Use of doubly protonated molecules in the analysis of cathedulins in crude extracts of khat (Catha edulis) by liquid chromatography/serial mass spectrometry.

Analysis of crude methanolic extracts of fresh khat (Catha edulis) by liquid chromatography/mass spectrometry (LC/MS) revealed the presence of 62 cathedulin alkaloids (compared with 15 published structures). Many cathedulins generated doubly protonated molecules following electrospray ionisation and the ratio of doubly to singly protonated species could be manipulated by adjusting the electrospray capillary position and source conditions. By selecting the doubly protonated species for serial mass spectrometric analysis (MS/MS), it was possible to use an ion trap mass spectrometer to observe singly charged product ions at lower m/z values than ion trap MS/MS analysis of M+H would have allowed. These spectra were particularly valuable in elucidating the acylation patterns of cathedulins where MS/MS analysis of M+H resulted in loss of a large neutral species to yield a small singly charged fragment below the lower limit for ion trapping. Acylation patterns for most of the 62 cathedulins are proposed from mass spectrometric analysis, and the data obtained for a major unreported cathedulin of mass 1001 Da suggest that it belongs to a new group of cathedulins having a cathate dilactone bridge but not an evoninate bridge.