title: Further studies on the fragmentation of protonated ions of peptides containing aspartic acid, glutamic acid, cysteine sulfinic acid, and cysteine sulfonic acid.
abstract: Here we examined the fragmentation, on a quadrupole ion-trap mass spectrometer, of the protonated ions of a group of peptides containing one arginine and two different acidic amino acids, one being aspartic acid (Asp) or glutamic acid (Glu) and the other being cysteine sulfinic acid [C(SO2H)] or cysteine sulfonic acid [C(SO3H)]. Our results showed that, upon collisional activation, the cleavage of the peptide bond C-terminal to C(SO2H) is much more facile than that of the peptide bond C-terminal to Asp, Glu, or C(SO3H). There is no significant difference, however, in susceptibility to cleavage of peptide bonds that are C-terminal to Asp, Glu, and C(SO3H). To understand these experimental observations, we carried out B3LYP/6-31G* density functional theory calculations for a model cleavage reaction of GXG --> b2 + Gly, in which X is Asp, Glu, C(SO2H), or C(SO3H). Our calculation results showed that the cleavage reaction is thermodynamically more favorable when X = C(SO2H) than when X = Asp or C(SO3H). We attributed the less facile cleavage of the amide bond after Glu to that the formation of a six-membered ring b ion for Glu-bearing peptides is kinetically not as favorable as the formation of a five-membered ring b ion for peptides containing the other three acidic amino acids. The results from this study may provide useful tools for peptide sequencing.
authors.affiliation: Department of Chemistry-027, University of California, Riverside, CA 92521-0403, USA.
authors.forename: Lijie
authors.initials: L
authors.lastname: Men
authors.forename: Yinsheng
authors.initials: Y
authors.lastname: Wang
mesh_terms: D000595:Amino Acid Sequence; D001224:Aspartic Acid / Q000032:analysis*; D003545:Cysteine / Q000031:analogs & derivatives* / Q000032:analysis*; D018698:Glutamic Acid / Q000032:analysis*; D009842:Oligopeptides / Q000737:chemistry; D010455:Peptides / Q000737:chemistry*; D021241:Spectrometry, Mass, Electrospray Ionization; D013451:Sulfonic Acids / Q000032:analysis*
chemical_list: D009842:Oligopeptides; D010455:Peptides; D013451:Sulfonic Acids; D001224:Aspartic Acid; D018698:Glutamic Acid; D003545:Cysteine; C013461:cysteine sulfinic acid