Experimental and theoretical investigations of infrared multiple photon dissociation spectra of arginine complexes with Zn²⁺ and Cd²⁺

Arginine (Arg) complexes with Zn²⁺ and Cd²⁺ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light from a free electron laser. Electrospray ionization generated complexes of deprotonated Arg with Zn²⁺, [Zn(Arg–H)]⁺, and Arg with CdCl⁺, CdCl⁺(Arg). Possible low-energy conformers of these species were found using quantum chemical calculations, and their calculated IR spectra were compared to experimentally measured IRMPD spectra. Calculations were performed at the B3LYP/6-311+G(d,p) level for Zn²⁺ complexes and B3LYP/def2-TZVP with an SDD effective core potential on cadmium for CdCl⁺ complexes. [Zn(Arg–H)]⁺ was found to adopt a charge-solvated, tridentate [N,CO⁻,N^ω′] structure where Zn²⁺ binds to the backbone amine, carbonyl oxygen, and side-chain terminal guanidine nitrogen (N^ω′). The CdCl⁺(Arg) species was suggested to be a mixture of a dominant (∼85%) charge-solvated, tridentate [N,CO,N^ω′] structure where the CdCl⁺ binds to the backbone amine, carbonyl, and side-chain imine (N^ω′) and a minor (∼15%) bidentate [N,CO⁻](N^ω′H₂⁺) zwitterionic structure where the metal center binds to the backbone amine and carbonyl oxygen with intramolecular proton migration from the hydroxyl to the N^ω′ guanidine nitrogen (as designated in parenthesis).