Characterization of Electrospray Ionization Mass Spectrometry for N-Diisopropyloxyphosphoryl Dipeptide Methyl Esters

A systematic study of the fragmentation pattern of N-diisopropyloxyphosphoryl (DIPP) dipeptide methyl esters in an electrospray ionization (ESI) tandem mass spectrometry (MS/MS) was presented. A combination of accurate mass measurement and tandem mass spectrometry had been used to characterize the major fragment ions observed in the ESI mass spectrum. It was found that the alkali metal ions acted as a fixed charge site and expelled the DIPP group after transferring a proton to the amide nitrogen. For all the N-phosphoryl dipeptide methyl esters, under the activation of a metal ion, the rearrangement product ion at m/z 163 was observed and confirmed to be the sodium adduct of phosphoric acid mono-isopropyl esters (PAIE), via a specific five-membered penta-coordinated phosphorus intermediate. However, no rearrangement ion was observed when a β-amino acid was at the N-terminal. This could be used to develop a novel method for differentiating isomeric compounds when either α- or β-amino acid are at the N-terminus of peptides. From the [M + Na]+ ESI-MS/MS spectra of N-phosphoryl dipeptide methyl esters (DIPP – Xaa1 – Xaa2 – OMe), the peaks corresponding to the [M + Na – Xaa1 – C3H6]+ were observed and explained. The [M + Na]+ ESI-MS/MS spectra of N-phosphoryl dipeptide methyl esters with Phe located in the C-terminal, such as DIPP–Val–Phe–OMe, DIPP–Leu–Phe–OMe, DIPP–Ile–Phe–OMe, DIPP–Ala–Phe–OMe and DIPP–Phe–Phe–OMe, had characteristic fragmentation. Two unusual gas-phase intramolecular rearrangement mechanisms were first proposed for this fragmentation. These rearrangements were not observed in dipeptide methyl ester analogs which did not contain the DIPP at the N-terminal, suggesting that this moiety was critical for the rearrangement.