Chiral Recognition of O-Phosphoserine by Mass Spectrometry

The authors described the application of the kinetic method of Cooks et. al. (W. Y. Shen, P. S. H. Wong, R. G. Cooks; Rapid Commun. Mass Spectrom., 1997, 11, 71) to the enantio-discrimination of O-phosphoserine (OPSer), to demonstrate the sensitivity and the reproducibility of the method and its potential in quantifying the enantiomeric excess of OPSer in biol. specimens. Singly charged trimeric cluster [M(ref)2A]+ (M = H, Na) were generated by electro-spraying aq. methanol solns. contg. a pure enantiomer of OPSer and a chiral ref. compd. [(R)-(-)- or (S)-(+)-(1-aminoethyl)phosphonic acid (ER or ES, resp.)], or O-phospho-L-threonine (OPThr). The use of ref. compds. with size and functionality similar to those of the OPSer analyte was dictated by the desirability to minimize entropy effects on [M(ref)2A]+ fragmentation. The CID of the selected trimeric cluster ions was carried out in a triple-quadrupole mass spectrometer. For comparative purposes, the CID spectra of both the homochiral [Na(OPThrL)2OPSerL]+ and heterochiral [Na(OPThrL)2OPSerD]+ ions were further investigated with a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The studies demonstrated that the kinetic method is sufficiently precise and accurate to be potentially applicable for quantifying the enantiomeric excess of OPSer in proteomic anal., that the method was fast and did not rely on any sample manipulation, and that chiral selectivity factors did not seem to depend on the specific instrumentation used.