Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Glutamine Complexes with Zn²⁺ and Cd²⁺

Complexes of glutamine (Gln) cationized with Zn²⁺ and Cd²⁺ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free-electron laser. Electrospray ionization yielded complexes of deprotonated Gln with Zn²⁺, [Zn­(Gln–H)]⁺, and intact Gln with CdCl⁺, CdCl⁺(Gln). For each complex, the spectra obtained were compared with those for low-energy conformers found using quantum chemical calculations to identify the structures present experimentally. Calculations were performed at the B3LYP/6-311+G­(d,p) level for [Zn­(Gln–H)]⁺ and at the B3LYP/def2-TZVP level with an SDD effective core potential on cadmium for CdCl⁺(Gln). The main binding motif observed for the Cd²⁺ complex was a charge-solvated, tridentate [N,CO,CO_sc] structure in which the metal binds to the backbone amino group and the carbonyl oxygens of the carboxylic acid and side-chain amide groups. The Zn²⁺ system similarly preferred a [N,CO^–,CO_sc] binding motif, where binding was observed at the carboxylate site along with the backbone amino and side-chain carbonyl groups. In both cases, the theoretically determined lowest-energy conformers explain the experimental [Zn­(Gln–H)]⁺ and CdCl⁺(Gln) spectra well.