Mass spectral study of hybrid peptides derived from ( R)-aminoxy ester and β-amino acids: The influence of aminoxy peptide bond (CO–NH–O) on peptide fragmentation under electrospray ionization conditions

Abstract

A new class of Boc-protected aminoxy hybrid peptides containing repeats of β-hAla-( R)-Ama-, and β-Caa-( R)-Ama- (β-hAla = β 3-( S)-hAlanine, ( R)-Ama = ( R)-aminoxy ester, and β-Caa = ( R)-C-linked carbo-β 3-amino acid) have been studied by electrospray ionization (ESI) ion-trap and quadrupole time-of-flight tandem mass spectrometry (Q-TOF MS/MS) of their protonated, cationized, and negative ions. MS 3 CID of protonated aminoxy peptides of β-hAla-( R)-Ama- yield intense β-amino acid characteristic retro-Mannich fragmentation. The b n + and [b n –methyl imine] + ( n = 3, 5) ions formed by cleavage of aminoxy peptide bond (CO–NH–O) are more intense than b n + ( n = 2, 4) formed by that of peptide bond (CO–NH–C) cleavage. Another characteristic ion observed is due to loss of H 3NO from y n + ions. The cationized (Li +, and Na +) peptides dissociate differently compared to protonated peptides. Intense cationized c n and z n ions are formed due to the cleavage of N–O bond. The deprotonated peptides also show abundant c n − and z n − ions ( n = 1, 3, 5) and do not form any y n − ions. All these results clearly indicate the influence of aminoxy peptide bond on fragmentation of these hybrid peptides.