Reaction centers 1962 Volume

Boichenko, V.A., Hou, J-M., and Mauzerall, D. (2001) Thermodynamics of electron transfer in oxigen photosynthetic reaction centers volume change, enthalpy and entropy of electron-transfer reactions in intact cells of the cyanobacterium Synechocystis PCC 6803, Biochemistry 40, 7126-7132. 

Two other steric parameters are independent of any kinetic data. Charton s v values are derived from van der Waals radii/ and Meyer s values from the volume of the portion of the substituent that is within 0.3 nm of the reaction center. The V values are obtained by molecular mechanics calculations based on the structure of the molecule. Table 9.7 gives v and value.s for some groups. As can be seen in the table, there is a fair, but not perfect, correlation among the Ei, v, and values. Other sets of steric values, (e.g., and have also been proposed. ... 

Steric repulsions come from two orbital-four electron interactions between two occupied orbitals. Facially selective reactions do occur in sterically unbiased systems, and these facial selectivities can be interpreted in terms of unsymmetrical K faces. Particular emphasis has been placed on the dissymmetrization of the orbital extension, i.e., orbital distortions [1, 2]. The orbital distortions are described in (Chapter Orbital Mixing Rules by Inagaki in this volume). Here, we review the effects of unsymmetrization of the orbitals due to phase environment in the vicinity of the reaction centers [3]. 

Figure 24. A relationship between free volume and the feasibility of a reaction in an organized media. Filled areas correspond to the shapes and sizes of reactants and products. Note the shape changes between the reactant and the product. Free volume around a reaction center is represented as unfilled regions. In all three cases shown here the total free volume present is much larger than needed for a reaction to occur, but it is not present at the correct location. Importance of location and directionality of free volume highlighted.

The simple geometric metering scheme shown in Fig. 11.4 has been used to develop a highly efficient microfluidic device for protein crystallization in ultra-small volume reactions. The crystallization chip implements 144 simultaneous metering and mixing reactions while consuming only 3.0 pL of protein solution. A layout of the chip (Fig. 11.5) shows 48 reaction centers (Fig. 11.4), each consisting of three pairs of microfluidic reaction chambers with relative volumes of 1 4, 1 1 and 4 1. 

DiMagno, T. J., and Norris, J. R., 1993, Initial electron transfer events in photosynthetic bacteria. In The Photosynthetic Reaction Center, (J. Deisenhofer and J. R. Norris, eds.) Volume 2, pp. 1059132, Academic Press, San Diego, USA. 

At pH 5, which is a reasonable estimate of the thylakoid internal volume pH, the water/oxygen couple has an oxidation-reduction potential of -t-0.93 V. The energy required to drive this reaction is supplied by photon absorption in photosystem II which produces the oxidized reaction center chlorophyll, P-680 ... 

Photon absorption in the PS II/OEC leads to charge separation in the PS II reaction center to generate the oxidized reaction center, P-680 (Ref. 18 Chapter 4, this volume.) The simplest scheme for subsequent electron transfer steps involves only the intermediate carrier, Z, and the Mn ensemble at the water-splitting site ... 

Nevertheless, other chromophores have been investigated and they have provided interesting insights, particularly porphyrin and Cso groups, since these serve as useful mimics of the cofactors present in the photosynthetic reaction center (Figure 37). Electron transfer involving porphyrins and fullerenes will be presented in more detail elsewhere in this Handbook Volume III, Part 2, Chapter 2 and Volume II, Part 1, Chapter 5 respectively), and so only a brief discussion is presented here. An excellent overview of photoinduced ET processes in Cso-based multichromophoric systems has been produced previously [116]. 

The Topological Steric Effect Index (TSEI) was proposed to describe the steric effects of substituents in terms ofthe relative specific volume ofthe reaction center screened by the atoms... 

Figure 4.23. Cross-over in reaction efficiency as a function of system geometry for M X M X N lattices. The vertical axis calibrates the eccentricity s = N/M and the horizontal axis calibrates the surface-to-volume ratio S/V (see text). To the right of the hatched area, random d = 3 diffusion to an internal, centrosymmetric reaction center in the compartmentalized system is the more efficient process. To the left of the hatched area, reduction of dimensionality in the d = 3 flow of the diffusing coreactant to a restricted d = 2 flow upon first encounter with the boundary of the compartmentalized system is the more efficient process. The lines delimiting the hatched region give upper and lower bounds on the critical crossover geometries.

This volume grew out of an American Chemical Society (ACS) symposium titled Bioenergetics. The ACS Division of Computers in Chemistry sponsored the symposium, whose goal was to bring together scientists from different disciplines to discuss current achievements and future directions in molecular-level simulations of electron and proton transfer. This volume provides a sampling of recently developed simulation methods, as well as their applications to prototypical biochemical systems such as the photosynthetic reaction center and bacteriorhodopsin. 

If the peptide aggregate is envisioned as a molten globule, one plausible mechanism (Marshall, unpublished Figure 19) for gating of a pore formed by a helical bundle embedded in the membrane is a simple reorientation of helices. a-Helices have orientations of side chains that favor nonparallel association with an angle of approximately 25° between helix axes to pack optimally (see discussion and references cited in Chou et al. (296)) as seen in the transmembrane helices of the photosynthetic reaction center (252) and of bacteriorhodopsin (297). The surface tension of the membrane tends to minimize the volume of the helical bundle. The gating potential could simply be sufficient to force the helical elements to align with the potential field with a concommitant separation of helices due to the increased steric interaction... 

Fig. 2. (A) Absorption spectra of photosynthetic reaction centers from Rhodobacter sphaeroides at low (solid line) and high light intensity (dashed line). All spectra were recorded with an integration time of 5 msec. (B) Time course of the bleaching under the lowest useful conditions. Compare the time scale to typical HPLC conditions at a flow rate of 1.5 ml/ min, and 8 /xl cell volume, the sample spends 330 msec in the cell of the detector.

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