Characterization of Nα-Fmoc-protected dipeptide isomers by electrospray ionization tandem mass spectrometry (ESI-MSn): effect of protecting group on fragmentation of dipeptides
Article first published online: 12 JUN 2011
Copyright © 2011 John Wiley & Sons, Ltd.
Rapid Communications in Mass Spectrometry
Volume 25, Issue 14, pages 1949–1958, 30 July 2011
How to Cite
Ramesh, M., Raju, B., Srinivas, R., Sureshbabu, V. V., Vishwanatha, T. M. and Hemantha, H. P. (2011), Characterization of Nα-Fmoc-protected dipeptide isomers by electrospray ionization tandem mass spectrometry (ESI-MSn): effect of protecting group on fragmentation of dipeptides. Rapid Commun. Mass Spectrom., 25: 1949–1958. doi: 10.1002/rcm.5076
- Issue published online: 12 JUN 2011
- Article first published online: 12 JUN 2011
- Manuscript Accepted: 2 MAY 2011
- Manuscript Revised: 22 APR 2011
- Manuscript Received: 7 MAR 2011
A series of positional isomeric pairs of Fmoc-protected dipeptides, Fmoc-Gly-Xxx-OY/Fmoc-Xxx-Gly-OY (Xxx = Ala, Val, Leu, Phe) and Fmoc-Ala-Xxx-OY/Fmoc-Xxx-Ala-OY (Xxx = Leu, Phe) (Fmoc = [(9-fluorenylmethyl)oxy]carbonyl) and Y = CH3/H), have been characterized and differentiated by both positive and negative ion electrospray ionization ion-trap tandem mass spectrometry (ESI-IT-MSn). In contrast to the behavior of reported unprotected dipeptide isomers which mainly produce y1+ and/or a1+ ions, the protonated Fmoc-Xxx-Gly-OY, Fmoc-Ala-Xxx-OY and Fmoc-Xxx-Ala-OY yield significant b1+ ions. These ions are formed, presumably with stable protonated aziridinone structures. However, the peptides with Gly- at the N-terminus do not form b1+ ions. The [M + H]+ ions of all the peptides undergo a McLafferty-type rearrangement followed by loss of CO2 to form [M + H–Fmoc + H]+. The MS3 collision-induced dissociation (CID) of these ions helps distinguish the pairs of isomeric dipeptides studied in this work. Further, negative ion MS3 CID has also been found to be useful for differentiating these isomeric peptide acids. The MS3 of [M–H–Fmoc + H]− of isomeric peptide acids produce c1–, z1– and y1– ions. Thus the present study of Fmoc-protected peptides provides additional information on mass spectral characterization of the dipeptides and distinguishes the positional isomers. Copyright © 2011 John Wiley & Sons, Ltd.