McLafferty‐type rearrangement in the collision‐induced dissociation of Li⁺, Na⁺ and Ag⁺ cationized esters of N‐acetylated peptides

Abstract

In this study we investigated the multi-stage collision-induced dissociation (CID) of N-terminally acetylated di-, tri- and tetrapeptides in the form of C-terminal ethyl, n-propyl, isopropyl, n-butyl and tert-butyl esters and cationized by the attachment of Li(+), Na(+) and Ag(+). While methyl ester versions of the metal cationized peptides primarily eliminate H(2)O following collisional activation and dissociation, the ethyl, propyl and butyl ester versions of the peptides exhibit a dissociation pathway consistent with gamma-hydrogen transfer to the C-terminal carbonyl group, with associated elimination of an alkene, in a McLafferty-type rearrangement. The rearrangement leaves a metal cationized, free-acid form of the peptide, as confirmed by comparing the multi-stage CID of rearrangement products generated from peptide esters with the CID of corresponding metal cationized free-acid peptides. The transfer of a gamma-hydrogen in the rearrangement reaction was confirmed by investigating the CID of ethyl esters for which the terminal methyl group was labeled with deuterium. We found that the rearrangement product was significantly more abundant, relative to other product ions, when derived from isopropyl and tert-butyl esters than from ethyl, n-propyl or n-butyl ester analogues.

Fragmentation pathway for glutamine identification: Loss of 73 da from dimethylformamidine glutamine isobutyl ester

“…This hydrogen migration is analogous to the McLafferty rearrangement except that it does not involve a radical cation; thus in this paper, it is referred to as a McLafferty-type rearrangement. A similar rearrangement was also documented for metal adducts of peptide esters [24]. ii) the side chain carbonyl carbon could be attacked by either of the oxygens from the newly-formed carboxylic terminus followed by the loss of NH 3 (17 Da) from the side chain (56ϩ17ϭ73).…”

Characterization of different poly(2‐ethyl‐2‐oxazoline)s via matrix‐assisted laser desorption/ionization time‐of‐flight tandem mass spectrometry

“…This kind of rearrangement has already been described in the ESI-MS/MS of carboxylate anions 29 and the esters of N- acetylated peptides. 30 In a complementary fashion, the double series C and C 0 could be explained by an a-cleavage of the backbone between the CH 2 groups followed by a cleavage of the side group resulting in the formation of a methylene imine end group (Scheme 2(e)). In addition to these related double series there are four additional product ion series, which can be assigned to structures that are formed by combination of the above mechanisms.…”

Investigation of c ions formed by N‐terminally charged peptides upon collision‐induced dissociation

“…demonstrated that McLafferty‐type rearrangement is possible for the [M + H] + ions of amide compounds . Moreover, there is an increasing amount of literature reporting this phenomenon in the dissociation process of even‐electron ions by using ESI‐CID methods . All of these findings stimulated our curiosity for a rationalized explanation between McLafferty‐type rearrangement and generation of these unusual c n ions.…”