Fragmentation of singly protonated peptides via interaction with metastable rare gas atoms.

The effects of metastable energy level and peptide sequence on the fragmentation patterns of singly charged peptide ions dissociated in collisions with metastable rare gas atoms were studied. Fragmentation spectra of singly charged peptide ions were shown to be more structure-informative and very different from those obtained in low-energy collision-induced dissociation. Unusual odd-electron radical a- and x-ions were observed. Several fragment ions corresponding to a side-chain loss were also observed, which allowed differentiation between leucine and isoleucine. The fragmentation mechanism depends on electronic energy transferred during interaction with metastable gas atoms and proceeds either via Penning ionization with formation of radical odd-electron doubly charged molecular cation or via high-energy excitation of internal degrees of freedom of the peptide cation.