Differentiation of positional isomers of hybrid peptides containing repeats of β -nucleoside derived amino acid (β-Nda-) and L-amino acids by positive and negative ion electrospray ionization tandem mass spectrometry (ESI-MSn)

Abstract

A new class of positional isomeric pairs of -Boc protected oligopeptides comprised of alternating nucleoside derived β -amino acid (β -Nda-) and L-amino acid residues (alanine, valine, and phenylalanine) have been differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-MS ⁿ ). The protonated dipeptide positional isomers with β-Nda- at the N-terminus lose CH₃OH, NH₃, and C₂H₄O₂, whereas these processes are absent for the peptides with L-amino acids at the N-terminus. Instead, the presence of L-amino acids at the N-terminus results in characteristic retro-Mannich reaction involving elimination of imine. A good correlation has been observed between the conformational structure of the peptides and the abundance of yn⁺ and b_n⁺ ions in MS ⁿ spectra. In the case of tetrapeptide isomers that are reported to form helical structures in solution phase, no y_n⁺ and b_n⁺ ions are observed when the corresponding amide -NH- participates in the helical structures. In contrast, significant y_n⁺ and b_n⁺ ions are formed when the amide -NH- is not involved in the H-bonding. In the case of tetra- and hexapeptides, it is observed that abundant bn+ ions are formed, presumably with stable oxazolone structures when the C-terminus of the bn⁺ ions possessed L-amino acid and the β-Nda- at the C-terminus appears to prevent the cyclization process leading to the absence of corresponding bn⁺ ions.