title: Investigation of several unique tandem mass spectrometric fragmentation patterns of NFDEIDR, an orcokinin analog, and its N-terminal dimethylated form.
abstract: Orcokinins are a family of myotropic neuropeptides widely present in various decapod crustaceans and insect species. The majority of the orcokinins identified to date share a conserved sequence of NFDEIDR at their N-termini. Electrospray ionization quadrupole time-of-flight tandem mass spectrometric (ESI-QTOF-MS/MS) analysis of doubly charged orcokinin precursor ions reveals the presence of a y(n - 1) + 10 peak, which is more intense than that for the y(n - 1) ion. To elucidate the identity of this novel fragment ion and understand the mechanism underlying this fragmentation, we employed a combined approach involving the use of isotopic N-terminal dimethylation, methyl esterification, and isotope-encoded NFDEIDR. Comparison of the fragmentation patterns of these chemically modified orcokinin analogs allowed the determination of the structure of the y(n - 1) + 10 ion as y(n - 1) + CO--H2O. The yx + CO--H2O ions, along with the yx + CO and yx + CO--NH3 ions, are also present in the MS/MS spectra of NFDEIDR and several other peptides. Additionally, we report two other unusual fragmentation ions in the MS/MS spectra of N-terminal dimethyl NFDEIDR (2+), which yields the novel fragment ions of the y(n - 1) + 38 ion and the [M+2H-59]2+ ion. These two ion series involve the neutral loss of the asparagine side chain. The same sets of ions are also present in other peptides with dimethyl-modified asparagines at the N-terminus. The competition between the side-chain loss and loss of dimethylamine is described. The loss of the side chain of N-terminal dimethyl Asp1 is reported as well. We also report for the first time the neutral loss of ammonia from the N-terminal amino group of Asn1 and the loss of CO2 from the side chain of aspartic acid.
authors.affiliation: Department of Chemistry, University of Wisconsin at Madison, Madison, WI 53705-2222, USA.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Fu
authors.forename: Lingjun
authors.initials: L
authors.lastname: Li
mesh_terms: D005557:Formaldehyde / Q000737:chemistry; D013058:Mass Spectrometry; D008745:Methylation; D015394:Molecular Structure; D009479:Neuropeptides / Q000737:chemistry*; D010455:Peptides; D014357:Trypsin / Q000378:metabolism
chemical_list: D009479:Neuropeptides; D010455:Peptides; C079067:orcokinin; D005557:Formaldehyde; D014357:Trypsin