Experimental and Theoretical Investigations of Infrared Multiple Photon Dissociation Spectra of Glutamic Acid Complexes with Zn2+ and Cd2+
Complexes of glutamic acid (Glu) cationized with Zn2+ and Cd2+ were examined by infrared multiple photon dissociation (IRMPD) action spectroscopy using light generated from a free electron laser. Complexes of deprotonated Glu with Zn2+, [Zn(Glu H)ACN]+ (where ACN=acetonitrile, CH3CN), and intact Glu with CdCl+, CdCl+(Glu) were formed. Notably, photodissociation induces Glu fragmentation rather than ACN loss in the Zn complex. In order to identify the structures formed experimentally, the experimentally obtained spectra were compared to those calculated from optimized structures at the B3LYP/6-311+G(d,p) level for [Zn(Glu H)ACN]+ and B3LYP/def2-TZVP level with an SDD effective core potential on cadmium for the CdCl+(Glu) system. The main binding motif observed for the heavy metal complex is a charge solvated, tridentate [N,COs,CO] structure where the metal binds to the backbone amino group and carbonyl oxygens of the side-chain and backbone carboxylic acid groups. The Zn2+ system similarly prefers a [N,CO–,COs] binding motif, where binding is observed at one oxygen of the backbone 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(Glu H)ACN]+ and CdCl+(Glu) spectra well.