Rate constants photosynthetic reaction center

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... 

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...

Bacterial photosynthetic reaction centers (PRC) have been among the most actively studied ET proteins since DeVault and Chance first measured C. vinosum tunneling rates in the early 1960s. In many cases, measurements of ET kinetics preceded determination of the three-dimensional structure of the membrane-bound protein assembly. It was not until the X-ray crystal-stracture determinations of the Rhodopseudomonas (Rps.) viridus and Rhodobacter (Rb.) sphaeroides PRCs that distances could be assigned to specific rate constants. The recent crystal structures of photosystems l and from cyanobacteria promise to clarify critical aspects of the ET mechanisms in oxygenic PRC. ... 

Boxer s group [2] first made a ns-laser photolysis apparatus with a super-conducting magnet. The sample was excited at 532 or 600 nm with a frequency-doubled YAG pumped dye laser (8ns, fwhm) and was probed at 860 nm with a laser diode. The maximum field of their magnet was 5 T. With this apparatus, they measured the quantum yield of triplet states (detected optically in quinone-depleted photosynthetic reaction centers (RCs) from R. spheroids, R-26 mutant, as a function of applied magnetic strength and temperature. The reaction scheme for qinone-depleted RCs is shown in Fig. 12.1. Here, the singlet and triplet radical-ion pair (RIP) are represented by [D" A ] and [D A ], respectively, and the rate constants of the S-T conversion of RIP, the recombination from [D A ], and the recombination from [D A ] are denoted by hsT, ks, and kj, respectively. 

Figure 9.7. The rate constant fegy as a function of the applied electric field for the initial stage of charge recomhination between the oxidized special pair donor and the reduced ubiquinone acceptor in bacterial photosynthetic reaction centers of Rb. sphaeroides at 80 K. (Reproduced from [243a] with permission. Copyright (1990) by the American Chemical Society.)...

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 electron-hole recombination processes in the ion radical pairs P+B"H and P+BH across the A branch of the bacterial photosynthetic reaction center (RC) are taking place in the same molecular framework in which the primary charge separation occurs. Consequently, the rate constants for the recombinations are correlated to the rate constants of the primary processes, and their investigation can help in the elucidation of the primary charge separation mechanism. 

Fig. 17.10. Dependence of maximum rate constant of ET on the edge-to-edge distance in photosynthetic reaction centers of bacteria and plant photosystem. The strai t line is related to the dependence of the attenuation parameter for spin exchange (Yse) in homogeneous non-conducting media. Filled circles (Likhtenshteln, G.I., J. Phochem. Photobiol. A Chem. 96, 79-92.1996). Reproduced with permission.

We now summarize in Fig. 11 the reaction-center structure and the known electron-transport reactions in purple bacteria. A simplified representation of the reaction-center and the light-harvesting complexes contained in the bacterial membrane is shown in Fig. 11 (A), followed by a column model and a cofactor model in Fig. 11 (B). The cofactor model is used to illustrate the various electron-transport steps with the associated rate constants in Fig. 11 (C), where the cofactors in the starting state (oxidized or reduced) are shown in solid black. When a cofactor first becomes reduced or oxidized, it is shown as an open symbol. We will also use this cofactor model and reaction sequence as a framework for introducing the remaining chapters throughout the section on photosynthetic bacteria. 

We have shown that the heterogeneity of PS II reaction centers is essential for explaining picosecond fluorescence kinetics data of intact photosynthetic systems. For the first time differences between a- and p-centers have been reported, concerning their primary photophysics. The significant differences in the rate constants and related parameters between PS II and PS Up can not be fully explained only with their difference in antenna size (8) and dieir difference in membrane potential (9). Of course even more complex models to fit the fluorescence decay data cannot be excluded. 

---