Conformational interconversion kinetics

This observation indicates that the multiple phases do not arise from static non-interconverting forms of the complex, and that they can best be interpreted by a kinetic scheme that involves dynamic conformational interconversion between conformational substates within state I. The analysis indicates that the... 

Clearly, the data contain information about both the equilibrium constant and the rate constants for the conformational interconversion. In this instance, the quantitative analysis was based upon the cyclic voltammetric data. The points in Figure 16.3 are the background-corrected experimental data, and the curves were computed by digital simulation with values of the equilibrium and rate constants selected to achieve best agreement with the experimental data. A given set of parameters was found to account for the data at a variety of scan rates, a necessary condition if the kinetic model is to be judged adequate. 

Variable-temperature NMR spectroscopy is a well-established technique in the study of conformational exchange kinetics in liquid amides. [7,58] Using DNMR spectroscopy, it is possible to measure the temperature dependence of interconversion rates in order to yield a complete set of kinetic parameters characterizing the internal rotation about the carbon-nitrogen bond. In the last several years, the development of gas-phase NMR methods has provided a valuable tool for the elucidation of the magnitude of intrinsic and environmental contributions associated with this conformational exchange. In order to characterize these contributions, a series of molecules with systematic variations in substituent size and polarity must be analyzed both in the liquid and gas phase. 

The kinetic processes associated with the conformational interconversions of two conformers of equal energy of [Mo204EDTA] " have been studied and the enthalpy of activation calculated to be 54 kJ moP. The rather high AH is accounted for by the semi-rigid nature of the glycinate chelate rings during the inversion process. 

As an example of the derivation of kinetic equations we will consider the hydrogenation of butadiene (J.Goetz, R.Touroude, D.Yu. Murzin, Kinetic aspects of selectivity and stereoselectivity for the hydrogenation of buta-1,3-diene over a palladium catalyst, Ind. Eng. Chem. Res., 35 (1996) 703). The overall reaction network, which was used for kinetic modeling is presented in Figure 4.2. In this scheme the addition of hydrogen to anti and syn - adsorbed diene molecules is assumed, producing but-l-ene trails- and cis but-2-ene are formed from anti and syn adsorbed diene respectively. There could also be a conformational interconversion of adsorbed buta-1,3-diene. 

Detailed experimental data on the rate constants associated adsorption/desorption kinetics or conformational interconversion of different forms of a protein chromatographed on -alkylsilicas are currently very sparse. The kinetics of de-naturation of several proteins on n-butyl-bonded silica surfaces have been reported. Fig. 18 for example, shows the dependence of peak area on the incubation time of lysozyme on the bonded phase surface, from which rate constants for interconversion on the stationary phase, i.e. were derived [63]. The graphical representations derived from quantitative numerical solutions of the probabihty distributions... 

The n-site Bloch-McConnell equations describe the evolution of nuclear spin magnetization in the laboratory or rotating frames of reference for molecules subject to chemical or conformational interconversions between n species with distinct NMR chemical shifts. Trott and Palmer used perturbation theory to approximate the largest eigenvalue of the Bloch-McConnell equations and obtain analytical expressions for the rotating-frame relaxation rate constant and for the laboratory frame resonance frequency and transverse relaxation rate constant. The perturbation treatment is valid whenever the population of one site is dominant. The new results are generally applicable to investigations of kinetic processes by NMR spectroscopy. 

These kinetic parameters are very little affected by solvent composition over the range 0 to 35% dioxan/ Activation parameters AH and have been reported for conformational interconversion of the chelate ring in the [Fe(CN)4(l,3-propanediamine)] anion AH is 7.5kJmol higher here than for the analogous 1,2-ethanediamine anion/ ... 

Inversion at the N center is coupled to conformational changes in a chelate ring. The kinetics of inversion at asymmetric N centers in complexes of tetraaza linear or macrocyclic ligands have received scant attention. There are five configurational isomers of the planar complex Ni([14]aneN4) +, Sec. 3.1.1. The interconversions between such structures are base catalyzed with second-order rate constants covering a small range from 1.2 x 10 to 2.4 x 10 M- s- Refs. 108-110. 

Kinetic information from the line width alterations of EPR spectra by changing the temperature has been obtained for a number of silacycloalkyl radicals [40,41]. For example, silacyclopentyl radical exists at low temperature (—119°C) in two equivalent twist conformations (11 and 12), which interconvert at higher temperature (15 °C). The Arrhenius parameters for such interconversion are logyl/s = 12.0 and = 21.3 kJ/mol. 

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