Kinetics of complex formation

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. 

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. 

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... 

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 stopped-flow and T-jump techniques were used by Mao and co-workers (34j) to investigate the kinetics of complex formation of [Cu(tren)(OH2)]2+ by HC03. In large excess of HC03 and pH = 6-9, the formation of the mononuclear bicarbonato/carbonato complex was too fast to be studied kinetically. The kinetically observed reaction was the reversible formation of the binuclear complex, [(Cu(tren))2(02C0)]2+ and this occurred as shown below ... 

The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-1 s-1.642 Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x 103 and 1.13 x 103 mol-1 s 1 for MDA- and VMDA ) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii Table 27), species with coordinated hydroxyl are labilized. 

The rates of complex formation and ligand substitution reactions of the polymer-bound Co(III) complexes depend on the dynamic property of the polymer domains. Reports on the kinetics of complex formation and ligand substitution of macromolecule-metal complexes are, however, relatively scarce. They include investigations on the complexation of poly-4-vinylpyridine with Ni2+ by the stopped conductance technique 30) and on a ligand substitution reaction of the polymer-bound cobalt(III) complexes 31>. 

Table 9.4 Solution reactions and the factors controlling the kinetics of complex formation...

The structural characteristics of cyclen and cyclam and their carboxylic and amidic derivatives in variously protonated states as well as their metal complexes were excellently reviewed by Guilard and co-workers <1998CCR1313>. As those derivatives are frequently used in the complexation of transition metal or lanthanide ions, the space arrangement, exact protonation sites, and presence of intramolecular hydrogen bonds are of interest from the point of view of kinetics of complex formation/dissociation. Since the number of related structures is very high, but no new remarkable information appeared since Guilard s review, derivatives of these macrocycles are not discussed in detail. 

Probably the best known polysaccharide complex is the blue, starch-iodine complex. This reaction between iodine and the amylose component of starch involves the polymerization of iodine atoms inside the linear cavity of helically coiled, amylose macromolecules. The kinetics of complex-formation for many starches follow the same rate-equation as that for pure amylose, and, thus, the presence of amylopectin does not significantly affect the reaction. The differences between the... 

Selective trapping of unstable dienes. This complex has the useful property of trapping the unstable diene tautomers of cycloocta,triene (2) and derivatives such as 5. The kinetics of complex formation with unstable dienes has been studied and arguments have been advanced to account for the selectivity observed. 

Thus, NMR spectroscopy has the potential to provide almost complete information on ligand-CD interactions (stoichiometry, binding constants, free energy, enthalpy and entropy of complex formation, dynamics and structure of the complexes) in a solution. The kinetics of complex formation is usually too fast on the NMR time scale and therefore difficult to follow using this technique. However, an information on the kinetics of complex formation may be also obtained from NMR data. 

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