Bacteriopheophytins, electron transfer reaction centers

In searching for a suitable electron acceptor, it seems reasonable to just mimic what is known about the initial acceptors in vivo. In photosynthetic bacteria it has been well established that bacteriopheophytin a is one of the first electron acceptors. In photosystem I of green plants a chlorophyll a dimer or monomer have been proposed as the acceptor. Either chlorophyll a or pheophytin a would be excellent choices as electron acceptors. However, the singlet lifetime of the pyrochlorophyll a dimer in toluene is only 4 ns. " For a diffusion-controlled electron transfer reaction between the dimer and one of the in vivo acceptors to take place in a few nanoseconds would require a 10 to 10 molar concentration of pheophytin a or chlorophyll a. The molar extinction coefficient of these molecules is on the order of 40,000. At a concentration of 10 M, the absorption of pheophytin a (or the chlorophyll a monomer) would be much too high. The solution of this dilemma is to link an electron acceptor such as pheophytin or chlorophyll to the dimer. Linking the dimer to an electron acceptor not only solves the diffusion problem, but also begins to mimic the photosynthetic reaction center. 

In the photosynthetic bacterial reaction center (RQ, the electron transfer reaction proceeds from the primary electron donor P, a dimer of bacteriochlorophyll, via an intermediate acceptor (a bacteriopheophytin molecule) to a primary quinone Qa and then to a secondary quinone Qb- In Rb. sphaeroides RC, both quinones are ubiquinone while in Rps. viridis RC, Qa is a mcnaquinone and Qfi is a ubiquinone. In addition, charge recombination between P+ and QA " or P" and Qb proceeds faster in Rps, viridis ( 1 msec and 100 msec, respectively [1]) than in Rb. sphaeroides ( 100 msec and a few sec, respectively [2]) RCs. Below lOOK, the electron is no longer transferred from Qa to Qb [3],... 

The bacterial photosynthetic reaction center protein (RC) catalyzes the conversion of light to electrochemical energy through a sequence of photon-initiated electron transfer reactions between redox cofactors held at fixed distances in the protein [1-4]. The primary processes are elicited by absorption of a photon by the primary donor, a dimer of bacteriochlorophyll ([BChIjg). The first excited singlet state of the dimer, [BChljg, transfers an electron over 10 A in 3 ps to a bacteriopheophytin (BPh). The BPh" in turn reduces a quinone bound at the site to form an anionic semiquinone in 0.2 ns (for a review, see [5]). Numerous experimental efforts have aimed to identify the factors which control the remarkable near unit quantum yield and temperature independence of the RC processes [6-9], often with an eye toward emulation in artificial photosynthetic devices [10]. Here, we examine the role of structural components of the quinone cofactor in determining electron transfer rates at the RC Q/y site. 

Further increase in Afi° causes a rate decrease. The inverted effect is now established with certainty. It corresponds to the region in which A,G > X. These ideas were strongly supported by P. L. Dutton, professor at the University of Pennsylvania, Philadelphia. Dutton studied electron-transfer reactions in bacterial photosynthetic centers. The first step in bacterial photosynthesis is a light excitation in bacteriochlorophyll. The excited electron is first transferred to bacteriopheophytin and then to a quinone, and again to another quinone. Electron transfer to the first quinone occurred at a distance of about 1 nm. Dutton and coworkers systematically changed quinones as electron acceptors and thereby also reaction distances. Additional distances were 0.46 and 2.34 nm. A variation of 2 nm (20 A) in the distance between electron donors and acceptors in protein changed the electron-transfer rate by a factor of 10. A linear distance dependence of the maximum electron trans-... 

Kirmaier, C., Cua, A. He, C. Holten, D. Bocian, David F. (2002) Probing M-branch electron transfer and cofactor environment in the bacterial photosynthetic reaction center by addition of a hydrogen bond to the M-side bacteriopheophytin, Journal of Physical Chemistry B 106, 495-503. 

Kellogg, E. C., Kolaczkowski, S., Wasielewski, M. R., and Tiede, D. M., 1989, Measurement of the extent of electron transfer to the bacteriopheophytin in the M-subunit in reaction centers of Rhodopseudomonas viridis. Photosynth. Res., 22 47n60. 

Figure 19.10. Electron Chain in the Photosynthetic Bacterial Reaction Center. The absorption of light by the special pair (P960) results in the rapid transfer of an electron from this site to a bacteriopheophytin (BPh), creating a photoinduced charge separation (steps 1 and 2). (The asterisk on P960 stands for excited state.) The possible return of the electron from the pheophytin to the oxidized special pair is suppressed by the "hole" in the special pair being refilled with an electron from the cytochrome subunit and the electron from the pheophytin being transferred to a quinone (Q ) that is farther away from the special pair (steps 3 and 4). The reduction of a quinone (Qg) on the periplasmic side of the membrane results in the uptake of two protons from the periplasmic space (steps 5 and 6). The reduced quinone can move into the quinone pool in the membrane (step 7).

Fig. 19.20 Stereoview of the photosynlhelic reaction center. The photoexcited electron is transferred from the special pair to another molecule of bacteriochlorophyll (BCD. then to a molecule of bacteriopheophytin (BPh), then to a bound quinone (Q), all in a period of 250 ps. From the quinone it passes through the nonheme iron (Fe) to an unbound quinone (not shown) in a period of about 100 p,s. The electron is restored to the hole in the special pair via the chain of hemes (He I, etc.) in four cytochrome molecules, also extremely rapidly ( 270 ps). The special pair here is rotated 90° with respect to Fig. 19.19. [From Deisenhofer, J. Michel, H. Huber, R. Trends Biochem. Sci. 1985. 243-248. Reproduced with permission.]...

Fig. 8. Kinetics of optical changes induced by 150-fs, 850-nm laser flashes (XoxJ in Rb. sphaeroides reaction centers. Changes are monitored at several wavelengths (top row) specifically absorbed by various pigment molecules ( tmon)- Solid traces are for the measured absorbance changes and the dotted lines represent the best fit corresponding to a relaxation time of 2.8-ps. Figure source Martin, Breton, Hoff, Migus and Antonetti (1986) Femtosecond spectroscopy of electron transform the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 Direct electron transfer from the dimeric bacteriochloro-phyllprimary donor to the bacteriopheophytin acceptor with a time constant of 2.8 0.2 ps. Proc Nat Acad Sci, USA. 83 958-960.

Fig. 9. (A) Absorption spectrum of Rb. sphaeroides used as a reference to show the Qx and Qy bands of the primary donor (P), BChl [B] and bacteriopheophytin [BO] (B) Femtosecond absorption changes at 920 (a), 785 (b) and 545 nm (c) vs. the delay time of the monitoring pulse measured at room temperature, and (C) absorption changes at 920 (a) and 794 nm (b) measured at 25 K. Figure source (A) see Fig. 7 (B) Holzapfel, Finkele, Kaiser, Oesterheldt, Scheer, Stilz and Zinth (1990) Initial electron transferin the reaction center from Rhodobacter sphaeroides. Proc Nat Acad Sci, USA 87 5170 (C) Zinth and Kaiser (1993) Time-resolved spectroscopy of the primary electron transfer in reaction centers of Rhodobacter sphaeroides and Rhodopseudomonas viridis. I n JR Norris and J Deisenhofer (eds) The Photosynthetic Reaction Center, Voi il, p 82. Acad Press.

J-L Martin, J Breton, AJ Hoff, A Migus, and A Antonetti (1986) Femtosecond spectroscopy of electron transfer In the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 Direct electron transfer from the dimeric bacteriochiorophyii primary donor to the bacteriopheophytin acceptor with a time constant of2.8 0.2ps. Proc Nat Acad Sci, USA 83 957-961... 

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.

The bacterial photosynthetic reaction center (RC)[1, 2] is a membrane protein composed of chromophores (bacteriochlorophylls, bacteriopheophytins and quinones) and three protein subunits named L, M and H. While proteins L and M form two branches of the RC (almost the mirror images of each other) and provide the necessary scaffolding to hold in place bacteriochlorophylls and bacteriopheophytins, the H subunit is in contact with the bacterial cytoplasm and binds the quinones in its interior. In the region of the RC near the pery-plasm is located a bacteriochlorophylls dimer, the so-called special pair (P). This chromophore is at the junction point between the L and M branches and is involved directly in the first photosynthetic electron transfer. 

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