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 Gin 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,COsc] 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-,COsc] 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.