Electron transfer rates protein dynamics

Thus the studies of the QA-involved electron tunneling reactions have shown that analysis of the reaction rate vs — AG° dependences provides information about the character of nuclear motions coupled to electron transfer in RC of photosynthetic bacteria. In particular, as kT becomes smaller than ha the dependence of electron transfer rate on — AG° becomes sensitive to the magnitude of ha. In other words, it is possible to find out from these experiments what aspects of protein and cofactor dynamics are important for the reaction. 

The complete analysis of electron transfer is typically performed in two steps. First the electronic coupling is calculated for a given nuclear configuration of the protein and then the dynamics is included and the averaging and other dynamics effects are incorporated into calculations of electron transfer rates. Various theories that combine pure electronic and dynamic effects are then utilized to calculate the rate of electron transfer [44,52,53]. In recent years, a number of theories and computational methods for electron tunneling in proteins have been developed and reviewed extensively. Of particular interest is the work of the Beratan and Onuchic groups, who pioneered the area of tunneling pathways [54]. 

The fact that the singlet-triplet mixing in radical pairs becomes faster at high fields, due to the increase of the Zeeman interaction, can also permit modelling of the sequential electron-transfer process of both the primary and secondary pairs. The importance of protein dynamics on the electron-transfer rate was noted in a 95 GHz study of bacterial photosynthetic reaction centres with slow electron-transfer rates. ... 

The first protein associations to be studied by Brownian dynamics simulation with detailed protein models are those of cytochrome c with its electron transfer partner cytochrome c peroxidase. Simulations of the association of cytochrome c and cytochrome b have also been performed. From these simulations, electron transfer rates are computed by coupling descriptions of the electron transfer event to diffu-sional encounter trajectories of the proteins. Hiese simulations have shown that ionic-strength dependencies can be reasonably reproduced when a realistic model is employed with a heterogeneous dielectric function, that qualitative insights into the effects of mutations can be obtained, and that... 

The standard formalisms for describing ET processes assume that in reactions such as Eqs. (1) and (2) there is but a single stable conformational form for each of the precursor and successor electron-transfer states. However, for a system that displays two (or more) alternative stable conformations with different ET rates, dynamic conformational equilibrium can modulate the ET rates. Major protein conformational changes can occur at rates that are competitive with observed rates of ET [9], and such gating [10] may occur in non-rigid complexes such as that between zinc cytochrome c peroxidase (ZnCcP) and cytochrome c (see below) or even within cytochrome c [5]. 

To investigate the effect of a protein on electron transfer and the energy conversion, the dual probes (R ) were incorporated to the hydrophobic pocket obovin serum albumin (Rubtsova et al., 1993 Vogel et al., 1994 Likhtenshtein, 1996 Lozinsky et al., 2001). Experimental temperature dependence on the rate constant of photoreduction kpr was found to be similar to that in the above-mentioned solvent. Values estimated from experiments of parameters of local molecular dynamics with the correlation frequency at... 

A eomparison of the calculated vs. experimental AG optimized tunneling rates for the productive charge separating electron transfers in two bacterial photosynthetic reaction centers shows that rate estimates have a standard deviation of 0.5 log units, or about a factor of 3 (Figure 5). Considering the experimental errors of determining AG and especially X, or even the uneertainties in R of a dynamic protein, it is not clear that a calculation any more involved than the one we have just described is usually justified. 

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