Metal ion binding to peptides: Oxygen or nitrogen sites?

Infrared multiple-photon dissociation (IRMPD) spectroscopy was used to probe the conformations of gas-phase metal-ion complexes between a series of five metal ions and six small peptide ligands. This report is presented in recognition and tribute for the Armentrout group's long and hugely productive interest in metal-ion binding to gas-phase ligands. The metal ions (K+, Ba2+, Ca2+, Mg2+, Ni2+) span a range of ligand binding strengths, and the ligands include several dipeptides and tripeptides, and one tetrapeptide. The weaker metal ions generally form charge-solvated (CS) complexes binding amide carbonyl oxygen, while the strongest metal ion, nickel, deprotonates the amide nitrogens, probably through iminol tautomerization, and binds to the amide nitrogens. The Amide II vibrational mode (1500–1550 cm−1) is found to be an excellent marker for the presence or absence of protons on amide nitrogens in a complex. The magnesium ion marks a boundary between these two structural motifs, forming iminol complexes with the dipeptides and switching to CS complexes for the tripeptides FGG and FGGF. Compared with solution-phase behavior, the iminol binding mode shown by Mg2+ for the smallest peptides is surprising, since this ion is considered as generally binding in a CS mode in solution. The present results for the larger peptides reconcile this surprising difference, showing that larger peptide ligands revert to the expected CS binding pattern for gas-phase Mg2+.