title: Differentiating N-terminal aspartic and isoaspartic acid residues in peptides.
abstract: Formation of isoaspartic acid (isoAsp) is a common modification of aspartic acid (Asp) or asparagine (Asn) residue in proteins. Differentiation of isoAsp and Asp residues is a challenging task owing to their similar properties and identical molecular mass. It was recently shown that they can be differentiated using ion-electron or ion-ion interaction fragmentation methods (ExD) because these methods provide diagnostic fragments c + 57 and z(•) - 57 specific to the isoAsp residue. To date, however, the presence of such fragments has not been explored on peptides with an N-terminal isoAsp residue. To address this question, several N-terminal isoAsp-containing peptides were analyzed using ExD methods alone or combined with chromatography. A diagnostic fragment [M + 2H - 74](+•) was observed for the doubly charged precursor ions with N-terminal isoAsp residues. For some peptides, identification of the N-terminal isoAsp residue was challenging because of the low diagnostic ion peak intensity and the presence of interfering peaks. Supplemental activation was used to improve diagnostic ion detection. Further, N-terminal acetylation was offered as a means to overcome the interference problem by shifting the diagnostic fragment peak to [M + 2H - 116](+•).
authors: Mass Spectrometry Resource, Department of Biochemistry, Boston University School of Medicine, 670 Albany Street, R504, Boston, Massachusetts 02118, USA.
authors: Nadezda P
authors: NP
authors: Sargaeva
authors: Cheng
authors: C
authors: Lin
authors: Peter B
authors: PB
authors: O'Connor
mesh_terms: D000595:Amino Acid Sequence; D000804:Angiotensin II / Q000737:chemistry; D001224:Aspartic Acid / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D056148:Chromatography, Reverse-Phase; D026581:Isoaspartic Acid / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods*; D010455:Peptides / Q000032:analysis*
chemical_list: D026581:Isoaspartic Acid; D010455:Peptides; D000804:Angiotensin II; D001224:Aspartic Acid