Tandem mass spectrometric analysis of alkali metal cation–tripeptide clusters

The impact of the electric field of metal ions on the vibrations and internal hydrogen bond strength in alkali metal ion di- and triglycine complexes

Using infrared predissociation spectroscopy of cryogenic ions, we revisit the vibrational spectra of alkali metal ion (Li+, Na+, K+) di- and triglycine complexes. We assign their most stable conformation, which involves metal ion coordination to all C=O groups and an internal NH⋯NH2 hydrogen bond in the peptide backbone. An analysis of the spectral shifts of the OH and C=O stretching vibrations across the different metal ions and peptide chain lengths shows that these are largely caused by the electric field of the metal ion, which varies in strength as a function of the square of the distance. The metal ion–peptide interaction also remotely modulates the strength of internal hydrogen bonding in the peptide backbone via the weakening of the amide C=O bond, resulting in a decrease in internal hydrogen bond strength from Li+ > Na+ > K+.

Experimental and computational studies of monovalent metal cation-peptide interactions in [M·GlyGlyHis]+ (M = Li, Na, K, Rb, Cs, and Ag) complexes

[M·GlyGlyHis]⁺ (M = Li, Na, K, Rb, Cs, and Ag) complexes were generated using the electrospray ionization method and were subjected to collision-induced dissociation. Metal ion loss is the primary dissociation channel for [Cs·GlyGlyHis]⁺ whereas other complexes yield metal-bound peptide sequence ions and dehydrated ions as the main products. [Li·GlyGlyHis]⁺ and [Ag·GlyGlyHis]⁺ also generate product ions that are not observed for other complexes. Density functional theory calculations suggest that metal ion-peptide ligand interaction occurs through covalent interactions in [Li·GlyGlyHis]⁺ and [Ag·GlyGlyHis]⁺, and through electrostatic attraction in [Na·GlyGlyHis]⁺, [K·GlyGlyHis]⁺, [Rb·GlyGlyHis]⁺, and [Cs·GlyGlyHis]⁺. The calculations also suggest that fragmentation behavior of these complexes is affected by charge transfer to the ligand and ion-ligand interaction energy, and to a lesser extent by the ion size.

Investigation of collision-induced dissociation products and structures of gas-phase [ M·GlyGlyHis-H]+ ( M = Fe, Ni, Cu, and Zn) complexes

Collision-induced dissociation is carried out for electrosprayed [Fe·GlyGlyHis-H]⁺, [Ni·GlyGlyHis-H]⁺, [Cu·GlyGlyHis-H]⁺, and [Zn·GlyGlyHis-H]⁺ complexes. [Fe·GlyGlyHis-H]⁺, [Ni·GlyGlyHis-H]⁺, and [Zn·GlyGlyHis-H]⁺ yield metal-bound peptide sequence ions and dehydrated ions as primary products, whereas [Cu·GlyGlyHis-H]⁺ generates a more extensive series of metal-bound sequence ions and a product arising from the unusual loss of a formaldehyde moiety; dehydration is significantly suppressed for this complex. Density functional theory calculations show that the copper ion-deprotonated peptide binding energy is substantially higher than those in other complexes, suggesting that there is a correlation between ion-ligand binding energy and their fragmentation behavior.