A carbonyl oxygen migration in electrospray ionization mass spectrometry and its application in differentiating alpha- and beta-alanyl peptides

A novel rearrangement reaction with a carbonyl oxygen migration was observed in the electrospray ionization tandem mass spectra of N-diisopropyloxyphosphoryl dipeptides and their analogues. A possible mechanism was proposed and supported by the MS/MS study, derivatization of different functional groups and deuterium labeling experiments. It was found that metal ions could catalyze the rearrangement through a five-membered ring intermediate. A strong affinity between the phosphoryl group and oxygen atom in the gas phase was proposed to result in this kind of rearrangement reaction, which might provide some basic information on the nature of phosphorylation in biochemistry. The replacement of N-terminal alpha-alanine by beta-alanine stopped the migration, which provides a simple method for differentiating the alpha- and beta-alanine residues at the N-terminus of peptides.