LOCATION OF THE ALKALI-METAL ION IN GAS-PHASE PEPTIDE COMPLEXES

Collision-induced decompositions of gas-phase (M + Cat)+ complexes between 55 peptides and Li+, Na+, K+, Rb+, and Cs+ were studied either as first field-free region reactions by using B/E scans or as third field-free region reactions by using MS-MS. Results from these and MS-MS-MS experiments provide evidence that (b(n-m) + Cat + OH)+ product ions arise from complexes that contain the metal ion bonded on the N-terminal side of the reaction site; the intramolecular nucleophilic reaction first proposed by Westmore and co-workers explains their formation. Furthermore, formation of (b(n-m) + Cat - H)+ product ions can be understood in terms of a 1,2-elimination that occurs from (M + Cat)+ complexes that likewise contain the metal ion bonded toward the N-terminus. Changes in metastable ion vs CID spectra with changes in side-chain structure support decompositions of complexes in which the metal ion is multidentate bonded to carbonyl oxygen atoms in the peptide backbone and to chelating side chains. All our results indicate that structures of gas-phase (M + Cat)+ complexes between peptides and alkali metal ions do not reflect solution-phase binding to a peptide zwitterion but more closely reflect the types of intramolecular, multisite binding interactions that might occur in less hydrophilic interiors of proteins.