A carbonyl oxygen migration of N-phosphoryl dipeptide methyl esters containing β-alanine or γ-amino butyric acid in electrospray ionization mass spectrometry

The electrospray ionization tandem mass spectra (ESI-MSn) of the sodium adducts of N-diisopropyloxyphosphoryl (DIPP) dipeptide methyl esters containing unnatural amino acids, such as DIPP-β-Ala-Phe-OMe and DIPP-γ-amino butyric acid-Phe-OMe, were studied. An unusual rearrangement reaction with a carbonyl oxygen migration through a six- or seven-membered ring tetracoordinate phosphorus transition state was observed. In addition, ESI-MSn of DIPP-l/d-α-Ala-Phe-OMe was also investigated. The transition state of their rearrangement was a five-membered ring pentacoordinate phosphorus transition state. It was the spatial configuration of phosphorus atom and the strong affinity between phosphorus and oxygen atoms that induced the carbonyl oxygen migration through different cyclic transition state. The results imply that the carbonyl oxygen migration in the ESI-MS/MS of dipeptide methyl esters was not dependent on the chain length of the amino acid at the N-terminus.