Formation of cyclic acylphosphoramidates in mass spectra of N-monoalkyloxyphosphoryl amino acids using electrospray ionization tandem mass spectrometry.

The fragmentation reactions of N-monoalkyloxyphosphoryl amino acids (N-MAP-AAs) were studied by electrospray ionization tandem mass spectrometry (ESI-MS). The sodiated cyclic acylphosphoramidates (CAPAs) were formed through a characteristic pentacoordinate phosphate participated rearrangement reaction in the positive-ion ESI-MS/MS and HR-MS/MS of N-MAP-AAs, in which the fragmentation patterns were clearly different from those observed in the corresponding ESI-MS/MS of N-dialkyloxyphosphoryl amino acids/peptides and N-phosphono amino acids. The formation of CAPAs depended on the chemical structures of N-terminal phosphoryl groups, such as alkyloxy group, negative charge and alkali metal ion. A possible integrated rearrangement mechanism for both P-N to P-O phosphoryl group migration and formation of CAPAs was proposed. The fragmentation patterns of CAPAs as novel intermediates in gas phase were also investigated. In addition, it was found that the formation of alpha-amino acid CAPAs was more favorable than beta- or gamma-CAPAs in gas phase, which was consistent with previous solution-phase experiments.