Abstract

Aqueous solution reactions in milli-molar concentrations of only glycine (Gly) with the zinc metal ion (Zn²⁺) and with the cadmium metal ion (Cd²⁺) independently have been carried out at room temperature. The formed metal-glycine complexes were tested with infrared (IR) spectroscopy, Ultraviolet and Visible (UV-Vis) spectroscopy, potentiometry and speciation calculations. The Zn-glycine reaction mixtures were further characterized by proton Nuclear Magnetic Resonance (1H-NMR) spectroscopy in D2O at variable pH-values. These aqueous solution reactions of Gly with Zn²⁺ and Cd²⁺ were carried out in 0.1M ionic strength (I) NaNO3 which is the ionic strength similar to that of mammalian blood. From the detailed potentiometric titration graphs and speciation at a wide pH-range, it appeared that the formations of both monomeric and dimeric metal complexes are the dominant species present in aqueous solutions. Our proposed data of the presence of the dimeric complex are in excellent agreement with the literature data that showed the crystal structure for the well-characterized tetramer Zn²⁺-glycinate complex. We think that if the dimer complex proposed herein is given extended time for crystallization, it will be the precursor to the well-characterized tetramer glycine complex previously published and established in the chemical literature.

Keywords:Aqueous solutions; Glycine; Cd²⁺; Zn²⁺; 1H-NMR; Speciation; UV-Vis