STRONG-INTERACTIONS OF ANIONIC PEPTIDES AND ALKALINE-EARTH METAL-IONS - BIS(PEPTIDE) COMPLEXES IN THE GAS-PHASE

Small peptides interact with alkaline earth metal ions under conditions of fast atom bombardment to form abundant metal-bis(peptide) complexes of the composition [2pept + Met2+ - 3H+]-. The structure of these gas-phase bis(peptide) complexes, as elucidated by their collisionally activated decompositions (CAD), involves metal ion binding to the deprotonated C-terminal carboxylate groups of both peptides and to a deprotonated amide nitrogen of one of the constituent peptides. Because the C-termini of both peptides in the metal-bis(peptide) complexes are strongly bound to the metal ion, the major product ions from the complex contain a metal ion bound to a doubly deprotonated peptide and to a C-terminal part of the second peptide. These features of the CAD may make these fragmentations useful in sequencing small peptides because there is little complication of fragment ions containing the N-terminus. Other decompositions involve side chain losses to give ions such as z - H, w, and v ions. This chemistry reflects the inherent strong metal bonding in the complex. The z - H ions are observed in the decomposition of metal-bound peptides that contain phenylalanine and alanine. They are not formed in the CAD of any other systems such as those of protonated peptides and metal-cationized peptides.