Electrospray ionization tandem mass spectrometry differentiation of N-phosphoryl-α-, β- and γ-amino acids

The fragmentation patterns of N-diisopropyloxyphosphoryl-l-α-Ala (DIPP-l-α-Ala), N-diisopropyloxyphosphoryl-d-α-Ala (DIPP-d-α-Ala), N-diisopropyloxyphosphoryl-β-Ala (DIPP-β-Ala) and N-diisopropyloxyphosphoryl-γ-amino butyric acid (DIPP-γ-Aba) were investigated by electrospray ionization tandem mass spectrometry (ESI-MS/MS). DIPP-d-α-Ala showed the same fragmentation pathways as DIPP-l-α-Ala. In the fragmentation of protonated DIPP-β-Ala, the characteristic fragment ion [M + H − 2C3H6 − H2O − CH2CO]+ appeared and could be used to distinguish β-Ala from l-α-Ala and d-α-Ala through tandem mass spectra, even though they possess the same molecular weight. In the fragmentation of protonated DIPP-γ-Aba, the break of PN bond occurred and an interesting protonated lactam ion with five-membered ring was generated. Furthermore, in the MS3 spectrum of [M + Na − 2C3H6]+ ion of DIPP-γ-Aba, a strong intensity of unique fragment ion, namely lactam-sodium adduct with five-membered ring, was observed, which could be considered as a mark for γ-amino acids. The stepwise fragmentations of their [M + Na]+ ions and [M − H]− ions showed that they all underwent a PN to PO bond migration through a five-membered or six-membered or even seven-membered ring transition state, respectively, which supported the great affinity of hydroxyl for phosphoryl group.