Magnesium stability constant

Covington AK, Danish EY (2009) Measurement of magnesium stability constants of biologically relevant ligands by simultaneously use of pH and ion-selective electrodes. J Solut... 

Calcium-selective electrodes have long been in use for the estimation of calcium concentrations - early applications included their use in complexometric titrations, especially of calcium in the presence of magnesium (42). Subsequently they have found use in a variety of systems, particularly for determining stability constants. Examples include determinations for ligands such as chloride, nitrate, acetate, and malonate (mal) (43), several diazacrown ethers (44,45), and methyl aldofuranosides (46). Other applications have included the estimation of Ca2+ levels in blood plasma (47) and in human hair (where the results compared satisfactorily with those from neutron activation analysis) (48). Ion-selective electrodes based on carboxylic polyether ionophores are mentioned in Section IV.B below. Though calcium-selective electrodes are convenient they are not particularly sensitive, and have slow response times. 

Preliminary rate measurements should allow one to make a plot of initial velocity Vq versus [metal ion], and this should provide information on the optimal metal ion concentration. (For many MgATP -dependent enzymes, the optimum is frequently 1-3 mM uncomplexed magnesium ion.) Then, by utilizing pubhshed values for formation constants (also known as stability constants) defining metal ion-nucleotide complexation, one can readily design experiments to keep free metal ion concentration at a fixed level. To compensate properly for metal ion complexation in ATP-dependent reactions, one must chose a buffer for which a stability constant is known. For example, in 25 mM Tris-HCl (pH 7.5), the stability constant for MgATP is approximately 20,000 M Thus, one can write the following equation ... 

It has been pointed out that the high and variable electric field at the surface of a charged polymer makes the quantitative description of its equilibria very complecated (3). In this work the following assumptions were made in order to calculate the stability constants of the complexes of middle PO3 units with magnesium ion. 

The amount of bound magnesium, NM, corresponds to the peak area of zone a. Since the amount of total ligand can be obtained by dividing Np by n, the amount of free ligand in zone a is (Np/n The stability constant of the complex can be defined as for-... 

Table 1. Stability constants of magnesium complexes for different types of phosphates.

Tables 2.12 and 2.13 list the logarithm of the stability constants for the complexes of these chelating agents with various metal ions. Note that with the exception of Chel-138, calcium and magnesium form rather stable complexes with these chelating agents Fe3+ forms the most stable chelate of any metal listed. Generally, ferric iron is followed by Cu2+, Zn2+, Mn2+, Fe2+, Ca2+, and Mg2+. The weak acid properties of these chelating agents must be considered in any evaluation of their behavior. Because they are weak acids, the hydrogen ion tends to compete with the metal ions for association with the active groups.

Blair, J. (1969). Magnesium and the aconitase equilibrium Determination of apparent stability constants of magnesium substrate complexes from equilibrium data. Eur.. Biochem. 8, 287-291. 

Storm, C. B., A. H. Corwin, R. Arellano, M. Marts, and R. Weintraub Stability Constants of Magnesium-Porphyrin-P3fridine Complexes Solvent and Sub-stitutent Effects. J.. m. Chem. Soc. 88, 2525 (1966). 

If the sample solution contains any ions of magnesium and alkaline-earth metals, they will replace zinc and form EDTA complexes since their apparent stability constants are much larger than zinc. The zinc set free will form a complex with the indicator changing the color to violet. The amount of zinc replaced is equivalent to the amoimt of magnesium, and alkaline-earth metals in the sample solution is determined in the second titration. 

At the surface, one should not expect magnesium and calcium to be able to replace zinc in its EDTA complex since the absolute stability constant of zinc is several orders of magnitude higher than the absolute stability constant of the Group II metals. This can be explained by the presence of ammonium ions and the relevant ion stability constants with Eriochrome Black. 

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