COLLISION-INDUCED IONIZATION OF NEUTRAL PEPTIDES

A four-sector mass spectrometer was employed to study fragments formed by the high-energy collision of neutral peptides with molecular oxyge n. Neutral peptides were generated in the gas phase from proton-bound dimers by the method of Wesdemiotis and co-workers [Anal Chem. 65, 159 4 (1993)]. B/E scanning was used in MS-2 to collect the fragment ions from neutral precursors. Molecular ions (radical ions) were not obtain ed in most of the spectra, but a variety of fragment ions were recorde d. High resolution and mass accuracy allowed most fragment ions to be assigned. Most fragment ions are of the same types as those produced f rom protonated peptides by collisional activation. Formation of gamma- type ions is significantly reduced in the processes induced by collisi onal ionization, compared with high- and low-energy collisional activa tion of protonated peptide precursors.