Potentiometric studies on the formation equilibria of ternary complexes of vanadium(III) with cysteine and some amino acids

Abstract

In this work, we present the results of the speciation of the ternary vanadium(III)–cysteine (H2Cys) complexes with the amino acids glycine (HGly), proline (HPro), α-alanine (HαAla), β-alanine (HβAla), histidine (HHis), aspartic acid (H2Asp), and glutamic acid (H2Glu), studied by the measurements of electromotive forces, emf(H), using 3.0 mol dm−3 KCl as the ionic medium at 25 °C. The experimental data were analyzed by means of the computational least-squares program LETAGROP, taking into account the hydrolysis of the vanadium(III) cation, the respective stability constants of the binary complexes, and the acid base reactions of the amino acids, which were kept fixed during the analysis. In the vanadium(III)–cysteine-amino acid (HB) systems, where HB represents the bidentate ligands, the formations of the ternary complexes [V(Cys)(HB)]+, V(Cys)(B), [V(Cys)(B)(OH)]−, and [V(Cys)(B)(OH)2]2− were observed. In the vanadium(III)-H2Cys-HHis system, the formations of the complexes [V(HCys)(HHis)]2+, [V(HCys)(His)]+, V(Cys)(His), and [V(Cys)(His)(OH)]− were detected; in the vanadium(III)-H2Cys-H2Asp system, the complexes [V(H2Cys)(HAsp)]2+, [V(HCys)(HAsp)]+, V(HCys)(Asp), [V(Cys)(Asp)]−,and [V(Cys)(Asp)(OH)]2− were detected; and finally, in the vanadium(III)-H2Cys-H2Glu system, the complexes [V(H2Cys)(HGlu)]2+, [V(HCys)(HGlu)]+, V(Cys)(HGlu), and [V(Cys)(HGlu)(OH)]− were observed. The species distribution diagrams as a function of pH are briefly discussed.