Metal Ion Complexes with HisGly: Comparison with PhePhe and PheGly

Gas-phase complexes of five metal ions with the dipeptide HisGly have been characterized by DFT computations and by infrared multiple photon dissociation spectroscopy (IRMPD) using the free electron laser FELIX. Fine agreement is found in all five cases between the predicted IR spectral features of the lowest energy structures and the observed IRMPD spectra in the diagnostic region 1500–1800 cm^–1, and the agreement is largely satisfactory at longer wavelengths from 1000 to 1500 cm^–1. Weak-binding metal ions (K⁺, Ba²⁺, and Ca²⁺) predominantly adopt the charge-solvated (CS) mode of chelation involving both carbonyl oxygens, an imidazole nitrogen of the histidine side chain, and possibly the amino nitrogen. Complexes with Mg²⁺ and Ni²⁺ are found to adopt iminol (Im) binding, involving the deprotonated amide nitrogen, with tetradentate chelation. This tetradentate coordination of Ni­(II) is the preferred binding mode in the gas phase, against the expectation under condensed-phase conditions that such binding would be sterically unfavorable and overshadowed by other outcomes such as metal ion hydration and formation of dimeric complexes. The HisGly results are compared with corresponding results for the PheAla, PheGly, and PhePhe ligands, and parallel behavior is seen for the dipeptides with N-terminal Phe versus His residues. An exception is the different chelation pattern determined for PhePhe versus HisGly, reflecting the intercalation-type cation binding pocket of the PhePhe ligand. The complexes group into three well-defined spectroscopic patterns: nickel and magnesium, calcium and barium, and potassium. Factors leading to differentiation of these distinct spectroscopic categories are (1) differing propensities for choosing the iminol binding pattern, and (2) single versus double charge on the metal center. Nickel and magnesium ions show similar gas-phase binding behavior, contrasting with their quite different patterns of peptide interaction in condensed phases