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 (Ca²⁺) and a deprotonated HisHis dipeptide, the complex adopts a charge solvation (CS) structure. Ca²⁺, 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 Ni²⁺ 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.