Present address: Department of Chemistry and Biochemistry, Northern Arizona University, PO Box 5698, S. Beaver Street, Bldg 20, Room 125, Flagstaff, AZ 86011–5698, USA.
A study of fragmentation of protonated amides of some acylated amino acids by tandem mass spectrometry: observation of an unusual nitrilium ion
Article first published online: 12 APR 2011
Copyright © 2011 John Wiley & Sons, Ltd.
Rapid Communications in Mass Spectrometry
Volume 25, Issue 9, pages 1119–1129, 15 May 2011
How to Cite
Talaty, E. R., Young, S. M., Dain, R. P. and Van Stipdonk, M. J. (2011), A study of fragmentation of protonated amides of some acylated amino acids by tandem mass spectrometry: observation of an unusual nitrilium ion. Rapid Commun. Mass Spectrom., 25: 1119–1129. doi: 10.1002/rcm.4965
- Issue published online: 12 APR 2011
- Article first published online: 12 APR 2011
- Manuscript Accepted: 24 JAN 2011
- Manuscript Revised: 21 JAN 2011
- Manuscript Received: 30 NOV 2010
A tandem mass spectrometric study of a series of secondary amides of acetylglycine and hippuric acid utilizing electrospray ionization (ESI) was conducted. Among the fragment ions observed was an unusual one, which we have determined to be a nitrilium ion having the structure or by loss of the full mass of glycine as a neutral fragment. A mechanism that we propose involves an initial protonation of the oxygen atom at the N-terminus, followed by cyclization to a five-membered imidazolium ring, and its subsequent collapse to the nitrilium ion. This mechanism is supported by extensive isotopic labels and considerable variation of substituents. A similar study of the amides of acyl β-alanine and acyl γ-aminobutyric acid revealed that the former furnishes the same nitrilium ion, but not the latter. Thus, a six-membered intermediate is also possible and capable of losing the full mass of β-alanine as a neutral fragment. When the size of the ring is forced to be seven-membered, this pathway is blocked. When this study was expanded to include a variety of N-acylproline amides, the nitrilium ion was observed in 100% abundance only when the acyl group was acetyl. Thus a proline effect (involvement of a strained bicyclic [3.3.0] structure) is being observed. Copyright © 2011 John Wiley & Sons, Ltd.