Enantiodiscrimination of chiral alpha-aminophosphonic acids by mass spectrometry

Diastereomeric clusters between first-group metal ions (M+) and chiral ?-aminophosphonic acids (A and B) have been readily generated in the gas phase by electrospray ionization (ESI) and their fragmentation investigated by mass spectrometry. The complexes studied had the general formula [MASB2]+ and [MARB2]+, where M = H, Li, Na, or K, AS and AR are the two enantiomers of a given acid A, and B is a reference ?-aminophosphonic acid of defined configuration. Collision-induced decomposition (CID) of [MASB2]+ and [MARB2]+ leads to fragmentation patterns characterized by [MAB]+/[MB2] abundance ratios which depend on the configuration of ligand A. These different spectral features were correlated to the different stability of the diastereomeric [MASB2]+ and [MARB2]+ complexes in the gas phase. The results have been discussed in the light of MM2 Molecular Mechanics Force Field calculations.