Nonexponential kinetics, electron-transfer

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. 

The excited-state kinetics of the chromoprotein were found to differ markedly from the one of the isolated chromophore in solution. A strong and fast biexponential decay is observed and seems to sign a specific deactivation channel, still to be properly identified. This process might well be an electron transfer from the chromophore to the protein, as earlier works had suggested [12]. It is additionally possible to suggest that the nonexponential nature of the fast decay could reveal a structural heterogeneity in the oxyblepharismin-protein complex. 

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... 

On the basis of a nonadiabatic electron-transfer theory, which exposes the homogeneous width of the nuclear factor from low frequency modes (phonons), and hole burning data we conclude that this nonexponentiality is not due to a distribution of values, f, for the relevant adiabatic electronic energy gap(s) 2. Dispersive kinetics from f in the low temperature limit are judged to be unlikely. Nevertheless, the expression (. 2) for the average electron-transfer rate constant suggests that samples which exhibit sufficiently different Fj-values for the P-band should have measurably different values for in... 

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