Does side chain water loss from protonated threonine yield N-protonated dehydroamino-2-butyric acid?

Abstract

Electrospray ionization tandem mass spectrometry (ESI-MS/MS) of protonated threonine, isotopically labelled threonine and threonine methyl ester, suggests that side chain water loss occurs via a neighbouring group participation reaction in which the free amino group displaces water from HO-protonated threonine. Ab initio calculations show that the resultant [M H À H 2 O]product ion is a protonated aziridine, which is in contrast to previously published results, which suggested N-protonated dehydroamino-2-butyric acid as the likely structure. # 1998 John Wiley & Sons, Ltd. Received 11 May 1998; Revised 10 June 1998; Accepted 12 June 1998 Tandem mass spectrometry is rapidly becoming an indispensible tool for the structural characterization of proteins, peptides, carbohydrates and oligonucleotides. While the common fragmentation pathways of such compounds have been examined, a more detailed understanding of the complex gas phase reactivities of these compounds (for example, the influence of potentially reactive side chains on 'sequence' ion versus 'non-sequence' ion formation) is required in order to facilitate the elucidation of unknown sequences. As part of our ongoing work into probing the gas phase reactions of biomolecules, we have recently examined both the bimolecular reactivity 1 as well as the collision induced dissociation (CID) reactions of several model peptide systems. 2 In the former studies we demonstrated that the various functional groups present in simple amino acids such as glycine and cysteine can act as intermolecular nucleophiles in gas phase bimolecular alkylation reactions.

Pathways for Water Loss from Doubly Protonated Peptides Containing Serine or Threonine

“…One pathway (Scheme 1) leads to formation of an aziridine ring on water loss. This pathway has been shown to be operative in loss of H 2 O from protonated serine and threonine [12][13][14]. A second pathway involves (Scheme 3) formation of an oxazoline upon elimination of H 2 O.…”

“…There have been several studies concerning the mechanism of water loss from protonated serine and threonine and their derivatives. For protonated serine and threonine, it has been shown that attack of the amino group in the hydroxyl-protonated form leads to water loss and formation of a protonated three-membered aziridine ring [12][13][14]. By contrast, in N-acetyl derivatives interaction of the carboxyl group with the side-chain protonated hydroxyl group leads to formation of a protonated five-membered oxazoline ring upon loss of water [13].…”

Characteristic neutral loss of CH3CHO from Thr-containing sodium-associated peptides

“…[25,26] Thr-containing peptides usually show many special characteristics upon CID and mechanisms into their fragmentation behavior have been reported. Loss of water upon CID is common for Thrcontaining peptides and has been proposed to form an aziridine ring [27][28][29] involving C α , C β , and the nitrogen of the amino group or of the peptide bond. It was also found that in the product ion spectra of doubly charged peptides, cleavages N-terminal to Ser and Thr show slightly enhanced tendency in comparison with cleavages C-terminal to these residues.…”

Protonated Dipeptide Losses from b5 and b4 Ions of Side Chain Hydroxyl Group Containing Pentapeptides

“…The water losses from the side chain of hydroxyl group containing amino acid residues have been extensively investigated by several groups [29][30][31][32][33][34][35][36]. In line with these works, a possible mechanism for the formation of m/z 279 fragment ion from protonated b 5 ion of SGGFL-NH 2 is shown in Scheme 1.…”