An experimental and theoretical dissection of potassium cation/glycine interactions

Abstract

The binding of K⁺ to glycine is examined in detail by studying the interaction of the potassium cation with glycine and ligands that contain the functional components of glycine both singly and in pairs. Bond dissociation energies of M⁺–L where L = glycine, ethanol amine, propionic acid, methyl ethyl ketone, 1-propanol, and 1-propylamine are reported. Experimentally, the bond energies are determined using threshold collision-induced dissociation of the M⁺–L complexes with Xe using a guided ion beam tandem mass spectrometer. Analyses of the energy dependent cross sections provide 0 K bond energies for the M⁺–L complexes. All bond energy determinations include consideration of unimolecular decay rates, internal energy of reactant ions, and multiple ion–molecule collisions. Ab initio calculations, including those for a 1-amino-2-propanone ligand, using MP2, density functional, and compound methodologies show good 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 a detailed understanding of the driving forces for the interaction of K⁺ with glycine and of the differences between the theoretical structures of Na⁺(glycine) and K⁺(glycine).