The formation constants of binary and ternary complexes of Thiobarbituric acid as primary ligand and L-tyrosine and L-histidine as secondary ligands have been examined in 40% (v/v) ethanol-water mixture at 27oC and 35oC and at ionic strength of 0.02 NaNO3 by potentiometric method. The ligands formed 1:1 binary complexes with the metal ions. The primary and secondary ligands simultaneously coordinated to the metal ions to form 1:1:1 ternary complexes. The difference in stability constants of binary and ternary complexes were determined by DlogK and RS%. Ternary complexes exhibited enhanced stability than the binary complexes. The stability of the complexes decrease with increase in temperature. The thermodynamic parameters such as Gibb’s free energy change (ΔG), entropy change (ΔS) and enthalpy change (ΔH) accompany the interactions were evaluated. The interactions were found to be spontaneous, exothermic, and entropically favoured.
An understanding of the principles of complex equilibria and species distribution in different solutions is important in expounding and correlating the interaction of different ligands with different metal ions in complex formation. Therefore, acid-base equilibria involved in the formation of binary and ternary complexes of Co (II), Cu (II) and Pb (II) with methionine (Met) and uracil (Urc) have been determined by potentiometric titration technique. The stability constants of the complexes were evaluated at 35 ± 0.1°C and 0.02 M ionic strength (kept constant with NaNO3) in aqueous and organic-aqueous media. The species distribution in solutions as a function of pH was determined using the Hyss program. The stability of the ternary complexes relative to the corresponding binary complexes of the secondary ligand is measured in terms ΔlogK and % RS values. The ternary complexes are observed to be more stable than binary complexes in the media except for [CuMetUrc] ternary complex in organic-aqueous medium where the ternary complex is less stable than the binary complex of the uracil. The overall stability of the ternary complexes was higher in organic-aqueous system than aqueous system. The stability of the complexes was found to be correlated with the covalent index of the metal ions and Jahn Teller distortion. pH-studies of these systems revealed an increase in the concentrations of the ternary complexes with increase in pH. The formation of binary complexes was shown to be favoured in physiological pH range (3-7) while that of the ternary complexes is observed to be favoured in the pH range 5-10.
Many diseases in nature have led to the death of many young and old. Many bacteria have developed resistance to the available antibiotics on the market. And many drugs employed in treating many diseases such as diabetes mellitus are expensive and are not locally available. Therefore, in order to search for more effective, inexpensive, and locally available drugs, this study synthesized and investigated the biological activities of Andrographis paniculata crude extract and its Co (II) and Ni (II) complexes. The crude extract and synthesized complexes were characterized using a solubility test, Infrared, and Ultraviolet-Visible spectroscopic analysis. Their antibacterial potentials were investigated against two gram-negative bacteria (Escherichia coli,Staphylococcus aureus) and three gram-negative bacteria (Proteus, Klebsiella, Pseudomonas) while their antidiabetic activities were examined against ?-amylase and ?-glucosidase enzymes. Acarbose was employed as a standard drug. The crude extract and its metal complexes showed different degrees of solubility in the employed solvents. Infrared analysis suggested coordination of the crude extract to the metal ions through the oxygen donor atom while the formation of the complexes was affirmed through the occurrence of d-d transitions in the visible region of the metal complexes. The metal complexes were found to display more antibacterial activity than the crude extract. Co (II) and Ni (II) complexes of the crude extract were found to exhibit better activities against ?-amylase and ?-glucosidase enzymes, respectively than the crude extract and acarbose. It is concluded that the metal complexes could be considered potential antibacterial and antidiabetic agents.
The analysis of coordination of chelating agents to metal ions in the formation of complexes under different experimental conditions provide information on the effects of the conditions on the extent of the coordination of the ligands to the metal ions for potential applications in bioinorganic and environmental Inorganic Chemistry. The effect of solvent on the coordination of L-methionine (primary ligand) and uracil (secondary ligand) to Co(II), Cu(II) and Pb(II) ions in the formation of binary and ternary complexes have been potentiometrically investigated in water and 40% (v/v) ethanol-water media at 27 °C and ionic strength of 0.02 mol dm-3was maintained by using NaNO3. The proton-ligand stability constants and formation constants of the binary and ternary systems were established under the experimental conditions. The formation constants of the ternary complexes were numerically compared with the corresponding binary complexes in terms of Δ log K and percentage relative stabilization (%RS). Speciation of the metal ions, binary and ternary complexes as a function of pH in the solutions was investigated by using HYSS program. The ligands were observed to form more stable ternary complexes than binary complexes. The proton-ligand stability constants the ligands and the formation constants of the metal complexes were found to be higher in 40% (v/v) ethanol-water than water. The distribution of the species revealed a decrease in the metal ion concentrations with increase in the concentrations of the binary and ternary as pH increased. The chelating agents interacted strongly with the metal ions and showed different selectivity towards the metals ions. These properties can be exploited in combating metal poisoning, separating the metals and expounding the bioinorganic chemistry of the metal ions and the ligands.
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