Photosystem I of green plants

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. 

On the other hand, if the correlated radical pair mechanism is operative, a pair of partially overlapping anti-phase doublets is expected.[36-38] The polarization pattern observed, E A E, is similar to that observed for P700 - A in Photosystem I of green plants, [24] and P865 -Q in bacterial reaction centers. [25] Recently, Stehlik et al. [39] have developed a simple theoretical model that can be used to simulate these spectra. This model focuses on the influence of/, D, and g-anisotropy on the EPR spectra of radical pairs. The latter two quantities are particularly useful in determining the distance between the radicals and their mutual orientation. The... 

In purple bacteria, the constituent bacteriochlorophyll of the primary donor is the same as the principal bacteriochlorophyll pigment, namely, BChl a. The same is true for the green bacteria and also for photosystem II of green plants. However, as discussed later in Chapter 28, the primary donor of photosystem I(PSI),P700, is now known to consist of a 13 epimer of Chi a, designated as Chi o ]. Since the reaction centers of PS I and heliobacteria are both of the FeS-type [refer to Chapter 1], it is reasonable to anticipate that the primary electron donor of heliobacteria might also be an epimer. 

The transformation of the Earth s atmosphere to an oxidizing environment through the photosynthetic oxidation of water to dioxygen (O2) (photosystem II of green plants see Photosystem I) resulted in the autoxidation of the reduced forms of the molecules of the Earth s crust, oceans, and atmosphere (over a period of about 750 million years) (equations 15-19). ... 

I. The photolysis of co-ordinated water in [ MnL-H2O) 2][C10j2 (L = dianion of tetradentate 02N2-donor Schiff bases). A model for the manganese site in photosystem II of green plant photosynthesis, J. Chem. Soc., Dalton Trans., p. 1391. 

It is interesting that in photosynthesis the energy loss in the primary photochemical step is 0.8 eV for photosystems I and II of green plants and algae and also for photosynthetic bacteria (18). Also Xg = 700 nm for green plant and algal photosynthesis, a value near the optimum for the 0.8 eV C curve in Fig. 2. 

Another approach to a combined system is the connection of the two systems through a quinone redox couple dissolved in an oil phase, as shown in Fig. 17.2. This system is analogous to the combination of photosystems I and II in the photosynthesis of green plants. Fig. 17.10 illustrates the structure of our model system, in which the oil-phase corresponds to the lipid bilayer membrane of chloroplast. Such a system is structurally identical to a liposome and has the possibility of development for use in a batch reactor. 

The photosynthetic apparatus of green plants and cyanobacteria oxidizes water and transfers electrons to NADP, with a net gain in electrochemical potential of 1.13 eV (at pH 7), utilizing the energy of two light quanta per electron. The complete system is contained in the chloroplasts, and is localized within the thylakoid membranes, with the exception of the electron carrier ferredoxin, which is in solution in the stroma, and serves to transfer electrons from the reducing end of photosystem I (PS I) to a membrane-bound flavoprotein which then reduces NADP, and of the copper protein plastocyanin (PC, the electron donor to PS I), which is in solution in the internal phase of thylakoids. 

Figure 29. Z-scheme of the photoinduced electron-transfer and dark enzymatic reactions operating in the photosynthesis of green plants. Mn = Mn-containing enzyme complex catalyzing water oxidation and O2 evolution Chi a and Chi b = photoactivated primary electron acceptors in photosystems I and II, respectively A and I = primary electron donors in photosystems I and II, respectively ADP = adenosine diphosphate ATP = adenosine triphosphate.

Photosynthetically active quinones include plastoquinone of green-plant photosystem II, ubiquinone and menaquinone in photosynthetic bacteria, and phylloquinone in photosystem I. Plastoquinone is present in green-plant photosystem II both as a tightly-bound and a loosely-bound electron carrier, designated Qa and Qb, respectively. Qa is photoreduced only to the semiquinone (PQ ) but Qb can accept two electrons, forming the plastohydroquinone (PQ-Hj) [see Chapters 5, 6 and 16 for further discussion]. Plastohydroquinone PQb H2 is the final reduction product of photosystem II and goes on to reduce the cytochrome bj complex as part of the electron transport and proton translocation processes [see Chapter 35 for detailed discussions]. 

We have seen the Z-scheme for the two photosystems in green-plant photosynthesis and the electron carriers in these photosystems. We have also described how the photosystems of green plants and photosynthetic bacteria all appear to function with basically the same sort ofmechanisms of energy transfer, primary charge separation, electron transfer, charge stabilization, etc., yet the molecular constituents of the two reaction centers in green plants, in particular, are quite different from each other. Photosystem I contains iron-sulfur proteins as electron acceptors and may thus be called the iron-sulfur (FeS) type reaction center, while photosystem 11 contains pheophytin as the primary electron acceptor and quinones as the secondary acceptors and may thus be called the pheophytin-quinone (0 Q) type. These two types of reaction centers have also been called RCI and RCII types, respectively. 

Both green-plant photosystem I and green sulfur bacteria have for sometime known to have the FeS-type reaction centers. Now a new group of photosynthetic bacteria, the heliobacteria, discovered about twenty years ago by Gest and Favinger, have also been found to belong to this type. The first dis-... 

The PS-1 reaction center is remarkably similar to the reaction center in photosynthetic bacteria and to photosystem 11 in green plants with respect to the apparent symmetrical arrangement of the major proteins and the associated pigment molecules and cofactors. For example, the two large heterodimerforming proteins that are encoded by the psaA and psaB genes, in photosystem I, are the counterparts of the L- and M-subunits of the photosynthetic bacterial reaction center and of the D1 and D2 subunits of the PS-11 reaction center. While both the PS-11 and purple bacterial reaction centers use pheophytin and quinones (plastoquinone, ubiquinone, or menaquinone) as the primary and secondary electron acceptors, the PS-1 reaction center is similar to that of green sulfur bacteria and heliobacteria in the use of iron-sulfur proteins as secondary electron acceptors. It may be noted, however, that the primary electron donor in all reaction centers is a dimer of chlorophyll molecules. 

Several laboratories now make use of the ultrafast continuum pump-probe technique in the study of ultrafast processes in biological molecules or molecular complexes.Notable molecules and complexes under study are the photosynthetic reaction centers of purple bacteria, the reaction centers of photosystems I and II of green plants. 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

The structure of the membrane of green sulfur bacteria appears to be fundamentally different. Most of the BChl a is contained in a water soluble BChl a protein which is attached to, rather than imbedded in the membrane (13). About one-fourth of the BChl a, however, forms part of a core complex (Fig. 3), which contains approximately 20 BCls a per reaction center (14). The complex also contains about 15 molecules of BChl c or a closely related pigment (15). Flash spectroscopic evidence indicates that one of these molecules acts as electron acceptor in the primary charge separation (16,17). The peptide composition of the core complex may suggest a structural relationship to the core of photosystem I of plants (18). 

Although studied in great detail, the action of chlorophyll is still not fully understood however, steady progress towards a more complete understanding has taken place over the past several decades. Two photosynthetic systems (photosystems I and II) are present in green plants - each incorporating a different chlorophyll type. When a photon is absorbed by a chlorophyll molecule, its energy is transformed and... 

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