Special pairs reaction centers

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.

The interiors of rhodopseudomonad bacteria are filled with photosynthetic vesicles, which are hollow, membrane-enveloped spheres. The photosynthetic reaction centers are embedded in the membrane of these vesicles. One end of the protein complex faces the Inside of the vesicle, which is known as the periplasmic side the other end faces the cytoplasm of the cell. Around each reaction center there are about 100 small membrane proteins, the antenna pigment protein molecules, which will be described later in this chapter. Each of these contains several bound chlorophyll molecules that catch photons over a wide area and funnel them to the reaction center. By this arrangement the reaction center can utilize about 300 times more photons than those that directly strike the special pair of chlorophyll molecules at the heart of the reaction center. 

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. 

While this electron flow takes place, the cytochrome on the periplasmic side donates an electron to the special pair and thereby neutralizes it. Then the entire process occurs again another photon strikes the special pair, and another electron travels the same route from the special pair on the periplasmic side of the membrane to the quinone, Qb, on the cytosolic side, which now carries two extra electrons. This quinone is then released from the reaction center to participate in later stages of photosynthesis. The special pair is again neutralized by an electron from the cytochrome. 

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.)...

Figure 12.22 Schematic diagram showing the flow of excitation energy in the bacterial photosynthetic apparatus. The energy of a photon absorbed by LH2 spreads rapidly through the periplasmic ring of bacterio-chlorophyll molecules (green). Where two complexes touch in the membrane, the energy can be transmitted to an adjacent LH2 ring. From there it passes by the same mechanism to LHl and is finally transmitted to the special chlorophyll pair in the reaction center. (Adapted from W. Kiihlbrandf, Structure 3 521-525, 1995.)...

Modeling of the reaction center inside the hole of LHl shows that the primary photon acceptor—the special pair of chlorophyll molecules—is located at the same level in the membrane, about 10 A from the periplasmic side, as the 850-nm chlorophyll molecules in LH2, and by analogy the 875-nm chlorophyll molecules of LHl. Furthermore, the orientation of these chlorophyll molecules is such that very rapid energy transfer can take place within a plane parallel to the membrane surface. The position and orientation of the chlorophyll molecules in these rings are thus optimal for efficient energy transfer to the reaction center. 

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... 

Organized molecular assemblies containing redox chromophores show specific and useful photoresponses which cannot be achieved in randomly dispersed systems. Ideal examples of such highly functional molecular assemblies can be found in nature as photosynthesis and vision. Recently the very precise and elegant molecular arrangements of the reaction center of photosynthetic bacteria was revealed by the X-ray crystallography [1]. The first step, the photoinduced electron transfer from photoreaction center chlorophyll dimer (a special pair) to pheophytin (a chlorophyll monomer without... 

On the planet Earth, the most important photoreaction occurs in green plants or in green or purple organisms. Their photochemical reaction centers contain a special pair of chlorins (cf. the purple bacterium Rhodobacter sphaeroides. Fig. 6.2). Solar photons cause electron transfer and generate a radical ion pair. Within two picoseconds, the negative charge is transferred to a second chlorin, and from it to a quinone. ... 

Structural studies of the reaction center of a purple bacterium have provided information about light-driven electron flow from an excited special pair of chlorophyll molecules, through pheophytin, to quinones. Electrons then pass from quinones through the cytochrome bci complex, and back to the photoreaction center. 

The origin of the 105-cnT1 mode is not known with certainty, but it is tempting to associate it with intradimer motion as is done for the very similar frequency observed in the special pair of reaction centers [15], Remarkably, Vos et al. [15] and Chachisvilis et al. [19] find that damping rate is independent of temperature. The absence of pure dephasing on a picosecond time scale is unexpected and implies very weak coupling of this motion to the protein photons. 

Fig. 19.20 Stereoview of the phctosynthetic reaction center. The photoexched electron is transferred from the special pair to another molecule of bactenochlorophyll (BCI), then to a molecule of hacteriopheophytin (BPh), then to a bound quinone (Q), ell in a period of 250 ps. From the quinone it passes through the nonheme iron (Fe) to an unbound quinone (rot shown) in a period of about 100 i. The electron is restored to the "hole" in the special pair via the chain oF hemes I 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 Bioihem. Sci. 1985. 243-248. Reproduced wiih permission.)...

The quinone QA (the secondary acceptor) is next reduced by the BPh radical in 200 ps with development of a characteristic EPR signal321 330 at g = 1.82. Over a much longer period of time ( 320 ns) an electron passes from the tetraheme cytochrome subunit to the Chl+ radical in the special pair.323/323a y ie relatively slow rate of this reaction may be related to the fact that the bacteriochlorophyll of the special pair is 2.1 nm (center-to-center) from the nearest heme in the... 

Cyclic photophosphorylation in purple bacteria. QH2 is eventually dehydrogenated in the cytochrome bc1 complex, and the electrons can be returned to the reaction center by the small soluble cytochrome c2, where it reduces the bound tetraheme cytochrome or reacts directly with the special pair in Rhodobacter spheroides. The overall reaction provides for a cyclic photophosphorylation (Fig. 23-32) that pumps 3-4 H+ across the membrane into the periplasmic space utilizing the energy of the two photoexcited electrons. 

These two molecules were the antecedents of a variety of oligomeric porphyrin and chlorophyll derivatives which were constructed as mimics of the reaction center special pair. Many of these systems exhibit interesting optical properties which allow modelling of the electronic interactions within the special pair and/or the antenna function of chlorophyll, which involves singlet-singlet energy transfer... 

A depending on the size of the lanthanide metals. Delocalization of electron density on four equivalent nitrogen atoms causes elongation of the Ln-N bonds at about 0.10-0.15 A compared to silylamides. The close proximity of the macrocyclic 7i-systems in sandwich complexes proved to be useful as structural and spectroscopic models for the bacteriochlorophyll [Mg(Bchl)]2, the special pair in the reaction center of bacterial photosynthesis [211,212]. The distance between the pyrrole rings in [Mg(Bchl)]2 is about 3 A. 

Primary steps of photoinduced electron transfer have been studied in plant reaction centers (PS-I and PS-II), by flash absorption and EPR. In PS-I two questions wereinvestigated i) the properties of the primary radical pair P-700+, A0 (kinetics of decay nature of A0, presumably a specialized chlorophyll a decay by recombination to populate the P-700 triplet state) and ii) the nature of the secondary acceptor A,. Extraction-reconstitution experiments indicate that A, is very probably a molecule of vitamin K,. 

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