Distance dependence electron transfer

Using mutant proteins as well as a variety of redox pairs and electron-transfer distances the validity of the Marcus equation with respect to the thermodynamic driving force and distance dependence has been verified.153 This is even true for cytochrome c mutants functioning in living yeast cells.146... 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

For the electron transfer reactions to amieyanin from the 0-quinol, 0-semiquinone and N-semiquinone, Fej is the slowest step in the overall reaction (Bishop and Davidson, 1998). The rates of these reactions vary predictably with driving force (i.e. the redox potential difference between the reactants) (Figure 9). Independent analyses by Eqs. 4 and 5 of the AG and temperature dependencies of the electron transfer rate of each of these reactions (Brooks and Davidson, 1994ab Bishop and Davidson, 1998) yielded identical values of k and H b, and predicted the electron transfer distance that is seen in the crystal structure. Furthermore, the rates of each of these reactions were relatively insensitive to pH and buffer composition (Table 1), as would be expected for an electron transfer reaction occurring within a protein complex. 

The electron transfer reaction from copper to heme within the ternary protein complex was also studied in solution by stopped-flow spectroscopy. Analysis by Marcus theory of the temperature dependence of the limiting first-order rate constant for the redox reaction (Davidson and Jones, 1996) yielded values for the of 1.1 eV and H b of 0.3 cm , and predicted an electron transfer distance between redox centers which was consistent with the distance seen in the crystal structure. Thus, the electron transfer event is rate-limiting for this redox reaction. Experiments are in progress to determine the validity of the predicted pathways for electron transfer shown in Figure 7. 

A simple theoretical approach has been developed to simulate the decay of a donor excited state in the presence of electronic energy transfer in polymer coils with arbitrary degrees of non-nearest neighbour, cross-chain transfer and transfer distance dependencies Excimer fluorescence from pyrene end groups... 

N. Koga, K. Sameshima, K. Morokuma, Ab initio MO calculations of electronic coupling matrix elements on model systems for intramolecular electron transfer, hole transfer, and triplet energy transfer distance dependence and pathway in electron transfer and relationship of triplet energy transfer with electron and hole transfer, J. Phys. Chem., 1993, 97, 13117-13125. 

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... 

However, the probability of electron transfer (tunneling) depends critically on the distance between the species participating in the electron exchange reactioa A reaction can take place between two molecules when they meet each other. It follows that the rate-determining step can be either the mass transport (mostly diffusion is considered, but effect of migration cannot be excluded) or the reaction (the actual rate of electron transfer in our case). For an electron exchange process coupled to isothermal diffusion, the following kinetic scheme may be considered ... 

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