Special pair . of bacteriochlorophylls

Fig. 13. Arrhenius plot of k(T) for electron transfer from cytochrome c to the special pair of bacteriochlorophylls in the reaction center of c-vinosum.

Figure 12.21 Schematic diagram of the relative positions of bacteriochlorophylls (green) in the photosynthetic membrane complexes LHl, LH2, and the reaction center. The special pair of bacteriochlorophyll molecules in the reaction center is located at the same level within the membrane as the periplasmic bacteriochlorophyll molecules Chi 875 in LHl and the Chi 850 in LH2. (Adapted from W. Kiihlbrandt, Structure 3 521-525, 1995.)...

Rapid multistep Coulombic energy transfer takes place as the excitation energy is transferred between the antenna chromophores and the special pair of bacteriochlorophyll molecules (P) in the reaction centre. 

Analogies of Rare Earth Porphyrin Doubledeckers with the Special Pair of Bacteriochlorophylls in Bacterial Photosynthesis... 

The energy conversion process begins with the excitation of the special pair of bacteriochlorophyll (Bchl) molecules which are located near the periplasmic side of the membrane. These Bchl species are in van der Waals contact with one another. The excitation may, of course, occur by direct absorption of a photon. More usually it is achieved via singlet-singlet energy transfer from antenna molecules. These antennas have been optimized for maximal absorption of sunlight under the environmental conditions experienced by the organism, and hence vary from species to species. They will be discussed in more detail in Section III. 

Here (BChl)2 stands for the special pair of bacteriochlorophyll molecules, UQ for ubiquinone, and Cyt for the cytochrome protein. Steps 1 and 3 involve excitation of bacteriochlorophyll and transfer of a pair of electrons to a ubiquinone molecule. Steps 2 and 4 restore the special pair to its initial state. Steps 5 and 6 transfer hydrogen ions outside the membrane wall and restore the cytochrome to its reduced form. The net reaction is the light-driven movement of hydrogen ions from inside the cell to outside the cell. 

Figure 19.9 Bacterial photosynthetic reaction center The core of the reaction center from Rhodopseudomonas viridls consists of two simitar chains L (red) and M (blue). An H chain (white) and a cytochrome subunit (yellow) complete the structure. Notice that the L and M subunits are composed largely of helices that span the membrane. Notice also that a chain of electron-carrying prosthetic groups, beginning with a special pair of bacteriochlorophylls and ending at a bound quinone. runs through the structure from top to bottom in this view. [Drawn from IPRC.pdb.]...

The reaction center of purple bacteria contains three protein subunits (L, M, and H) located in the plasma membrane (Figure 8-35). Bound to these proteins are the prosthetic groups that absorb light and transport electrons during photosynthesis. The prosthetic groups include a special pair of bacteriochlorophyll a molecules equivalent to the reaction-center chlorophyll a molecules in plants, as well as several other pigments and two quinones, termed Qa and Qb, that are structurally similar to mitochondrial ubiquinone. 

M subunit (white) each form five transmembrane a helices and have a very similar structure overall the H subunit (light blue) is anchored to the membrane by a single transmembrane a helix. A fourth subunit (not shown) is a peripheral protein that binds to the exoplasmic segments of the other subunits. (Bottom) Within each reaction center is a special pair of bacteriochlorophyll a molecules (green), capable of initiating photoelectron transport two voyeur chlorophylls (purple) two pheophytins (dark blue), and two quinones, Qa and Qb (orange). Qb is the primary electron acceptor during photosynthesis. [After M. H. Stowell etal., 1997, Science 276 812.]... 

The photosynthetic reaction center stores light energy by effecting electron transfer to reduce an electron transfer cofactor and form a proton gradient across the membrane. The arrangement of electron transfer cofactors is indicated in Figure 2 and includes a special pair of bacteriochlorophyll molecules, two accessory bacteriochloroophylls, two bacteriopheophytins, two quinone electron acceptors, and a non-henae iron. The reaction center functions... 

Figure 2. Arrangement of the electron transfer cofactors in the photosynthetic reaction center protein from the bacterium Rhodobacter sphaeroides. The figure shows the special pair of bacteriochlorophylls (top, in green and light blue), two accessory bacteriochlorophyll molecules (dark blue), two bacteriopheophytins (red), the primary quinone (Qa), the secondary quinone (Qb), and the non-heme iron.

Within the bacteria RCs, photoexcitation of the RC drives single electron transfo from the special pair of bacteriochlorophylls to the primary electron acceptor, one of two bacteriopheophytin molecules. The reduced bacteriopheophytin transfers the electron on to a primary ubiquinone-10 acceptor (see figure 1), U(J, to form the semiquinone anion, Then, UQ transfers the electron to a secondary ubiquinone-10, UQ., to form UQ,. After the initial charge separation, the special pair is re-reduced, UQ, accepts a... 

The radical cation P680 + oxidizing potential, recently estimated to be 1.3-1.4 V, exceeds that required for water splitting and is the highest of all types of reaction centers. The two Chls denoted Pdi and Pd2, equivalent to the special pair of bacteriochlorophylls (Bchl) in the anoxygenic bacterial reaction center (BRC), are rotated by 20° to the... 

Key rates of the photoinduced electron transfer process in the reaction center are noted in Figure 13.16. The excited special pair of bacteriochlorophylls (Figure 13.16) transfers an electron to an intermediate bacteriochlorophyll in ca. 3 ps. In ca. 1 ps, a second transfer takes place to the bacteriopheophytin, which is the last tetrapyrrolic chromophore involved in the electron transport. Electron transfer will then continue via a melaquinone Qm (200 ps), a nonhemic iron, and a ubiquinone Q j (200 //s). Charge compensation occurs in ca. 270 ns from the set of cytochromes. The left side of the reaction center does not participate in the electron transport, but its major role is probably to preclude back electron transfer... 

The primary reaction of bacterial photosynthesis - an electron transfer via several prosthetic groups in the so-called reaction center - proceeds extremely rapid on the time-scale of picoseconds. A series of recent experiments gave the following picture of this charge transfer process (data taken for reaction centers from Rhodobacter (Rb.) sphae-roides /1-4/) After excitation of the lowest excited singlet state of the primary electron donor (a "special pair" of bacteriochlorophyll molecules) the excited electronic state P lives for approximately 3.5ps. The decay of P is related with the electron transfer away from P. From several time-resolved experiments it was concluded that this first charge transfer carries the electron directly to the bacterio-pheophytin H /I,2/. Only very recently we could demonstrate the existence of an additional short-lived intermediate prior to the reduction of the bacteriopheophytin H /4/. We interpreted this intermediate as P+B , i.e. the state where the electron from the special pair P has reduced the monomeric bacteriochlorophyll B to the anion radical B . In the final picosecond reaction the electron arrives (with a time constant of 200 ps) at the quinone Qa. It is the purpose of this paper to present additional experimental data supporting a sequential electron transfer via the accessory bacteriochlorophyll. 

Fig. 3. The Q-cycle operating with the reaction center of the pigment system of the purple sulfur-, purple non-sulfur and green non-sulfur bacteria, B870 = special pair of bacteriochlorophyll (BChl) a or b B870 = B870 in the S, excited state BChl = bacteriochlorophyll a or b molecules directly reduced by B870 BPheo = bacteriopheophytin a or b Q = bound quinone [ubiquinone (UQ) in Rhodobacter sphaeroides menaquinone (MQ) in Rhodobacter viridis and Chloroflexus spp.l Qb = mobile quinone [UQ in Rb. sphaeroides and Rb. virldis MQ in Chloroflexus spp.l which exchanges with QbHj at the sites marked with an X QbHj = fully reduced (quinol) form of Qg QbH- = semiquinone form of Qb Fe-S = Rieske iron-sufur center b(Fe ) or c(Fe +) = reduced forms of cytochromes b or o b(Fe ) or c(Fe ) = oxidized forms of cytochromes b or c X = Qb/QbH2 exchange site.

Fig. 6.7 FTIR difference spectrum (light-minus-dark) of the absorbance changes associated with electron transfer from the special pair of bacteriochlorophylls (P) to a quinone (Qa) in photosynthetic reaction centers of Rhodobacter sphaeroides. The negative absorption changes result mainly from loss of absorption bands of P the positive changes, from the absorption bands of the oxidized dimer (P ). These measurements were made with a thin film of reaction centers at 100 K. The amplitudes are scaled arbitrarily. Adapted from [101]...

Johnson, E.T., Mtih, F., Nabedryk, E., Williams, J.C., Allen, J.P., et al. Electronic and vibronic coupling of the special pair of bacteriochlorophylls in photosynthetic reaction centers fi om wild-type and mutant strains of Rhodobacter sphaeroides. J. Phys. Chem. B 106, 11859-11869 (2002)... 

The primary electron donor D in reaction centers (RCs) of photosynthetic bacteria is a dimer ( special pair ) of Bacteriochlorophyll (BChl) molecules (Fig. 1). The structure and spectral (EPR, ENDOR) characteristics of the dimer have been investigated extensively in RCs of Rhodopseu-domonas viridis [1-4] and Rhodobacter sphaeraides [5-9]. 

Boxer has reviewed the theoretical and experimental evidence bearing on the proposed contributions of the monomeric bacteriochlorophyll to super-exhange coupling, in the primary electron transfer step, between the electronically excited special pair of bacteriochlorophylls and the pheophytin acceptor of the reaction centers in photosynthetic bacteria.Sugawara et al have used a density matrix theoretical approach to show that the protein influences the primary charge separation event in the photosynthetic reaction center. 

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