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

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journal contribution
posted on 2015-09-03, 00:00 authored by Georgia C. Boles, Rebecca A. Coates, Giel Berden, Jos Oomens, P. B. Armentrout
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,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.