Potentiometric determination of stability constants and thermodynamic data for ternary Cd(II) complexes with 2-aminomethyl benzimidazole (AMBI) and other bioactive ligands

Abstract

The complexing properties of 2-(aminomethyl)-benzimidazole with cadmium(II) were investigated pH-metrically at 25 ^oC and at ionic strength of 0.1 mol.dm^-3 (NaNO₃). Ternary complex formation equilibria of cadmium(II) complexes involving (AMBI) and some bio-relevant ligands (L = amino acid and peptides) have been investigated. Ternary complexes of amino acids and peptides are formed by a simultaneous mechanism. Amino acids form the complex Cd(AMBI)L, whereas amides form two complex species Cd(AMBI)L and Cd(AMBI)(LH_-1). The stability of ternary complexes was quantitatively compared with their corresponding binary complexes in terms of the parameters ΔLog K, Log β_Stat and Log X. The effect of the side chains of amino acid ligands (Δ_R) on complex formation was discussed. The values of ΔLog K indicate that the ternary complexes containing aromatic amino acids are significantly more stable than the complexes containing alkyl- and hydroxyalkyl-substituted amino acids. This may be taken as evidence for a stacking interaction between the aromatic moiety of AMBI and the aromatic side chains of the bioactive ligands. The concentration distributions of various species formed in solution were also evaluated as a function of pH. The thermodynamic parameters ΔH^o, ΔG^o and ΔS^o calculated from the temperature dependence of the equilibrium constants were investigated for the interaction of Cd(II)-AMBI with glycine as a representative example of amino acids.