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

Abstract

A characteristic neutral loss of 44 Da is observed in the MS/MS spectra of Thr-containing sodiated peptides. A combination of tandem mass spectrometry and quantum chemical calculations calculated at the B3LYP/6-311G (d, p) level of ab initio theory is used to elucidate this fragmentation pathway. The high resolution mass spectrometry data indicate this neutral loss is acetaldehyde lost from the side chain of Thr rather than CO2. The intensity of this neutral loss can be enhanced when Thr residue is far from the C-terminus and when the C-terminus is esterified as well. The mechanism of the acetaldehyde loss is proposed to adopt a McLafferty-type rearrangement reaction, which involves a proton transfer from the hydroxyl of Thr side chain to its C-terminal neighboring carbonyl oxygen inducing the cleavage of the Ca-Cβ bond. This mechanism is further supported by examining the fragmentation of a [GT(tBu)G + Na](+) peptide derivative and by comparing the product ion spectra of [M + Na-44](+) of [GTGA + Na](+) with [M + Na](+) of [GGGA + Na](+). A similar neutral loss of HCHO can also be detected in Ser-containing peptides. Our computational results reveal that the most stable [GTG + Na](+) ion is present as a tridentate charge-solvated structure and the dissociation leading to the 44 loss is dynamically and energetically favorable.

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

“…The characteristic c n ions and their formation pathways presented in this work will surely enrich the mass spectrum interpretation protocols and be of benefit to the critically evaluation of key results from protein identification by de novo sequencing. Furthermore, the proposed mechanism is not only in accordance with the previous findings on such amide compounds but also compliments findings with countless other natural products having similar structural characteristics . When compared with the charge‐directed fragmentation reactions, less attention is paid to charge‐remote pathways.…”

“…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.…”

“…In our previous research regarding the formation of novel b n ‐44 ions, CH 3 CHO loss from Thr residue via McLafferty‐type rearrangement was proposed . The hydroxyl proton of Thr transfers to the C‐terminal adjacent amide oxygen inducing the cleavage of the C α –C β bond, as shown in path II (Scheme ).…”

Neutral losses of sodium benzoate and benzoic acid in the fragmentation of the [M + Na]+ ions of methoxyfenozide and tebufenozide via intramolecular rearrangement in electrospray ionization tandem mass spectrometry

“…In some cases, the Na + only acts as a charge tag but is not involved in the fragmentation: so‐called charge‐remote fragmentation . In other cases, the Na + plays an important role in the fragmentation, which can affect the CID process . The effect of Na + is usually compared with another alkali metal cation, Li + , and they can lead to different fragmentation behavior because of the stronger inductive effect of Li + than that of Na + .…”

Characteristic NH3 and CO losses from sodiated peptides C‐terminated by glutamine residues

“…It has been demonstrated that sodiated peptides often undergo fragmentation pathways that differ from those of protonated peptides; thus they would provide complementary peptide sequencing information. Our previous studies indicated that peptides accommodate Na + by the available carbonyl O and amino N atoms to disperse the charge, thus bending the peptide to generate special fragmentation. Thus, sodiated peptides fragment in a somewhat “charge‐remote” way (that charge is fixed in a local area and dissociation is not always initiated by the flow of a charge carrier) .…”