Bacteriopheophytins, electron transfer

For convenience of discussion, a schematic diagram of bacterial photosynthetic RC is shown in Fig. 1 [29]. Conventionally, P is used to represent the special pair, which consists of two bacterial chlorophylls separated by 3 A, and B and H are used to denote the bacteriochlorophyll and bacteriopheophytin, respectively. The RC is embedded in a protein environment that comprise L and M branches. The initial electron transfer (ET) usually occurs from P to Hl along the L branch in 1—4 picoseconds (ps) and exhibits the inverse temperature dependence that is, the lower the temperature, the faster the ET. It should be noted that the distance between P and Hl is about 15 A [53-55]. 

There is rapid ( ps) electron transfer to a bacteriopheophytin molecule (BP). 

There is charge separation by electron transfer from bacteriopheophytin to a quinine (QA) and then on to a second quinine (QB) ... 

Figure 5.7 Electron transfer processes in the first stages of photosynthesis. The energy of light E absorbed by the antenna chlorophylls is transferred to the special pair (BChl)FC is the ferrocytochrome, BPh the bacteriopheophytin and QFe, Q are quinones...

Fig. 25. Electron transfer pathways in the RC isolated from Rhodobacter Sphaeroides strain R-26. (BChl)2 is dimer of the bacterioehlorophyll, BPh is the bacteriopheophytin, Q, -ubiquinone. The electron transfer rates are for the native RC with ubiquinone —10 as Q,v at room temperature. The rates given in parenthesis were determined below 100 K [194-197]...

Returning to the photosynthetic system it now seems plausible that the explanation for the short charge-separation times in the primary steps must be found in the nature of the medium between the redox-sites involved. In this connection it is interesting to note that saturated hydrocarbon chains (i.e. phytyl sidechains) extend from the special pair and from the menaquinone towards the intermediate bacteriopheophytin (see Fig.l). At this moment it is not clear, however, whether in rhodopseudomonas viridis any of these phytyl sidechains plays the role of a molecular wire (see also Kuhn, 1986) that we attribute to the hydrocarbon bridges in l(n). For rhodopseudomonas sphaeroides a fivefold decrease in the rate of the reverse electron transfer from the quinone (ubiquinone) to the bacteriopheophytin was recently reported to result upon removal of the isoprenoid sidechain from the quinone (Gunner et al., 1986). 

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. 

In purple bacteria a number of different lines of evidence led to the conclusion that bacteriopheophytin (BPh) acts as an electron carrier between the primary donor and Qa (Chapter 3). When is reduced illumination results in the photoaccumulation of reduced bacteriopheophytin, detected by its characteristic absorption changes and by an EPR signal split due to its interaction with Q Fe. At temperatures too low for rapid photoaccumulation of BPh to take place, illumination results in formation of a triplet state of the primary donor P-870 which has a polarization pattern characteristic of its formation by recombination of a radical pair. When BPh is reduced this triplet state cannot be formed. The most direct proof that BPh acts as a primary acceptor comes from the direct observation by absorption spectroscopy of BPh reduction within a few picoseconds after the flash. The BPh is reoxidized in 200 ps by electron transfer to or, if is already reduced, by recombination in 14 ns (see Chapter 3). 

Figure 22. (a) A fragment of the photosynthetic reaetion center with the three eomponents Speeial Pair (SP), Accessory Baeteriochlorophyll (BCh), and Bacteriopheophytin (BPh). Photoindueed electron transfer from SP to BPh takes place at a i lobal rate of (3 ps). (b) Prineiple of the modular mimetic approach. D and A are the donor and aeceptor porphyrins, respectively. M is an eleetron transfer mediator whose introduction between D and A can be realized via coordination of a metal center (solid circle) which will also bind the chelating part of the bisporphyrin spacer. 

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... 

Clearly, this spectroscopic phenomenon was a serendipitous event because there was no corresponding selection applied during the generation of FluA. Also, the other mutants that were selected along with it did not show fluorescence quenching to the same high extent. Nevertheless, it is remarkable that such an efficient electron transfer process, which is even faster (by a factor 3-4) than the one measured between bacteriochlorophyll and bacteriopheophytin in the bacterial reac-... 

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

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.

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