STUDY OF BINARY AND TERNARY COMPLEXES OF MERCAPTOSUCCINIC ACID AND L-ORNITHINE WITH ESSENTIAL METAL IONS IN ETHYLENE GLYCOL-WATER MEDIA

During this study, the stability constants of the metals with mercaptosuccinic acid (MSA) and L-ornithine (L-Orn), as well as their ternary system have been studied in 0-60% ethylene glycol (EG)-water medium using the Metrohm 877 titrino plus auto-titrator between a pH ranges 0-14 at a temperature of 298 K. The modified Calvin-Wilson titration method was used to figure out the relative compositions of chemical species that were formed with metal ions and ligands. The MINIQUAD75 computer program was used to figure out the stability constants of the complexes. Some statistics have been discussed about the formed binary metal complexes and their ternary system, as well. The structures were proposed based on the observations.


INTRODUCTION
"In reaction with ligands, a metal ion can form a range of species dependent on the metal ion and other circumstances". 1,2 Speciation investigations can indicate species formation nature and degree with change in pH. 3,4 Recent work has focused on aqueous and aqueous-organic binary and ternary complexes of important biological ligands. In aqueous-organic solvents, compounds evolve in new ways. Mercaprosuccinic acid (MSA) and L-Ornithine (L-Orn) are potentially important tridentate ligands that can form strong complexes with numerous metal ions with chemotherapeutic applications in a range of solvents. [5][6][7][8] Despite extensive literature research [9][10][11][12] in this sector, nothing is known about MSA and L-Orn stability constants in ethylene glycol(EG)-water media. In the current study, the stability constants of binary complexes along with their ternary complexes of selected divalent metal ions with MSA and L-Orn have been reported in an aqueous-EG medium. This study gives insight into information about the formation of several species as a role of pH. EXPERIMENTAL 0.05mol L -1 MSA (Himedia, India) and L-Orn (Himedia, India) solutions were prepared with hydrochloric acid to improve the solubility. Ethylene Glycol (Merck, India) was used as received. All metal ion solutions were prepared using G.R. Grade chloride salts and an acidic strength of 0.05 mol L -1 was maintained using HCl to inhibit metal salt hydrolysis. Standard approaches were used for the preparation and standardization of all solutions and data were subjected to ANOVA 13 to identify any errors by the Gran plot approach. 14,15 STUDY OF BINARY AND TERNARY COMPLEXES G. Rama Swamy et al.

Approach
pH metric titrations were conducted in 0-60%v/v EG-Water media by using an auto-titrator Metrohm 877 titrino plus Switzerland in conjunction with a pH-sensitive electrode under the temperature of 298 K as described elsewhere. 16

RESULTS AND DISCUSSION
Binary Metal-Ligand Complexes Chemical species formed by metal-ligand interactions were measured using a modified 17 Calvin-Wilson titration approach. On the electrode response, a correction factor was determined using SCPHD. 18 MINIQUAD75 computer programme 19,20 was used to refine stability constants.

Modeling of Chemical Species
Tables-1 and 2 show the results of the models that suit best the M(II)-MSA and M(II)-L-Orn binary systems, respectively. The possible forms and their distribution of protonated and un-protonated complexes observed that suit best for the metal-ligand hypothesis has been shown in Fig.-

Inter Comparison of Stabilities of M(II)-L Binary Complexes
The gradual decrease in the free metal (FM) distribution curve compared with the free ligand (FL) signifies the development of binary complexes. Copper(II) has the maximum stability among all M(II)-L binary complexes with MSA and L-Ornithine (Fig.-2b). This is consistent with the "Irving-Williams stabilities order". 21

Role of Dielectric Constant
The present investigation indicated that the stability constants increased linearly with increasing solvent composition. Because the solvent has a lower dielectric constant than water, the metal-ligand complexes are more stable as illustrated in Fig.-2c.

Ternary Complexes of MSA and L-Orn Modelling of Chemical Species
The best fit model for ternary complexes with MSA and L-Orn has been chosen by considering all statistical parameters given in Table-3. MLX, MLX2, and ML2X species were detected. Overall stability constants (log β) with minimal standard deviation (SD) in specifies the exactness of these constraints.

Inter Comparison of Stability Constants
As illustrated in Fig.-5b, the lower size of Cu(II) causes the stability constants to be higher than those of Co(II), Ni(II), and Zn(II) ternary complexes.

Role of Dielectric Constant
Being a protophilic and structural forming nature of EG, eliminates water from the metal ions coordination sphere and creates them highly reactive to ligands resulting in the complexes being more stable. Thus, log values should be directly related to the reciprocal of the medium's dielectric constant (1/D), as shown in the present study (Fig.-5c).

Proposed Structure
The structures of the ternary complexes are hypothesized and sketched using Chem Draw 18.1 as shown in Fig.-6. The linear increase in ternary complex stabilities with the percentage of solvent is due to electrostatic forces. The study also examines metal obtainability/transportation in biofluids and metal toxicity. Because Cu(II) is smaller than other studied metal ions, its stability constants are larger.