Bacteriochlorophyll dimer

Reaction center kinetics. After an 0.8-ps or shorter flash of light the decay of the singlet excited state of the bacteriochlorophyll dimer in isolated reaction centers can be followed by loss of its characteristic fluorescence.328 329 The lifetime of this excited state in R. sphaeroides is only 4 ps indicating a rapid occurrence of the initial electron transfer of Eq. 23-31. 

In the excited singlet state the dimer of bacteriochlorophyll possesses a redox potential of 930 mV, which is sufficient to reduce the intermediate primary acceptor J. The rate of electron transfer from the excited state of bacteriochlorophyll dimer P to J is quite high (k 1011 s-1). The high rate of electron transfer ensures a high quantum yield (0 1) of the charge separation process... 

The rate of another important process, the recombination of the primary product of the charge separation, i.e. the reduced primary acceptor (QA ) and oxidised primary donor, bacteriochlorophyl dimer (P+), falls from 103 to 102 s 1 when dynamic processes with uc = 103 s 1 monitored by the triplet labelling method occur. Very fast electron transfer from P+ to bacteriochlorophyl (Bchl) and from (Bchl) to QA does not depend on media dynamics and occurs via conformationally non-equilibrium states (Fig. 3.16). 

Bogatyrenko, V.R., Sabo, Ya., Chamorovskii, S.K., Zakharova, N.I., Kononenko, A.A. and Kulikov A.V. (1991) Study of localization of bacteriochlorophyl dimer and cytochrome c in reaction centers from Chromatium minutissium by ESR, Biofizika 36, 289-290. 

Kulikov A.V., Bogatyrenko, V R., Melnikov, A.V., Syrtzova, LA. and Likhtenshtein, G.I. (1979) Determination of distance between cation radical of bacteriochlorophyl dimer and anion of quinone in photosynthetic reaction center from R. rub mm. Biofisika 24, 178-185. 

FIGURE 7. Two redox cofactor chains meet at the bacteriochlorophyl dimer in the photosynthetic reaction center of Rp. viridis. Electron transfer takes place by tunneling between cofactors diat are spaced by no more dian 14, assuring overall elech on transfer rates in the msec or faster range, even though a total distance of 70 is crossed by the c heme chain. 

Allen, J. P., and Williams, J. C., 1995, Relationship between the oxidation potential of the bacteriochlorophyll dimer and electron transfer in photosynthetic reaction centers. J. Bioenerg. Biomemb., 27 2759283. 

Bixon, M., Michel Beyerle, M. E., and Jortner, J., 1988, Formation dynamics, decay kinetics and singlet-triplet splitting of the (bacteriochlorophyll dimer)-positive (bacteriopheophytin)-negative radical pair in bacterial photosynthesis. Isr. J. Chem., 28 1559168. 

The bacteriochlorophyll dimer (special pair), accessory bacteriochlorophyll, and bacteriopheo-phytin are denoted, respectively, as P,B, and H. 

Fig. 4.2. Optical and ESR spectra of electron carriers in the RC of purple photosynthetic bacteria. (A and B) Primary electron donor (D,) bacteriochlorophyll dimer. (A) Light-dark optical spectrum (recorded at 30 °C) and (B) ESR spectrum of D, in Rps. sphaeroides. The ESR spectrum is y/2-times narrower than the corresponding spectrum of the Bchl cation, indicating a dimeric structure (from Ref. 3).

It is assumed, that the electronic excitation is delocalized via strong exciton coupling over the whole ensemble of 4 bacteriochlorophylls, whereas the positive charge remaining after photochemical electron transfer is restricted to a bacteriochlorophyll-dimer. 

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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