Spectroscopic Evidence for Lactam Formation in Terminal Ornithine b₂⁺ and b₃⁺ Fragment Ions

Abstract

Infrared multiple photon dissociation action spectroscopy was performed on the AlaOrn b 2 + and AlaAlaOrn b 3 + fragment ions from ornithine-containing tetrapeptides. Infrared spectra were obtained in the fingerprint region (1000-2000 cm −1 ) using the infrared free electron lasers at the Centre Laser Infrarouge d'Orsay (CLIO) facility in Orsay, France, and the free electron lasers for infrared experiments (FELIX) facility in Nijmegen, the Netherlands. A novel terminal ornithine lactam AO + b 2 + structure was synthesized for experimental comparison and spectroscopy confirms that the b 2 + fragment ion from AOAA forms a lactam structure. Comparison of experimental spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory shows that AO + b 2 + forms a terminal lactam protonated either on the lactam carbonyl oxygen or the N-terminal nitrogen atom. Several low-lying conformers of these isomers are likely populated following IRMPD dissociation. Similarly, a comparison of the experimental IRMPD spectrum with calculated spectra shows that AAO + b 3 + -ions also adopt a lactam structure, again with multiple different protonation sites, during fragmentation. This study provides spectroscopic confirmation for the lactam cyclization proposed for the Bornithine effect^and represents an alternative b n + structure to the oxazolone and diketopiperazine/macrocycle structures most often formed.

Fragmentation reactions of protonated α,ω‐diamino carboxylic acids: The importance of functional group interactions

“…The proposed formation of a lactam upon loss of H 2 O 21,28,34,40 ( a → c , Scheme 1) required reaction of the side‐chain amino group with the carbonyl carbon instead of an accompanying loss of CO, as is generally observed for protonated α‐amino acids 7–10,12–22 . The analogous cyclization reaction of Orn residues in peptides has been designated as “the ornithine effect.” 55,56 …”

Quantum chemical mass spectrometry: Ab initio study of b2‐ion formation mechanisms for the singly protonated Gln‐His‐Ser tripeptide

“…In a comment 41 on this publication, the fact that we had limited our study to amide oxygen protonation with subsequent formation of the lactam b 2 ‐ion structure was criticized. Although ab initio calculations in the context of MS are usually performed on (very) simple systems such as substituted amino acids 37,42,43 and di‐ or tripeptides based on glycine and alanine, 44–50 in some cases containing a specific residue to be studied (mainly arginine, 51,52 histidine, 36–39 proline, 38,53 cysteine, 54 and more recently ornithine 55 ), we performed additional calculations to obtain more insight into the amide bond protonation and the b 2 ‐ion structure by intentionally choosing one of the 10 tripeptides for which the MPM had previously been investigated. In a comprehensive analysis, all possible mechanisms for all possible b 2 ‐ion structures after both amide oxygen and amide nitrogen protonation have been investigated.…”