Photosynthetic reaction center electron-transfer rates

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. 

Study the AG° dependence at different temperatures. This was done by Dutton and co-workers for the electron transfers from Bph to and Qa to (Bchl)2 in Rhodobacter sphaeroides photosynthetic reaction centers [139,140], which take place over about 13 A and 25 A, respectively [18], The rates were measured between 10 K and 300 K in series in which quinone substitutions provide AG° ranges of 0.5 eV and 0.8 eV for the two reactions respectively. The following conclusions were deduced from a thorough analysis of the experimental results ... 

A microscopic theory for describing ultrafast radiationless transitions in particular for, photo-induced ultrafast radiationless transitions is presented. For this purpose, one example system that well represents the ultrafast radiationless transaction problem is considered. More specifically, bacterial photosynthetic reaction centers (RCs) are investigated for their ultrafast electronic-excitation energy transfer (EET) processes and ultrafast electron transfer (ET) processes. Several applications of the density matrix method are presented for emphasizing that the density matrix method can not only treat the dynamics due to the radiationless transitions but also deal with the population and coherence dynamics. Several rate constants of the radiationless transitions and the analytic estimation methods of those rate... 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

Figure 4-7. Electronic factors in the rate constant calculated for the electron transfers in the bacterial photosynthetic reaction centers of (a) Rhodopseudomonas viridis, and (b) Rhodobactor sphaeroides...

FIGURE 7. Two redox cofactor chains meet at the bacteriochlorophyl dimer in the photosynthetic reaction center of Rp. viridis. Electron transfer takes place by tunneling between cofactors diat are spaced by no more dian 14, assuring overall elech on transfer rates in the msec or faster range, even though a total distance of 70 is crossed by the c heme chain. 

Gudowska-Nowak, E., 1994, Effects of heterogeneity on relaxation dynamics and electron transfer rates in photosynthetic reaction centers. J. Phys. Chem., 98 5257n 5264. 

Maximum electron-transfer rate ( max) vs. edge-to-edge distance (d) for proteins. Photosynthetic reaction center rates are shown as circles and ZnP to rates in modified myoglobins and cytochromes c are shown as triangles. Adapted from Reference 80. 

Electron Transfer Rates in Bacterial Photosynthetic Reaction Centers"... 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

The recent determination of the crystallographic structure of a bacterial photosynthetic reaction center has provided an important framework for treating electron transfers in these systems. As in electron transfers in simpler systems, a knowledge of both the structure and the thermodynamics is needed for understanding the experimental results on reaction rates at the fundamental level. In this lecture we first describe the theory of simple electron transfer reaction rates, and then consider how the resulting interaction between theory and experiment may assist us in treating electron transfers in the photosynmetic reaction center. 

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