Characterization of Nα-Fmoc-protected ureidopeptides by electrospray ionization tandem mass spectrometry (ESI-MS/MS): differentiation of positional isomers
Article first published online: 4 NOV 2010
Copyright © 2010 John Wiley & Sons, Ltd.
Journal of Mass Spectrometry
Volume 45, Issue 12, pages 1461–1472, December 2010
How to Cite
Ramesh, M., Raju, B., Srinivas, R., Sureshbabu, V. V., Narendra, N. and Vasantha, B. (2010), Characterization of Nα-Fmoc-protected ureidopeptides by electrospray ionization tandem mass spectrometry (ESI-MS/MS): differentiation of positional isomers. J. Mass Spectrom., 45: 1461–1472. doi: 10.1002/jms.1862
- Issue published online: 19 DEC 2010
- Article first published online: 4 NOV 2010
- Manuscript Accepted: 18 OCT 2010
- Manuscript Received: 11 AUG 2010
- electrospray ionization;
- tandem mass spectrometry;
- structure elucidation;
- positional isomers
Four pairs of positional isomers of ureidopeptides, FmocNH-CH(R1)-φ(NH-CO-NH)-CH(R2)-OY and FmocNH-CH(R2)-φ(NH-CO-NH)-CH(R1)-OY (Fmoc = [(9-fluorenyl methyl)oxy]carbonyl; R1 = H, alkyl; R2 = alkyl, H and Y = CH3/H), have been characterized and differentiated by both positive and negative ion electrospray ionization (ESI) ion-trap tandem mass spectrometry (MS/MS). The major fragmentation noticed in MS/MS of all these compounds is due to NCH(R)Nbond cleavage to form the characteristic N- and C-terminus fragment ions. The protonated ureidopeptide acids derived from glycine at the N-terminus form protonated (9H-fluoren-9-yl)methyl carbamate ion at m/z 240 which is absent for the corresponding esters. Another interesting fragmentation noticed in ureidopeptides derived from glycine at the N-terminus is an unusual loss of 61 units from an intermediate fragment ion FmocNH = CH2+ (m/z 252). A mechanism involving an ion-neutral complex and a direct loss of NH3 and CO2 is proposed for this process. Whereas ureidopeptides derived from alanine, leucine and phenylalanine at the N-terminus eliminate CO2 followed by corresponding imine to form (9H-fluoren-9-yl)methyl cation (C14H11+) from FmocNH = CHR+. In addition, characteristic immonium ions are also observed. The deprotonated ureidopeptide acids dissociate differently from the protonated ureidopeptides. The [M − H]− ions of ureidopeptide acids undergo a McLafferty-type rearrangement followed by the loss of CO2 to form an abundant [M − H − Fmoc + H]− which is absent for protonated ureidopeptides. Thus, the present study provides information on mass spectral characterization of ureidopeptides and distinguishes the positional isomers. Copyright © 2010 John Wiley & Sons, Ltd.