Gas-Phase Interactions of Transition-Metal Ions and Di- and Tripeptides: A Comparison with Alkaline-Earth-Metal-Ion Interactions

Di- and tripeptides containing amino acids with nonacidic or nonbasic side chains bond with transition metals to give [tripept + Met²⁺ − 3H⁺]⁻ that exist in various structural forms in the gas phase. This is in sharp contrast to the nearly homogeneous structure population of the corresponding alkaline-earth-metal-bound tripeptides in which the C-terminal COOH group and the two amide NH groups are deprotonated and bond to the metal ion. The structure-determining factor is the difference in ligand affinity of the metal ions. Alkaline-earth-metal ions are oxyphilic, whereas transition-metal ions favor nitrogen ligands in solution. That property is carried over to the gas phase and may be regarded as an intrinsic property. Because transition metals favor nitrogen ligands, dipeptides form abundant metal-bound peptides of these metals. Collisionally activated decompositions (CAD) measured on a tandem mass spectrometer show that transition-metal-bound tripeptides fragment both at the C-terminus by losing CO₂ and H₂CO₂ and at the N-terminus, where alkaline-earth-metal-bound tripeptides decompose. Transition-metal complexes containing serine, threonine, phenylalanine, and methionine lose elements of the side chain. For peptides containing several amino acids with functionalized side chains, fragments from each are observed, in contrast with the predominant loss of a single side chain from corresponding alkaline-earth complexes. Another contrast is bis(peptide) complexes which are formed less readily with transition metals than with alkaline-earth-metal ions.