Electron Transfer

In order to establish a high quantum yield of electron transfer, that is a conversion of nearly every photon of light into a charge- separated state, bacterial reaction centers have a series of closely spaced acceptors [35]. The presence of several acceptors close together allows for a rapid forward electron transfer with a minimum of unproductive charge recombination reactions. The rates ofelectron transfer between these cofactors, as delineated by Marcus and coworkers [53], can be expressed in terms of several factors, including the free energy difference, the reorganization energy, the coupling between the electron donor and acceptor, and protein dynamics.

For most processes, increasing the free energy difference between the final and initial states increases the rate. Experimentally, it can be difficult to measure such energy differences. However, reaction center mutants with different oxidation/ reduction midpoint potentials for the bacteriochlorophyll dimer provide the opportunity to determine the effect of shifting the energies on the electron transfer rates [54]. For example, using transient optical spectroscopy, the charge recombination rates for the primary and secondary quinones were measured and found to increase with increasing free energy differences [55] .

In the Marcus theory, electron transfer rates are exponentially dependent on the difference between the free energy difference and the reorganization energy, which represents the energy needed to change the initial state into the final state. The difference between these two factors is a measure of the activation energy of a process. When the reorganization energy is much larger than the free energy difference, the activation energy is large and the rate is slow. As the free energy difference approaches the reorganization energy, the activation energy decreases and the rate becomes faster. In wild-type reaction centers, the charge recombination rate from the secondary quinone is much slower than that from the primary quinone because the reorganization energy for charge recombination from the secondary quinone is much larger, due to its more hydrophilic environment.

Electron transfer rates are also proportional to the coupling, which is a measure of the interactions between the electron donor and acceptor. While the coupling depends upon a number of factors, the predominant determinant is the distance between donor and acceptor. To avoid the impact of the free energy difference, the maximal rate (which occurs when the difference between the free energy difference and the reorganization energy is zero), is used when comparing rates for different reactions. In proteins, the maximal rates of electron transfer show an approximate exponential dependence upon distance for proteins [56, 57]. For the primary and secondary quinones in the reaction center, the maximal charge recombination rates were estimated to be equal, which is consistent with their nearly identical distances to the bacteriochlorophyll dimer.

The initial rate of electron transfer from the excited bacteriochlorophyll dimer to the bacteriopheophytin has a weak dependence on the free energy difference, with wild- type reaction centers having a small activation energy [ 31]. While the presence of two nearly symmetrical branches would predict comparable couplings, and hence rates along the two branches, in wild-type reaction centers the initial electron transfer is dominantly along the A-branch of cofactors. Key to the directionality is the initial electron transfer of an electron from the excited state of the bacteriochlorophyll dimer preferentially to the A-side bacteriopheophytin in 3 ps. The reason for the functional asymmetry of this initial electron transfer has been investigated extensively using transient optical spectroscopic measurements of wild type and mutant reaction centers [35]. These studies indicate that the energy difference for transfer along the A-branch is more favorable than for the B-branch and that this energy difference can be manipulated by mutagenesis. Although the yield of electron transfer directly along the B-branch is very low in wild type, mutations of the reaction center can increase the direct reduction of QB up to 30% [58, 59], The influence of the protein environment on the initial electron transfer is also illustrated by the observation that charge separation along the A branch or the B branch can be switched by using the pH to tune the ionization state of pro-tonatable residues [60] .

Finally, protein dynamics also play a role in the electron transfer process, although direct measurement of the dynamics has been experimentally difficult. By monitoring the tryptophan absorption change, which is a marker of protein relaxation in the reaction center, the dynamics associated with the initial transfer of the electron from the bacteriochlorophyll dimer to the bacteriopheophytin have been shown to modulate the initial rate of electron transfer [61].

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