Long-range electron transfer rates

Concept The rates of long-range electron transfer (ET) and excitation energy transfer (EET) processes between a pair of chromo-phores (redox couple) may be strongly facilitated by the presence of an intervening non-conjugated medium, such as saturated hydrocarbon bridges, solvent molecules and n-stacks, e.g.,... 

The general framework of the quantum mechanical rate expression for long-range electron transfer processes in the very weak or non-adiabatic regime will be presented in Sect. 2 with an emphasis on the inclusion of superexchange interactions. The relation between the simplest case of direct donor-acceptor interactions, on the one hand, and long-range electronic interactions important in proteins, on the other, is considered in terms of the elements of electron transfer theory. 

The Landau-Zener expression is calculated in a time-dependent semiclassical manner from the diabatic surfaces (those depicted in Fig. 1) exactly because these surfaces, which describe the failure to react, are the appropriate zeroth order description for the long-range electron transfer case. As can be seen, in the very weak coupling limit (small A) the k l factor and hence the electron transfer rate constant become proportional to the absolute square of A ... 

A very practical comprehensive rate expression appropriate for long-range electron transfer in proteins and other large molecules yet which retains ease of computation by anyone and on the smallest computer is obtained by assuming one high frequency harmonic mode (inner sphere reorganization) and one very... 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

Experimental evidence for long-range electron transfer in polypeptides and proteins had been early accrued.The value of using a metal center as a marker is apparent from the above. The approach can be extended to electron transfer between two proteins which are physiological partners. Metal substitution (e. g. Zn for Fe) can be used to alter the value of AG° and permit photoinduced initiation. The parabolic behavior predicted by (5.86) has been verified for the electron transfer rate constant vs AG° within the adduct between cyt c and cyt bj." ... 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

Direct evidence for long range electron-transfer in biological systems was first observed by Gray et al.50,51) and Isied et al.481 using [Ru(NH3)5]3+ substituted metallo protein. Histidine-83 of blue copper (azurin) was labeled with Ru(III)(NH3)5 50). Flash photolysis reduction of the His-83 bound Ru(III) followed by electron-transfer from the Ru(II) to Cu2+ was observed with a rate constant of 1.9 s 1. The result shows that intramolecular long distance (approx. 1 nm) electron-transfer from the Ru(II) to the Cu2 + of the azurin takes place rapidly. 

Figure 42 Stern-Volmer plots for fluorescence quenching of PBAC by Co(phen)3+ in the presence of 0.008% BAZrP. Using 200 nsec as the singlet lifetime of PBAC, the rate constant for quenching is calculated to be 3 X 1012 M-1sec. This is much too fast for a dynamic process and may involve long-range electron transfer. (From Ref. 17. Copyright 1995 Overseas Publishers Association.)...

A study of long-range electron transfer from BPh to QA in both the native and reconstructed RCs was reported in Ref. [259]. The rate of electron transfer from BPh" to QA was determined at 14 K, 35 K, 113 K and 298 K. The majority of quinones used for reconstruction has in situ polarographic midpoints lower than that for UQ10. The electron transfer rate was determined from EPR measurements of the quantum yield of (BChl)2 QA radical-ion pair. 

Many other papers on long-range electron transfer between two reactive sites of modified proteins were published [270-288] after the above mentioned pioneering works. Most of them dealt with photoinduced electron tunneling from triplet states of closed shell Mg(II) and Zn(II) porphyrins to Fe(III) or Ru(III). In agreement with the prediction of Marcus theory the rate constants for the majority of these intraprotein electron transfer reactions were found to increase as the free energy of reaction decreased. However for one of the reactions disagreement with this theory was observed [285],... 

Blue copper proteins are involved in electron transfer (ET) reactions in organisms ranging from bacteria to humans. In order to understand the contribution of the electronic structure to the ET function of these sites, it is useful to examine the three primary terms that contribute to the rate of ET, as described by the semi-classical Marcus equation (1) see also Long-range Electron Transfer in Biology). 

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