Nonexponential kinetics

It is generally understood that the reactive intermediates are generated in a random distribution of different microenvironments, each with its own energy barrier. The complex decay of this dispersion of rates leads to the nonexponential kinetics. Thus, disappearance plots are dominated at early times by reaction of those species in fast sites , which have lower energy barriers. As these sites are cleared, the distribution of rates over time becomes more reflective of sites with higher barriers. Finally, at longer times, the decay curves are dominated by the slowest sites. It is often observed that plots of In [intensity] versus or are approximately linear. It... 

In photosynthetic systems, some electron transfer processes exhibit nonexponential kinetics at low temperature, which are generally attributed to the existence of different conformations of the system. While the differences between the reaction rates corresponding to these conformations do not exceed a factor of four in some cases [157,158,159], they are sufficient to lead to different quantum yields in others [160, 161]. Sometimes, the heterogeneous character of the kinetics disappears at room temperature, which probably reflects a fast exchange between the conformations that are frozen at low temperature [157, 158]. A systematic study of all these effects, similar to that performed in Ref [159], could give useful information about the nature of the conformational differences. 

In the context of chemical reactions that are subject to dispersive kinetics as a result of structural disorder, the above model suggests that a widening of the intermediate region between the Arrhenius law and low-temperature plateau should occur. The distribution of barrier heights should also lead to nonexponential kinetic curves (see Section 6.5). 

The excited-state decay kinetics of Ru(bpy)2(dcb) + -Ti02 immersed in neat acetonitrile, probed by transient absorption spectroscopy, exhibited nonexponential kinetics. By minimizing the excitation irradiance, near exponential kinetics were observed for excited-state decay. However, at high excitation irradiance, second-order kinetics were found to fit the experimental data well. These observations are consistent with competitive first- and second-order processes attributed to radiative and nonradiative excited-state deactivation, Eq. 21, proceeding in parallel with excited-state annihilation, Eq. 22 ... 

The reason for the nonexponential kinetics of solid-state chemical reactions lies in the existence of the rate constant distribution determined by the set of different configurations of the reactants, incommensurate to the solid lattice (see, e.g., ref. 177). In the case of low-temperature reactions the existence of the set of configurations can be phenomenologically accounted for by introduction of the equilibrium distance distribution R. As shown in the literature [138, 178, 179], introduction of this distribution into the discussed model of low-temperature reactions enables us to quantitatively describe the... 

Note that the long-time rate and the mean rate differ by a factor of in 2 0.7. This discrepancy indicates that in general the mean reaction rate can be a poor characteristic of nonexponential kinetics. For activated reactions, the kinetics is practically exponential and the mean reaction rate coincides with the long-time rate. Note also that for a reversible reaction, the decay is usually normalized with respect to the stationary population, P co) = [1 + exp(AG)] ... 

Experimental data on distance dependence continue to be gathered from studies of the nonexponential kinetics observed in rigid media and a new method has recently been claimed, based on the simultaneous analysis of kinetic and ESR data. The major development in recent years, however, has been the study of unimolecular electron transfer rates in specially synthesized binuclear complexes of known structure. Early work mostly involved systems with nonrigid, or not quite rigid, bridging groups, so that some doubt remained as to the operative electron transfer distance. In recent work this limitation has been removed in... 

Figure 3 Two-state (D = F) versus three-state (D = I = F) folding, (a) Free energy surface for a two-state folder as a function of the reaction coordinate q. (b) Free energy surface for a three- state folder as a function of the reaction coordinate q. An additional minimum corresponding to an intermediate state I is present, (c) Single exponential kinetics of folding for a two-state folder, (d) Nonexponential kinetics of folding for a three-state protein, (e) Linear chevron plot for a two state folder, (f) Chevron plot with rollover for a three-state folder.

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