737:chemistry*; D044367:Phase Transition; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D014364:Tryptophan / Q000737:chemistry
D001557:Benzenesulfonates; D043844:Crown Ethers; D005740:Gases; D007477:Ions; D017953:Isocyanates; D017879:Isothiocyanates; D010455:Peptides; C487390:chlorosulfonyl isocyanate; C015762:18-crown-6; C015308:4-isothiocyanatobenzenesulfonate; D014364:Tryptophan