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Experimental and Theoretical Dissection of Sodium Cation/Glycine Interactions

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journal contribution
posted on 28.09.2002, 00:00 by R. M. Moision, P. B. Armentrout
The binding of Na+ to glycine is examined in detail by studying the interaction of the sodium cation with glycine and five molecules that contain the functional components of glycine both singly and in pairs. Bond dissociation energies of Na+−L where L = glycine, ethanol amine, propionic acid, methyl ethyl ketone, and 1-propylamine are reported, and L = 1-propanol is available in the literature. Experimentally, the bond energies are determined using threshold collision-induced dissociation of the Na+−L complexes with Xe using a guided ion beam tandem mass spectrometer. Analysis of the energy-dependent cross sections provides 0 K bond energies for the Na+−L complexes. All bond energy determinations account for unimolecular decay rates, internal energy of reactant ions, and multiple ion−molecule collisions. Ab initio calculations at the MP2(full)/6-311+G(2d, 2p)//MP2(full)/6-31G* and CBS-QB3 levels, which also include 1-amino-2-propanone, show reasonable agreement with the experimental bond energies and with the few previous experimental values available. The combination of this series of experiments and calculations allows the binding strength of individual functional groups and the influence of chelation to be thoroughly explored. This permits the driving forces for the interaction of Na+ with glycine to be understood in some detail. Specifically, glycine is a bidentate ligand with Na+. The primary binding site is the carbonyl with a bond energy reduced by an inductive effect of OH in the carboxylic acid group. Chelation to the amino group enhances the bonding although the increase is mediated by steric constraints imposed by the sp2 hybridization at the carbon center of the carboxylic acid group.