Reaction center . bacterial

Photoreactions that produce chemical energy by excitation of BChl or Chi molecules take place in RCs. The process is referred to as the primary charge separation. Purple bacteria use a type of photosynthesis that, to some extent, resembles green plant photosynthesis in PSll. In the 1980s, two purple bacteria, Rhodopseudomonas viridis and Rhodobacter sphoeroides, reached a prominence that few had expected from species living at the bottom of ponds and similar places. Two German chemists, Johann Deisenhofer and Hartmut Michel, managed to dissolve the protein from the membrane, crystallize it, and determine its structure. 

Earlier time-resolved spectroscopic work had allowed time constants to be determined. In 1978, the Russian photochemist V. A. Shuvalow determined a time constant for charge separation. At this time, no details were known about the geometry of the system where this photoreaction takes place. 

The first determined stracture of an RC was thus the bacterial one. In fact, this was the first membrane protein whose structure could be determined. The electron is transported in the RC with the help of chromophores of the same type used in the antenna system. The free energy is used to build up a proton gradient across the membrane. 

The RCs of the purple bacteria, R. viridis and Rb. sphaeroides, turn out to be similar. In the former case, a cytochrome subunit is accompanying the subunits L, M, and H, which contain the chromophores. The cytochrome subunit contains four heme groups organized in such a way that the electrons can transfer through these heme groups and reduce the photoxidized chromophores. In other words after excitation, there is a hole in the highest occupied molecular orbital (HOMO) and this hole is filled by an electron coming in from the cytochrome system. 

The prosthetic chromophore groups show a high degree of local two-fold symmetry with the symmetry axis perpendicular to the membrane plane. It has been possible to determine that ET takes place in only one of the two almost equivalent sides, from the SP, via an accessory BChl to BPh to a quinone on the active side. Index A is used for the active side and B for the inactive side. The A chromophores belong to the L subunit and the B chromophores to the M subunit. 

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BittI R, van der Est A, Kamlowski A, Lubitz W and Stehlik D 1994 Time-resolved EPR of the radical pair bacterial reaction centers. Observation of transient nutations, quantum beats and... 

Vos M H, Jones M R, Hunter C N, Breton J and Martin J-L 1994 Coherent nuclear dynamics at room temperature in bacterial reaction centers Proc. Natl Acad. Sci. USA 91 12 701-5... 

Jean J M, Chan C-K and Fleming G R 1988 Electronic energy transfer in photosynthetic bacterial reaction centers Isr. J. Chem. 28 169-75... 

Figure 12.13 Photosynthetic pigments are used hy plants and photosynthetic bacteria to capture photons of light and for electron flow from one side of a membrane to the other side. The diagram shows two such pigments that are present in bacterial reaction centers, bacteriochlorophyll (a) and ubiquinone (b). The light-absorbing parts of the molecules are shown in yellow, attached to hydrocarbon "tails" shown in green.

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

Ceccarelli, M. Marchi, M., Simulation and modeling of the Rhodobacter spaeroides bacterial reaction center, J. Phys. Chem. B 2003,107, 1423-1431... 

The spectroscopy and dynamics of photosynthetic bacterial reaction centers have attracted considerable experimental attention [1-52]. In particular, application of spectroscopic techniques to RCs has revealed the optical features of the molecular systems. For example, the absorption spectra of Rb. Sphaeroides R26 RCs at 77 K and room temperature are shown in Fig. 2 [42]. One can see from Fig. 2 that the absorption spectra present three broad bands in the region of 714—952 nm. These bands have conventionally been assigned to the Qy electronic transitions of the P (870 nm), B (800 nm), and H (870 nm) components of RCs. By considering that the special pair P can be regarded as a dimer of two... 

In the study of the ultrafast dynamics of photosynthetic bacterial reaction centers, we are concerned with the photoinduced electron transfer [72]... 

Figure 3. The bacterial reaction-center protein model from Rhodopseudomonas sphaeroides the structure and positioning of components are highly speculative.

B. A. Heller, D. Holten, C. Kirmaier, Effects of Asp Residues near the L-Side Pigments in Bacterial Reaction Centers , Biochemistry 1996, 35,15418-15427. 

As pointed out in Ref. [4], no entropy variation appears in the description given by the harmonic model, apart from the weak contribution arising from the frequency shifts of the oscillators. The applications of this model are then a priori restricted to redox reactions in which entropic contributions can be neglected. We shall see in Sect. 3 that the current interpretations of most electron transfer processes which take place in bacterial reaction centers are based on this assumption. 

Some authors have described the time evolution of the system by more general methods than time-dependent perturbation theory. For example, War-shel and co-workers have attempted to calculate the evolution of the function /(r, Q, t) defined by Eq. (3) by a semi-classical method [44, 96] the probability for the system to occupy state v]/, is obtained by considering the fluctuations of the energy gap between and 11, which are induced by the trajectories of all the atoms of the system. These trajectories are generated through molecular dynamics models based on classical equations of motion. This method was in particular applied to simulate the kinetics of the primary electron transfer process in the bacterial reaction center [97]. Mikkelsen and Ratner have recently proposed a very different approach to the electron transfer problem, in which the time evolution of the system is described by a time-dependent statistical density operator [98, 99]. 

It is evident that the preceding considerations do not apply to all biological electron transfer systems. Even in the bacterial reaction center, the transfer between the two quinones Qa Qbj which takes place over 18 A [18], is characterized in Rhodobacter sphaeroides by a large entropic contribution, which has been attributed to the high solvent exposure of Qg [126]. By using the activation energy value reported in Ref. [126], two very different X values may be deduced from Eq. (23) = 0.1 eV and Aj = 2.5 eV. The previous considerations... 

Fig. 4. Schematic representation of the bacterial reaction center (R. sphaeroides). The two branches are noted A, B in studies on R. viridis. Center-to-center distances are reported from Refs. [18, 21], The approximate position of the cytochrome is indicated. The simplified notations P, B, H are used in the text...

A recent study (Booth PJ, Crystall B, Giorgi LB, Barber J, Klug DR, Porter G (1990) Biochim. Biophys. Acta 1016 141) has shown that the free energy difference of the primary electron transfer is dominated by entropic contributions in photosystem II reaction centers as in bacterial reaction centers (Woodbury NWT, Parson WW (1984) Biochim. Biophys. Acta 767 345), so that the interpretation of the rate temperature dependenee should be revised. 

Until a recent x-ray diffraction study (17) provided direct evidence of the arrangement of the pigment species in the reaction center of the photosynthetic bacterium Rhodopseudomonas Viridis, a considerable amount of all evidence pertaining to the internal molecular architecture of plant or bacterial reaction centers was inferred from the results of in vitro spectroscopic experiments and from work on model systems (5, 18, 19). Aside from their use as indirect probes of the structure and function of plant and bacterial reaction centers, model studies have also provided insights into the development of potential biomimetic solar energy conversion systems. In this regard, the work of Netzel and co-workers (20-22) is particularly noteworthy, and in addition, is quite relevant to the material discussed at this conference. 

J. Breton and A. Vermeglio (eds.), The Photosynthetic Bacterial Reaction Center -Structure and Dynamics , Plenum Press, New York and London, 1988. 

Zheng, Z., Dutton, P.L. and Gunner, M.R. (2010) The measured and calculated affinity of methyl- and methoxy-substituted benzoquinones for the Q(A) site of bacterial reaction centers. Proteins Struct. Fund. Bioinform., 78 (12), 2638-2654. 

Fig. 1. A. Noise level expressed in milli optical density, obtained after 1 minute of data acquisition. B. Time dependent absorption change of the keto group of the primary donor of the bacterial reaction center, at 1685 cm 1 and 1715 cm 1 upon excitation at 600 nm, noise level 30 pOD, measured in the Lissajous scanner. The solid line through the data points is a fit with = 3.8 ps, t2 = 16 ps, t3 = 4 ns and t5 = oc. The time scale is linear up to 3 ps and logarithmic thereafter.

In Purple Bacterial Reaction Centers, Electrons Move from P870 to Bacteriopheophytin and Then to Quinones... 

Although the structures of the plant reaction centers are not yet known in detail, photosystem II reaction centers resemble reaction centers of purple bacteria in several ways. The amino acid sequences of their two major polypeptides are homologous to those of the two polypeptides that hold the pigments in the bacterial reaction center. Also, the reaction centers of photosystem II contain a nonheme iron atom and two molecules of plastoquinone, a quinone that is closely related to ubiquinone (see fig. 15.10), and they contain one or more molecules of pheophytin a and several... 

The electron acceptors on the reducing side of photosystem II resemble those of purple bacterial reaction centers. The acceptor that removes an electron from P680 is a molecule of pheophytin a. The second and third acceptors are plastoquinones (see fig. 15.10). As in bacterial reaction centers, electrons move one at a time from the first quinone to the second. When the second quinone becomes doubly reduced, it picks up protons from the stromal side of the thylakoid membrane and dissociates from the reaction center. 

Fig. I. Arrangement of the chromophores, electron donors, and electron acceptors in the bacterial reaction center of Rhodobacter sphaeroides [2f], The horizontal lines at the top and bottom of the figure represent the approximate location of the surfaces of the lipid bilayer membrane...

The abundance of cyclic tetrapyrroles in the bacterial reaction center naturally suggests the preparation of triads bearing two porphyrin moieties. A number of such systems have been described. Building upon their earlier work with D-A type systems [19, 66], Sanders, Beddard and coworkers have reported triad 14, which consists of two doubly-linked porphyrins, one of which bears a pyromellitimide acceptor [66]. The pyromellitimide moiety is nearly as good an electron acceptor as benzoquinone (E0 = —0.55 V vs. —0.51 V for benzoquinone). Triad 14,... 

Kirmaier, C. Holten, D. In The Photosynthetic Bacterial Reaction Center-Structure and Dynamics, Breton, J., Vermeglio, A., Eds. Plenum New York, 1988 p 219. 

Fig. 1.6. Photosynthetic bacterial reaction center for Rsp. viridis. The chromophores are indicated but not the protein part of the structure, helices, etc., holding the whole unit...

One of the enigmatic problems of photsynthesis is the drastic difference between the rate of photelectron transfer in the active (M) and inactive branches of bacterial reaction centers. The quantum mechanical calculation (Kolbasov and Scherz, 2000) showed that the square of electronic matrix element VA2 for the electron transfer from the excited primary donor, P, to bacteriochlorophyl in the active brunch is larger by three order of magnitude than that in the inactive part Vb2. Therefore, the electron transfer rate in the RC inactive L-brunch should be essentially slower than that in the M-brunch. 

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