FRAGMENTATIONS OF GAS-PHASE COMPLEXES BETWEEN ALKALI-METAL IONS AND PEPTIDES - METAL-ION BINDING TO CARBONYL OXYGENS AND OTHER NEUTRAL FUNCTIONAL-GROUPS

Collision-induced fragmentations of gas-phase (M + Cat)+ complexes between 55 structurally divers peptides and 5 alkali metal ions, Li+, Na+, K+, Rb+, and Cs+, are reported. Mechanisms for fragmentation indicate that neither N-terminal (a(n-m) + Cat - H)+ nor C-terminal (y(n-m) + Cat + H)+ fragment ions arise from complexes that contain the alkali metal ion bonded to a deprotonated (zwitterionic) carboxylate terminus. In cases in which there is no strongly interacting side chain, the metal ion is most likely bonded to an amide oxygen. Another reaction, which necessitates the metal ion being bonded toward the N terminus as opposed to being bonded to a carboxylate anion, differentiates C-terminal amino acids such as leucine and isoleucine. Decompositions of (M + Ca)+ complexes of C-terminal amides generally provide more extensive sequence information than C-terminal carboxylates. Fragmentation patterns, which change with increasing size of the alkali metal ion, are related to more favorable coordinative multisite binding between the large metal ions and several amide oxygens, in analogy to know chemistry. The fragmentations thus do not reflect aqueous-phase bonding to a zwitterionic species but instead reflect the types of interactions that could occur between a metal ion and binding sites in less hydrophilic interiors of proteins.