Figure 3.

Neutral loss of 98 Da from pHis peptides occurs predominantly through the C-terminal or a side chain carboxylate. (a) Proposed model showing loss of phosphoric acid from pHis peptides via the C-terminal carboxylate during CID. (b) CID MS/MS of the [M + 2H]²⁺ pHis peptide TSpHTSIMAR-C¹⁸O¹⁸OH ion showing the prominent neutral loss of 100 Da. Inset into the spectrum is a high resolution MS spectrum of the precursor species at m/z 544.23763 (left panel) and a high resolution MS/MS spectrum showing species corresponding to the primary loss of 100 Da and less prominent losses of 80, 118, and 116 Da (right panel). (c) Proposed model showing loss of phosphoric acid from pHis peptides via a side chain carboxylate during CID. (d) CID MS/MS of the [M + 2H]²⁺ pHis peptide SpHEFMNK-C¹⁸O¹⁸OH ion showing the prominent neutral loss of 98 Da. Inset into the spectrum is a high resolution MS spectrum of the precursor species at m/z 488.69006 (left panel) and a high resolution MS/MS spectrum showing species corresponding to the primary loss of 98 Da and less prominent losses of 80, 118, and 116 Da (right panel).