Speciation of Co(II), Ni(II) and Cu(II) Complexes with L-Glutamic Acid in Dioxan–Water Mixtures

Abstract

Chemical speciation of Co(II), Ni(II) and Cu(II) complexes of L-glutamic acid in the presence of Dioxan-Water mixtures at an ionic strength of 0.16 mol L^–1 at 303 K was studied pH–metrically. Glu is taken as a model compound for amino acid residues and dioxan is used to mimic the low dielectric constant at the active site cavities in bioactive molecules like enzymes and proteins. The active forms of the ligand were LH₃⁺, LH₂, LH^– and L^2–. The models containing different numbers of species were refined using the computer program, MINIQUAD75. The predominant species detected were ML₂H₂, ML₂H^–, ML₂^2–, MLH⁺ and ML. The best fit chemical models were arrived at based on statistical parameters. The trend in variation of complex stability constants with the medium composition was explained on the basis of changes in the dielectric constant of the solution. Effect of errors in the ingredients on the stability constants was also studied. Chemical speciation is discussed based on the distribution diagrams.

Keywords: Complex equilibria, chemical speciation, L-glutamic acid, essential metals, dioxan