DEHYDRATION OF PEPTIDE [M-PHASE(H]+ IONS IN THE GAS)

The loss of water from protonated peptides was studied using [O-18]-la beling of the C-terminal carboxyl group. The structures (including the location of the isotopic label) of first-generation product ions were examined by sequential product ion scanning (MS3 and MS4) using a hyb rid sector/quadrupole mass spectrometer. Water loss may involve carbox ylic acid groups, side-chain hydroxyls, or peptide backbone oxygens. A lthough one of these three pathways often predominates, more than one dehydration route can be operative for a single peptide structure. Whe n peptide backbone oxygen is lost, the dehydration can occur at one or two primary sites along the backbone, with the location of the site(s ) varying among peptides. When water loss involves the C-terminal carb oxyl group, the resulting ion may undergo extensive intraionic oxygen isotope exchange. This evidence for complex intraionic interactions fu rther emphasizes the significance of gas-phase conformation in determi ning the fragmentations of peptide ions.