Chiral Recognition of O-Phosphoserine by Mass Spectrometry This work was supported by the Ministero della Università e della Ricerca Scientifica e Tecnologica (MURST) and the Consiglio Nazionale delle Ricerche (CNR). The authors express their gratitude to F. Angelelli for technical assistance.

Identification and quantification of the enantiomeric excess of O-phosphoserine (OPSer) residues from post-translational modified proteins is now possible by the application of a very reproducible and sensitive mass spectrometric method. This technique is based on the different mass-spectral fragmentation patterns of the diastereomeric cluster ions generated in water/methanol solutions of the analyte (OPSerD/L) with a suitable chiral compound (OPThrL) by collision-induced dissociation (CID; see mass spectra).