Stability study of novel zinc complexes of pharmaceutical relevance by CE

CE was used as a stability-indicating assay for zinc mixed-ligand complexes with promised biological activity. CE measurements indicated that Zn(RCOO)2Ln compounds (where R = H, CH3, C2H5, or C3H7; L = caffeine, nicotinic acid, thiourea, or phenazone; and n = 1 or 2) in water tend to decompose at room temperature. For thiourea and phenazone compounds, the degradation products (free zinc ion, carboxylate anion, and the corresponding neutral ligand) were found to increase within a few hours of storage in water. More stable complexes of caffeine and nicotinic acid showed no appreciable changes in CE behavior during the same period of time and only decomposed notably after 10 hr. The stability differences of zinc complexes were characterized in terms of apparent half-life values and rate constants of decomposition reaction. Relationships between the stability and structure of the complexes are discussed, and theoretical interpretation is presented.