Differentiation of three pairs of Boc-β,γ- and γ,β-hybrid peptides by electrospray ionization tandem mass spectrometry

✍ Scribed by V. Ramesh; R. Srinivas; G. V. M. Sharma; P. Jayaprakash; A. C. Kunwar

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 445 KB
• Volume: 43
• Category: Article
• ISSN: 1076-5174
• DOI: 10.1002/jms.1393

✦ Synopsis

Abstract

A new series of Boc‐N‐β^3^, γ^4^‐/γ^4^, β^3^‐isomeric hybrid peptides (containing repeats of β^3^‐Caa and γ^4^‐Caa's, Caa = C‐linked carbo β^3^‐/γ^4^‐amino acids derived from D‐xylose) have been differentiated by both positive and negative ion electrospray ionization (ESI) ion‐trap and high resolution quadrupole time‐of‐flight/tandem mass spectrometry (Q‐TOF MS/MS). MS^n^ of protonated isomeric peptides and [M + H − Boc + H]^+^ produce characteristic fragmentation involving the peptide backbone, the Boc‐group, and the side chain. The positional isomers are differentiated from one another by the presence of y~n~^+^, b~n~^+^, and other fragment ions of different m/z values. It is observed that the peptides with β‐Caa at the N‐terminus produce extensive fragmentation, whereas γ‐Caa gave rise to much less fragmentation. Peptides with γ‐Caa at the N‐terminus lose NH~3~, whereas this process is absent for the carbopeptides with β‐Caa at the N‐terminus. Two pairs of dipeptide diastereomers are clearly differentiated by the collision‐induced dissociation (CID) of their protonated molecules. The loss of 2‐methylprop‐1‐ene is more pronounced for Boc‐NH‐(R)‐β‐Caa‐(R)‐γ‐Caa‐OCH~3~ (6) and Boc‐NH‐(R)‐γ‐Caa‐(R)‐β‐Caa‐OCH~3~ (12), whereas it is insignificant or totally absent for its protonated diastereomeric pair Boc‐NH‐(S)‐β‐Caa‐(S)‐γ‐Caa‐OCH~3~ (1) and Boc‐NH‐(S)‐γ‐Caa‐(S)‐β‐Caa‐OCH~3~ (7). Further, ESI negative ion tandem mass spectrometry has also been found to be useful for differentiating these isomeric peptide acids. Copyright © 2008 John Wiley & Sons, Ltd.