In this study, the chemical speciation of mixed ligand complexes of Ca(II) and Mg(II) with L-glutamine and succinic acid was studied in varying compositions of Dimethyl Formamide (DMF) in aqueous solution maintaining an ionic strength of 0.16 mol L^_1 (NaCl) at 303 K. The stability constants of ternary complexes were refined with MINIQUAD75 using the primary alkalimetric data. The best-fit models were selected based on statistical parameters and residual analysis. The extra stability of ternary complexes compared to their binary analogues was believed to be due to electrostatic interactions of the side chains of ligands, charge neutralization, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of DMF and the plausible equilibria for the formation of species are discussed. The probable structures of the complexes are also given. The bioavailability of the metal ions is explained based on the speciation.

The specificity and selectivity of enzyme-substrate reactions were achieved in vivo by manipulating the Equivalent Solution Dielectric Constant (ESDC) at the active site. Further, intramolecular and ligand-ligand stacking interactions in mixed ligand complexes are favored in water-organic media. Thus, the resulting reduced ESDC at the active sites is a consequence of solute-solute interactions, quadrupole and dipole-dipole interactions among the ligands and hydrogen bonding. Although the contributions of free energy changes associated with these individual reactions are small compared to the total free energy change, their role is unequivocally established to be of utmost importance. Hence, knowledge of the ESDC at the active site can throw some light on the mechanism of the reaction. Sigel (1987) pointed out that extensive collection of equilibrium data with model compounds, in the media of widely different solvent properties was the only way to study their effect. This data can provide a solid basis to rationalize the effect of dielectric constant (Sigel et al., 1985) on properties of the metal ion complexes. Hence, modeling studies (Ronald and Rao, 2002a and 2002b; Babu et al., 2003; Sailaja et al., 2004; and Rao and Sudarsan, 2006) involving ternary complexes have gained popularity in different aquo-organic mixtures with varying dielectric constants.

The role of calcium(II) and magnesium(II) in biological systems is well recognized (Gunther, 1981; Scott et al., 1990; Anderson and Welsh, 1991; Somlyo and Somlyo, 1994; Roger and Robert, 1995; and Service, 1996), and any variation in their concentrations leads to metabolic disorders. Calcium is associated with blood coagulation, neuromuscular and membrane excitability, neurotransmitter secretion, transmission of nerve impulses, maintenance and function of cell membranes, cellular adhesiveness, activation of enzyme reactions and hormone secretion.

Chemical Speciation, Calcium(II) and Magnesium(II), L-Glutamine, Succinic Acid Speciation, DMF, Dimethyl Formamide, Equivalent Solution Dielectric Constant, ESDC, Neurotransmitter secretion, Metabolic disorders, Primary alkalimetric data, Thermodynamic stabilities, Electrostatic interactions.