Electrospray tandem mass spectrometry of alkali-cationized BocN-carbo-α,β- and -β,α-peptides: differentiation of positional isomers†

Dissociation pathways of a series of alkali-cationized hybrid peptides, viz., Boc-α,β- and -β,α-carbopeptides, synthesized from C-linked carbon-β 3 -ammo acids [Caa (S)] and α-alanine (L-Ala), have been investigated by electrospray ionization tandem mass spectrometry. The positional isomers (six pairs) of the cationized α,β- and β,α-peptides can be differentiated by the collision-induced dissociation (CID) spectra of their [M + Cat-Boc + H] + ions which give characteristic series of alkali-cationized C- (x n + , y n + , z n + ) and N-terminal (a n + , b n + , c n + ) ions. Another noteworthy difference is cationized β,α-peptides eliminate a molecule of ammonia whereas this pathway is absent for α,β-peptides. This is useful for identifying the presence of a β-amino acid at the N-terminus. The CID spectra of [M + Cat-Boc + H] + ions of these peptide acids show abundant rearrangement [b n +17 + Cat] + (n=1 to n-1) ions which is diagnostic for distinguishing between α- and β-amino acid at the C-terminus. MS n experiments of [b n + Li-H] + ions from these hybrid peptides showed the loss of CO and 72 u giving rise to [an + Li-H] + and cationized nitrile product ions which render support to earlier proposals that b n + or [b n + Cat-H] + ions have protonated or cationized oxazolinone structures, respectively.