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

Abstract

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-,Nω'] structure where Zn2+ binds to the backbone amine, carbonyl oxygen, and side-chain terminal guanidine nitrogen (Nω'). The CdCl+(Arg) species was suggested to be a mixture of a dominant (∼85%) charge-solvated, tridentate [N,CO,Nω'] structure where the CdCl+ binds to the backbone amine, carbonyl, and side-chain imine (Nω') and a minor (∼15%) bidentate [N,CO-](Nω'H2+) zwitterionic structure where the metal center binds to the backbone amine and carbonyl oxygen with intramolecular proton migration from the hydroxyl to the Nω' guanidine nitrogen (as designated in parenthesis).