Proton Mobility in b₂ Ion Formation and Fragmentation Reactions of Histidine-Containing Peptides

Abstract

Abstract. A detailed energy-resolved study of the fragmentation reactions of protonated histidine-containing peptides and their b 2 ions has been undertaken. Density functional theory calculations were utilized to predict how the fragmentation reactions occur so that we might discern why the mass spectra demonstrated particular energy dependencies. We compare our results to the current literature and to synthetic b 2 ion standards. We show that the position of the His residue does affect the identity of the subsequent b 2 ion (diketopiperazine versus oxazolone versus lactam) and that energy-resolved CID can distinguish these isomeric products based on their fragmentation energetics. The histidine side chain facilitates every major transformation except trans-cis isomerization of the first amide bond, a necessary prerequisite to diketopiperazine b 2 ion formation. Despite this lack of catalyzation, trans-cis isomerization is predicted to be facile. Concomitantly, the subsequent amide bond cleavage reaction is rate-limiting.

Difference of Electron Capture and Transfer Dissociation Mass Spectrometry on Ni2+-, Cu2+-, and Zn2+-Polyhistidine Complexes in the Absence of Remote Protons

“…Since the peptide bond is weakened by the protonation of the backbone amide nitrogen, formation of b and y' ions is often mediated by a mobile proton [35]. Recently, the protonated histidine residue has been reported to mediate peptide bond cleavage by proton mobilization [36]. Consequently, the mechanism of peptide bond cleavage in the charge-reduced [His 3 + Cu] + complex was considered in terms of proton mobilization, even though the [His 3 + Cu] + complex does not contain any remote protons.…”

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

“…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. (Note that, until now, the most detailed energetic analysis of the same tripeptide is that of Nelson et al who limited themselves to the study of nitrogen protonation. 39 ) Our results for QHS show that amide nitrogen protonation is energetically favored over amide oxygen protonation and that the b 2 ‐ion structure is probably a mixture of the oxazolone and lactam structures.…”

“…Many studies, presented mainly by Wysocki, 35,36 O'Hair, 37,38 and Nelson and their coworkers, 39 dealt with the structural investigation of b 2 ‐ions containing a histidine residue, but they studied only simple systems to probe the formation of the “fused bicyclic five‐membered ring structure” resulting from the attack of the imidazole side chain onto the peptide backbone. Indeed, dipeptides and tripeptides were chosen in which one histidine was surrounded by only aliphatic (glycine, alanine, leucine) or aromatic residues (phenylalanine, tyrosine) or by the nonpolar proline to prevent any influence of a side‐chain reactive moiety on the dissociation pathways.…”

“…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.…”

An Unprecedented Retro‐Mumm Rearrangement Revealed by ESI‐MS/MS, IRMPD Spectroscopy, and DFT Calculations

“…[6] All of thesef indings suggestt he occurrence of ar etro-Mumm rearrangement under collision-inducedd issociation (CID) con-ditions.N otably,t he product ion at m/z 410, formed through the loss of tert-butylamine, is analogous [13] to the b-type product ion of peptides. The b-type ions usually possess oxazolone [14] structures and are known to undergo further CO loss to generate a-type fragments (m/z 382 in the presentcase). [15] The reverseM umm rearrangementh as never been reported before.…”