Magnesium, adenosine triphosphate, and catecholamine complexes: application of new means of identifying solute-solute interactions of biological significance using NMR chemical shift data.

The interaction of magnesium ions with biomolecular species is fundamental to the regulation of many biological processes. A mathematical technique to investigate such interactions is described. The concept of a pseudo-formation curve is discussed and the relevance and applicability of such constructions to the study of solute-solute interactions is outlined. Analysis of curves based on the published variation of chemical shift data of various nuclei of adrenaline and dichloroisoprenaline in aqueous solution, in the presence of increasing concentration of the complex Mg-ATP, is described. Unambiguous indentification of specific interactions and the stability of the species produced is shown to be feasible. Some variation of the values of formation constants deriving from different nuclei reflects the significance of molecular environmental factors on the parameters used.

Diffusion coefficients and complex equilibria in solution-V Limitations to the validity of the diffusion equations.

The ligand-concentration dependence of the limits of applicability of equations involving diffusion parameters and ligand numbers is discussed. Estimation of - n from preliminary values of formation constants derived from the pseudo-formation curve often provides proof of the direct correlation between -n and Deltai(d) for a proportion of the ligand concentration range, but not all of it. The limitation is especially noticeable for systems where full co-ordination is approached well within the range of experimental values of ligand concentration.

Diffusion coefficients and complex equilibria in solution-IV Experimental determination and manipulation of diffusion data.

An experimental method for effectively measuring the change of diffusion current of a metal ion in the presence of increasing concentration of a complexing ligand is described and applied to five metal-ligand systems. The method has been applied to cases of reversible and irreversible electrochemical behaviour and is shown to provide reliable formation-curve data. The systems selected have formation constants which range over some ten orders of magnitude and in most cases the values obtained agree satisfactorily with those reported in the literature. In one case equilibrium data have been calculated for the first time.

Diffusion coefficients and complex equilibria in solution-III Graphical evaluation of formation constants from diffusion coefficients.

The values of the individual diffusion coefficients of the complexes participating in a metal-ligand system of equilibria may be estimated when provisional values of the formation constants have been calculated from diffusion currents. In this paper a new graphical method, involving average diffusion coefficients observed for metal-ligand systems, is described. The method is applied to four such systems, chosen for the particular characteristics shown by them.

Diffusion coefficients and complex equilibria in solution-I theory: Diffusion coefficients of complex species.

It is shown that in principle it is possible to obtain values of the formation constants of metal complexes in solution from values of diffusion coefficients. It is also confirmed that, in a system of successively formed complexes, there is a linear relationship between the square root of the diffusion coefficients and the co-ordination numbers of the complexes formed. In some instances such analysis has shown that revision of some previously reported formation constant data is required.

Diffusion coefficients and complex equilibria in solution-II Approximate evaluation of formation constants.

It is shown, for four systems containing complexes of widely differing stabilities, that a very fair assessment of the values of formation constants is obtainable from the data pairs Deltai(d) log [X] (the pseudo-formation curve). This is seen as a preliminary exercise to the more precise analysis which is possible in terms of mean diffusion coefficients which feature in equations reported earlier.

A polarographic cell system with a novel temperature-controlled reference electrode for the determination of kinetic parameters of electrode reactions.

An improved experimental arrangement for the determination of kinetic data relating to polarographic reductions is described. The significant feature is the constant-temperature reference electrode; its construction and use in this context are described. A comparison is made of the more realistic results obtained by these means, with those obtained with a more conventional cell where the temperature of the reference electrode is varied with that of the working solution.

Computer calculation of activation energies of irreversible electrode processes at a dropping mercury electrode.

A computer program has been written for calculation of the activation energies of irreversible electrode processes at a dropping mercury electrode, based on the variation of current and half-wave potential with temperature. The program calculates the half-wave potential, the transfer coefficient, and the activation energies of the diffusion process and the irreversible electrode process. It has been written in both ALGOL and FORTRAN.