Long-range ET in proteins

There is now a well-defined theory-based method for separating these two factors By using suitable substituents, AG° can be varied. Either by extrapolation or otherwise, a condition can be attained where AG° = —A, and then the AG = 0. The resulting rate constant in equation (1.3) then provides directly the value of //daP, after an approximate estimate of A is made from the data on the AG° dependence. 

Comment In this electronic example, the data did prompt the theory but did not pose a paradox in themselves, since off-resonant ET was or could be anticipated. Instead, the problem was how to treat this extremely large electronic system. Ultimately, two ways were found to do this, and comparison of //DA s was then made with the measured ET rates, after an analysis based on equations (1.3) and (1.4) to obtain the //daI s. 

In the late 1940s, the 1950s and later, radioactive isotopic tracers were used extensively to study the mechanistic paths of chemical reactions. One such use was in isotopic exchange reactions [19], such as the reaction, equation (1.5), in an aqueous medium, 

The ET rate was measured by following the appearance of the isotope (the starred atom) in the other valence state, using selective precipitation or other techniques to separate the two redox forms at various reaction times. 

I realized that the answer to this problem was that suitable fluctuations of orientations of the solvent dipoles in the neighborhood of the reactants had to precede the ET. In the TS, the ensemble of configurations of solvent dipoles would then be neither an equilibrium one for the reactants, nor an equilibrium one for the products. Instead, for appropriate configurations of the ensemble of solvent molecules the electron could transfer and satisfy both the Franck-Condon principle and the energy conservation. 

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Long-range ET rates have been measured in c/ccp complexes [73, 74] the reactions between cyt c and Feccp [ES is the oxidation product of Fe(II)ccp and peroxide it has two oxidizing equivalents, namely, Fe(IV)0 and a protein-based organic radical cation] are given in Eq. (3) ... 

Because of the rapid decay of the electronic coupling with distance, very long-distance (>40 A) ET reactions rarely occur in a single step. Instead, extremely long range ET involves an array of multiple redox centers, mobile electron carriers, or large-scale motion of redox-active domains (46). All intermolecular ET reactions require either protein-protein docking or formation of an encounter complex in which the two protein cofactors are... 

Investigations of protein-protein ET reactions have provided important insights into biological electron flow [10-14]. Natural systems, however, often are not amenable to the systematic studies that are required for evaluation of the key ET parameters 2 and Nab- A successful alternative approach involves measurements of ET in metalloproteins that have been labeled with redox-active molecules [15-19]. By varying the binding site and chemical composition of the probe molecule, it has been possible to elucidate the factors that control the rates of long-range ET reactions in proteins. 

The classical theory developed by Marcus and Hush can offer no explanation for this, any more than it can for the occurrence of long-range ET, first observed in an ET protein by Chance and Nishimura (1960) and in a rigid glass by Miller (1975), or the sensitivity of ET rates to the nature of any molecules or molecular groups lying between the donor and acceptor, established by the pioneering work of Taube et al. (1953) and Tanbe and Myers (1954) on the inner-sphere ET reactions of transition-metal complexes. 

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