Kinetics of complexation

The exact mechanisms of the complex formation and dissociation processes are not known. The overall process is represented by equation (X). Conformational changes may occur. Bimolecular processes might contribute, especially in solvents of low polarity (see, however, 143). A limited number of exchange rates have been reported, based mainly on NMR data (Table 13). Exchange kinetics are of prime importance in transport processes, where, however, more complex mechanisms may be operative than in the systems described here (see below andp. 145). 

Although the measurements do not all refer to the same solvent, some characteristic features may be extracted from the data.6) 

The dissociation rates of the complexes of the macrobicydic ligand 30 are more than three orders of magnitude slower than those of the 

All association rates are much slower than a diffusion controlled process, which would have a rate of about 109 M 1 sec-1 (134). However, the rates and free energies of activation seem to vary less from one system to another for the association than for the dissociation process. 

The association and dissociation rates are especially slow for the AEC complexes of the relatively rigid ligand 30, which is thus efficiently poisoned by Ba2+ and Sr2+ cations. 

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The chaimel-flow electrode has often been employed for analytical or detection purposes as it can easily be inserted in a flow cell, but it has also found use in the investigation of the kinetics of complex electrode reactions. In addition, chaimel-flow cells are immediately compatible with spectroelectrochemical methods, such as UV/VIS and ESR spectroscopy, pennitting detection of intennediates and products of electrolytic reactions. UV-VIS and infrared measurements have, for example, been made possible by constructing the cell from optically transparent materials. 

From a theoretical point of view the study of the kinetics of coupled catalytic reactions makes it possible to investigate mutual influencing of single reactions and the occurrence of some phenomena unknown in the kinetics of complex reactions in the homogeneous phase. This approach can yield additional information about interactions between the reactants and the surface of the solid catalyst. 

The studies mentioned in this brief account did not concern the kinetics of complex reactions taking place on the so-called polyfunctional catalysts, which were treated by Weisz (56) the theory of the use of these catalysts has been further worked out for some consecutive or parallel reactions carried out in the reactors with a varying ratio of catalyst components along the catalyst bed [e.g. (90, 91, 91a) ]. Although the description of these reactions, not coupled in the sense defined in Section III, is beyond the scope of this treatment, we mention here at least an experimental... 

Temperature-jump kinetics. The kinetics of complexation of lutetium(III) with anthrani-late ion was studied by the use of a temperature-jump method.25 The principal reaction is... 

Chapter 1 treated single, elementary reactions in ideal reactors. Chapter 2 broadens the kinetics to include multiple and nonelementary reactions. Attention is restricted to batch reactors, but the method for formulating the kinetics of complex reactions will also be used for the flow reactors of Chapters 3 and 4 and for the nonisothermal reactors of Chapter 5. 

Vynckier, E., and Froment, G. F., Modeling of the kinetics of complex processes based upon elementary steps , in Kinetic and Thermodynamic Lumping of Multicomponent Mixtures (G. Astaiita and S. I. Sandler, Eds.) Elsevier, Amsterdam (1991) 131-161. 

The high in vivo stability of DOTA complexes makes it a desirable ligand framework for BFCAs relative to acyclic analogs however, complex formation with DOTA and its analogs can be slow. The slow kinetics of complex formation with DOTA-type ligands does not pose problems with nuclides such as 177Lu (t /2 6.64 d) however, improved reaction conditions may... 

The macrotetrolide group (Figure 9.1) also form complexes with the alkali metals. Once again, such species preferentially complex with potassium the potassium complex of monactin in methanol at 30°C exhibits a stability constant of 2.5 x 105 mol-1 dm3 whereas the value for the sodium derivative is considerably lower at 1.1 x 103 mol-1 dm3. The kinetics of complex formation by compounds of the type so far discussed with alkali metals is known to be fast. For example, members of the... 

There appear to be two major ways by which ionophores aid ions to cross membrane barriers. Ionophores such as valinomycin and nonactin enclose the cation such that the outside of the complex is quite hydro-phobic (and thus lipid-soluble). The transport behaviour thus involves binding of the cation at the membrane surface by the antibiotic, followed by diffusion of the complexed cation across the membrane to the opposite surface where it is released. Such carrier type ionophores can be very efficient, with one molecule facilitating the passage of thousands of ions per second. A prerequisite for efficient transport by this type of ionophore is that both the kinetics of complex formation and dissociation be fast. 

While studying the formation kinetics of complexes gives useful mechanistic information about the reactivity of the iron center when bound to a particular siderophore, it is not necessarily a good model for how environmental iron will react in the siderophore system of interest. In biological systems,... 

None of the N2S2 or N3S systems described above is suitable for the postformed labeling of biomolecules (i.e. coupling a chelator to the biomolecule, followed by incorporation of the radiometal into the chelator) because the unfavorable kinetics of complexation or transchelation require relatively harsh conditions (strong acid/alkali, heat) which the biomolecule would not tolerate. In order to combine the advantages of in vivo stability in the amido systems with the better complexation... 

The quantum yield of the mant fluorophore in mant-GDP or mant-GTP increases approximately by 100 % when the molecule changes from the aqueous environment into the nucleotide binding pocket of the Ras protein. Therefore the kinetics of complex formation between nucleotide free Ras and the mant analogues of GDP or GTP could be detected easily in a stopped flow system by an increase in fluorescence signal. 

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 ... 

For a more extensive discussion of the kinetics of complexation with cryptands and natural antibiotics, the reader is referred to a recent review of Liesegang and Eyring (1978). 

The kinetics of complex formation with Zn2+ can be followed by monitoring the change in the fluorescence intensity [ 17g]. In the case of 1, the change in the fluorescence intensity with time indicates a biphasic kinetics with the incorporation rate constants k1 = 4.9 x 105 M 1 s 1 followed by a first order process... 

Information on the kinetics of complex ion reactions in high acid is sparse, partly because of the instability problems, but also because of the difficulties in the interpretation of results in such a complex medium. 

The rate at which solvent molecules are exchanged between the primary solvation shell of a cation and the bulk solvent is of primary importance in the kinetics of complex formation from aquocations. In both water exchange and complex formation, a solvent molecule in the solvated cation is replaced with a new molecule (another water molecule or a ligand). Therefore, strong correlations exist between the kinetics and mechanisms of the two types of reactions. 

The fact that adenine appears to be the target for MYKO 63 through a dialkylating process on the N(7) and NHj sites is consistent with the very low kinetics of complexation of MYKO 63 to DNA mentionned above Lawley and Brookes Maxam and Gilbert and Goodwin et al. have clearly demonstrated that methylation on adenine happens very slowly, five times slower, for example, than on guanine. 

There is thus a complicated interplay of the thermodynamics and kinetics of complexation, lipophilicity, diffusion and interface phenomena. Synthetic ligands are of special interest since, in principle, stepwise variations in the different parameters may be brought about via structural modification of the carrier. An efficient and selective carrier should have the following features (see also 16, 134) ... 

The design of a specific cation carrier thus presents two main additional requirements lipophilicity and a compromise between thermodynamics and kinetics of complexation. 

From the above list, one can see that kinetics of complex heterogeneous reactions are intimately related to important geological processes such as igneous rock formation, volcanic eruptions, and metamorphism. 

The non-linear theory of steady-steady (quasi-steady-state/pseudo-steady-state) kinetics of complex catalytic reactions is developed. It is illustrated in detail by the example of the single-route reversible catalytic reaction. The theoretical framework is based on the concept of the kinetic polynomial which has been proposed by authors in 1980-1990s and recent results of the algebraic theory, i.e. an approach of hypergeometric functions introduced by Gel fand, Kapranov and Zelevinsky (1994) and more developed recently by Sturnfels (2000) and Passare and Tsikh (2004). The concept of ensemble of equilibrium subsystems introduced in our earlier papers (see in detail Lazman and Yablonskii, 1991) was used as a physico-chemical and mathematical tool, which generalizes the well-known concept of equilibrium step . In each equilibrium subsystem, (n—1) steps are considered to be under equilibrium conditions and one step is limiting n is a number of steps of the complex reaction). It was shown that all solutions of these equilibrium subsystems define coefficients of the kinetic polynomial. 

A single-route complex catalytic reaction, steady state or quasi (pseudo) steady state, is a favorite topic in kinetics of complex chemical reactions. The practical problem is to find and analyze a steady-state or quasi (pseudo)-steady-state kinetic dependence based on the detailed mechanism or/and experimental data. In both mentioned cases, the problem is to determine the concentrations of intermediates and overall reaction rate (i.e. rate of change of reactants and products) as dependences on concentrations of reactants and products as well as temperature. At the same time, the problem posed and analyzed in this chapter is directly related to one of main problems of theoretical chemical kinetics, i.e. search for general law of complex chemical reactions at least for some classes of detailed mechanisms. 

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