Stability constants of mixed ligand complexes of vanadium(III) with cysteine and the amino acids serine, threonine, methionine and phenylalanine

The formation of the ternary complexes of Vanadium (III)-l-cysteine with the amino acids serine (HSer), threonine (HThr), methionine (HMet) and phenylalanine (HPhe) in aqueous solution was studied by means of emf(H) electromotive force measurements under the conditions: 3.0 M KCl and temperature (25.0 ± 0.1) °C, in order to know the speciation of these systems. The data obtained through potentiometric measurements were analyzed using the computer program generalized least squares LETAGROP, which indicate the formation of ternary complexes of the type [V(Cys)(HL)]+, V(Cys)(L) and the hydrolysis products [V(Cys)(L)(OH)]− and [V(Cys)(L)(OH)2]2 −, where L represents the amino acids studied (L = Ser, Thr, Met, Phe ). The spectrophotometric studies indicate the presence of six-coordinated and heptacoordinated complexes for the V(III)-H2Cys-HThr system. The Δ log K″ indicated that the ternary complexes are more stable than the binary ones.