Determination of binary stability constant of the complexes of Ni(II) and Mn(II) ions with cysteine

Abstract

Conventional chelators have been reported to be toxic, non-biodegradable, and rigid towards the recovery of bonded metal ions. The drawbacks with these chelators necessitate a search for their alternatives such as amino acids. Therefore, binary complexes of biologically important transition metal ions; Ni(II) and Mn(II) with cysteine have been studied potentiometrically at about 27 ℃ in aqueous medium using Irving-Rossotti titration technique. The results obtained were used to evaluate the proton-ligand and metal-ligand stability constants. The proton-ligand stability constants were; log K₁H 8.4 (pKa for thiol group) and log K₂H 10.7 (pKa value for NH_2-group). The binary metal-ligand stability constant values for1:1 (M: L) complexes, log K as evaluated were found as Ni(II) > Mn(II); which is in agreement with the Irving–Williams order of the divalent metals of 3d series. Hence, these binary metal complexes could be applied as a medium of transporting chemotherapeutic drugs to target sites or detoxifying poisonous substances which possesses donor atoms with chelating capability. This finding also agreed with previous claimed that cysteine could be applied to transport metals to or away from target sites.