Proton migration and its effect on the MS fragmentation of N-acetyl OMe proline: MS/MS experiments and ab initio and density functional calculations

The comparison of low energy gas-phase collision-induced dissociation (eV CID) and low energy surface-induced dissociation (eV SID) of protonated N-acetyl O-methoxy (N-acetyl OMe) proline shows that this ion has two main fragmentation pathways. One of them is the loss of methanol that occurs at low internal energy. The other one is a higher energy process, the loss of ketene from the protonated molecule. Theoretical calculations at three different levels provide further support for the energy requirements of these fragmentation reactions. These calculations also suggest that several protonated forms may coexist together after the ion activation and several fragmentation pathways may be operative, especially for the ketene loss.