Reduction potentials photosynthetic

A decade after the discovery of the Rieske protein in mitochondria (90), a similar FeS protein was identified in spinach chloroplasts (91) on the basis of its unique EPR spectrum and its unusually high reduction potential. In 1981, the Rieske protein was shown to be present in purified cytochrome Sg/complex from spinach (92) and cyanobacteria (93). In addition to the discovery in oxygenic photosynthesis, Rieske centers have been detected in both single-RC photosynthetic systems [2] (e.g., R. sphaeroides (94), Chloroflexus (95)) and [1] (Chlo-robium limicola (96, 97), H. chlorum (98)). They form the subject of a review in this volume. 

While the oxidation reduction potential of the ferredoxins is —0.2 V to —0.4 V and that of the rubredoxins is about —0.05 V, a protein from the photosynthetic bacterium Chromatium has a redox potential of +0.35 V. This is the high potential iron protein, or HIPIP. 

Figure 18. In [2]catenane 20 + [86], upon excitation of its [Rutbpy), p moiety, a very fast electron transfer process to a bipyridinium unit occurs. Owing to the catenane structure, the two bipyridinium units do not possess the same reduction potential (half-wave potential values versus SCE for the inside and outside units are indicated) such a redox asymmetry could mimic that of the cofactors in the bacterial photosynthetic reaction center.

B Kok (1961) Partial purification and determination of oxidation reduction potential of the photosynthetic chiorophyii complex absorbing at 700 m i. Biochim Biophys Acta 48 527-533... 

The photochemically oxidized reaction-center chlorophyll of PSII, Peso, is the strongest biological oxidant known. The reduction potential of Peso is more positive than that of water, and thus it can oxidize water to generate Q2 and H ions. Photosynthetic bacteria cannot oxidize water because the excited chlorophyll a in the bacterial reaction center is not a sufficiently strong oxidant. (As noted earlier, purple bacteria use H2S and H2 as electron donors to reduce chlorophyll in linear electron flow.)... 

The a, j3, and Soret bands occur at 554.5, 524, and 422 nm. Its reduction potential, like that of all high potential photosynthetic cytochromes, is about 100 mV higher than that of respiratory c. The potential at pH... 

The most thoroughly investigated high-potential iron-sulfur protein (HiPIP) is that isolated for Chromatium vinosum (a purple photosynthetic bacterium). The reduction potential of the iron-sulfur cluster is 350 mV, and the overall charge on the protein is -3 the Fc4S4 cluster is buried within the 9500-Da protein, ligated to cysteines at positions 43,45,63, and 77 (Fig. 12) (1, 18). Several studies have been made of the oxidation and reduction of HiPIP by inorganic complexes (40, 62, 131, 153). 

In controlled laboratory mesocosms, Spartina patens, a dominant brackish marsh species found along the U.S. Gulf coast, and rice (0. sativd) showed a decrease in net photosynthesis in response to reduced soil redox potentials. Net photosynthesis decreased when soil redox potential or Eh was below -100 mV (Kludze and DeLaune, 1995b). A similar reduction in photosynthetic rates was observed in 0. sativa with increase in intensity of reduction (Figure 7.31). However, wetland plants... 

The energetics of the electron transport reactions with the participation of chlorophyll is described in detail in the review by Silly.86 Briefly, the role of light in photosynthesis is that the electron transport reaction from the donor (water, in the case of higher plants) to the reaction center where chlorophyll is located and then to the acceptor becomes possible when the chlorophyll molecule undergoes excitation. This process yields products with a high reduction potential. What animals receive in a ready-made form, in plants is synthesized under the action of light. These transformations have been sufficiently well studied for true chlorophyll solutions in photochemical oxidation and reduction reactions.85 However, in natural conditions chlorophyll in the reaction center of the photosynthetic apparatus is inserted in the membrane of thylakoids, and, therefore, enters into photochemical reactions at the interface. 

Charge separation probably occurs mainly as a result of one of the chlorophyll pair, with some involvement of the accessory chlorophyll. Rather than pheophytin acting as an acceptor, in this case the electron is transferred to the chlorophyll. Since the oxidation/reduction potential of the acceptor chlorophyll is negatively shifted, it plays a role in the subsequent photosynthetic reduction, hi the case of photosystem II (Fig. 3d), the interplanar and Mg Mg distances are further increased to 5.0 and 10.0 A [15], respectively, and the two chlorophylls in the pair are now regarded as almost independent. 

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