Complexes dissociation kinetics

Schmitt, D., Saravia, F., Frimmel, F. H., and Schuessler, W. (2003). NOM-facilitated transport of metal ions in aquifers Importance of complex-dissociation kinetics and colloid formation. Water Res. 37, 3541-3550. 

Activation energy values for the recombination of the products of carbonate decompositions are generally low and so it is expected that values of E will be close to the dissociation enthalpy. Such correlations are not always readily discerned, however, since there is ambiguity in what is to be regarded as a mole of activated complex . If the reaction is shown experimentally to be readily reversible, the assumption may be made that Et = ntAH and the value of nt may be an indication of the number of reactant molecules participating in activated complex formation. Kinetic parameters for dissociation reactions of a number of carbonates have been shown to be consistent with the predictions of the Polanyi—Wigner equation [eqn. (19)]. 

Let us now consider the the reverse of the binary complex dissociation reaction that we just described. We now turn our attention to the kinetics of association between an enzyme molecule and a ligand. The association reaction is described as follows ... 

From the examples given above it is clear that the development of a novel approach for real time measurements of the dissociation kinetics of reaction intermediates would greatly assist the unravelling of complex multistep processes associated with the transformation of even simple molecules at transition metal centers. While techniques are available for such measurements over a limited range of times, none of the methods are sufficiently general to be useful for extensive measurements. 

Kinetics evaluation software generates the values of ka (rates of complex formation) and kd (rates of complex dissociation) by fitting the data to interaction models. In a sensorgram, if binding occurs as sample passes over a prepared sensor surface, the response increases and is registered upon equilibrium, a constant signal is reached. The signal decreases when the sample is replaced with buffer, since the bound molecules dissociate. 

The dissociation kinetics of macrocyclic complexes have received considerable attention, especially during investigations of the nature of the macrocyclic effect. Before discussing the dissociation of cyclic ligand species, it is of benefit to consider some aspects of the dissociation of open-chain ligand complexes. 

The dissociation kinetics of the nickel chloride complex of the 15-mem-bered, S2N2-macrocycle of type (279) has also been investigated in the presence of hydrochloric acid (Lindoy Smith, 1981). This complex has a similar trans-octahedral structure to that of the 02N2-donor systems just discussed. For the sulfur-containing complex, two consecutive (acid-independent) first-order steps were observed, with the second being slower than the first. The data are in accordance with the scheme ... 

It is evident that competitive equilibria alone cannot explain the in vivo behavior of Gd111 complexes and kinetic factors also have to be considered. The excretion of low molecular weight chelates from the body is very rapid (119,126), whereas the dissociation and transmetallation of the Gdm complexes is relatively slow. Therefore, the system is not in equilibrium even in the case of linear chelates and kinetic factors are important. We should mention that for renal impaired patients the elimination rate becomes much slower which might lead to an... 

Shuman and Michael [10] applied a rotating disk electrode to the measurement of copper complex dissociation rate constants in marine coastal waters. An operational definition for labile and non-labile metal complexes was established on kinetic criteria. Samples collected off the mid-Atlantic coast of USA showed varying degrees of copper chelation. It is suggested that the technique should be useful for metal toxicity studies because of its ability to measure both equilibrium concentrations and kinetic availability of soluble metal. 

Formation and dissociation kinetics of nine Ni(II)-macrocyclic tetra-thiaether complexes (eight macrocyclic, one linear in acetonitrile) have been compared with those for Cu(II) analogues and for Ni(II) complexes with macrocylic tetramines (262). Whereas for the tetramine complexes conformational changes may be apparent in the kinetics this is not the case for the tetrathiaether complexes, where there is no kinetic evidence for slow conformational changes after initial bonding of the ligand to... 

Kinetics of H+-promoted dissociation of the Ni2+ complex of a tetra-dentate aza-oxa-cryptate derived from tren, conducted in acidic aqueous acetonitrile, indicate that its dissociation rate is smaller than that of [Ni(tren)(H20)2]2+, despite the much higher thermodynamic stability of the tren complex a kinetic cryptate effect is invoked to rationalize this (288). 

For a single microorganism with a given ro, different metals may have remarkably different behaviour, due to the metal-specific association and dissociation kinetics of the complexes involved. Take, for example, an organism of... 

The preceding sections have demonstrated the considerable quantitative understanding of biouptake that can be attained by models with a sound theoretical basis. We have shown solutions for a range of conditions, ranging from relatively simple limiting cases to more involved situations involving kinetically limited metal complex dissociation fluxes. In this section, we highlight key points that should be considered in future refinements of biouptake models. 

De Jong, H. G. and van Leeuwen, H. P. (1987). Voltammetry of metal-complex systems with different diffusion-coefficients of the species involved. 2. Behaviour of the limiting current and its dependence on association/dissociation kinetics and lability, J. Electroanal. Chem., 234, 17-29. 

If the metals bound in complexes exchange with biological ligands, the dissociation kinetics of these complexes, the ligand-exchange kinetics and the association kinetics with the biological ligands must be considered. Simple dissociation kinetics of complexes are related to their thermodynamic stability constants by the relationship ... 

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