The effect of the amphoteric properties of amino acids in the zwitterionic form on the structure of iodine complex compounds in aqueous solutions

Abstract

The donor–acceptor interactions in three model systems: water–glycine (a), water–glycine–KI3 (b), water–glycine–KI3–LiCl–ethanol (c) were investigated by UV-, IR-spectroscopy, and the quantum-chemical DFT-B3PW91/midi method.UV- and IR spectrum data for the considered systems agree well with the results of calculations of the spectral characteristics of glycine cluster that consists of six molecules, and models that simulate the influence of the zwitterionic form of glycine on the structure of complex iodine compounds in the systems (b–c).Localization of a negative and a positive charge at the ends of a zwitterion makes it possible for a cluster of glycine zwitterions to be formed in water. In such cluster glycine molecules are located under each other so that the protonated group is always under (or above) the carboxy group. Glycine fragments are bound by strong hydrogen bonds and also by hydrogen bonding interactions with water molecules. In the water–glycine–KI3–LiCl–ethanol system the cluster of glycine zwitterions splits the iodide ion into I− and I2 (I− reacts with the protonated amino group, and I2 with the carboxy group). This creates conditions for the formation of an iodine complex compound, in which molecular iodine reveals the acceptor properties towards glycine, and the donor properties towards the LiCl–ethanol complex.