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 w...