A study of fragmentation of protonated amides of some acylated amino acids by tandem mass spectrometry: observation of an unusual nitrilium ion

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  CH 3 ‐C ≡ N ⊕ ‐Ph or Ph‐C ≡ N ⊕ ‐Ph 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.