Gas-phase complexes of Ni2+ and Ca2+ with deprotonated histidylhistidine (HisHis): A model case for polyhistidyl-metal binding motifs.

In the complex formed between the calcium cation (Ca 2+ ) and a deprotonated HisHis dipeptide, the complex adopts a charge solvation (CS) structure. Ca 2+ , a weak binding main group metal cation, interacts with the oxygens of the peptide carbonyl moiety and the deprotonated C-terminus. In contrast, the much stronger binding Ni 2+ cation deprotonates the peptide nitrogen and induces an iminolate (Im) ligand structure in the [Ni(HisHis-H)] + complex ion. The combination of infrared multiple-photon dissociation (IRMPD) spectroscopy and quantum chemistry evidence these two representative binding motifs. The iminolate coordination pattern identified and characterized in the [Ni(HisHis-H)] + complex serves as a model case for nickel complexes of poly-histidyl-domains and is thereby also of interest to better understand the fundamentals of immobilized metal ion affinity chromatography as well as of Ni co-factor chemistry in enzymology. [ABSTRACT FROM AUTHOR]