Redox Potentials of Chlorophyll

Reactions of Chlorophyll Inserted in the Membrane with the Redox Components in an Aqueous Solution under Illumination 

The chain of redox processes taking place under illumination at the interfaces between a bilayer lipid membrane and electrolyte causes a photopotential to be set up.92 95 The stationary values of the photopotential depend on the redox potentials of the components in aqueous solutions. The spectral dependence of the photopotential corresponds approximately to the absorption spectrum of the pigment, while the photopotential depends on the 

The most common way of investigating redox reactions is to use the methods enabling one to follow the fate of the products of redox reactions taking place in the membranes of liposomes and microemulsions.98 101 The use of these systems provides the possibility of actively controlling the potential at the interface. However, the redox potentials of the process components are chosen on the basis of the table data. 

When the membranes which contain chlorophyll and are in contact with redox systems are illuminated, a photopotential connected with the formation of the cation CHL+ is generated in aqueous solutions. The first stage of the process is the appearance of an excited chlorophyll molecule  

The further fate of the excited molecule depends on the concrete conditions of the experiment. As an example we can take the processes occurring in liposome membranes101 containing sodium ascorbate on the inside and Cu2+ on the outside. After an excited CHL molecule appears at the interface between the membrane and the water containing an oxidizer, the following reaction takes place  

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In contrast to Photosystem II, which operates in a highly oxidizing regime. Photosystem I is much more reducing. The redox potentials of the early electron acceptors in Photosystem I are approximately -1V, with the excited state of the photoactive chlorophyll P700, estimated to be -1.26 V (Table 3). 

Fig. 7 (A) shows the results of data obtained with the PS-I particle poised at a moderate redox potential of -200 mV. Thedifference spectra produced by exciting with 710-nm, 35-ps pulses were recorded at 5 ns and 880 ps after the flash and presented as traces (a) and (b), respectively, in Fig. 7 (A). The difference spectra are practically the same and represent AA[P700 -P700], with a major bleaching at 700 nm and a minor one at 683 nm. In contrast to the difference spectra obtained by excitation at 532 nm, there is negligible contribution due to excited antenna chlorophyll. The results indicate that at 880 ps, the primary electron acceptor had already been reoxidized, as little absorbance change attributable to the reduction of Aq remained. Meanwhile, P700 had not yet been re-reduced even 5 ns after excitation.

Publications of Mees, Homer and Tomlinson in the 1960s on general herbicidal properties indicated that the phytotoxic action is connected with chlorophyll and light. The authors presumed, on the basis of the relationships between the reducibility of the single compounds and the phytotoxic action, that a reduction to a stable free radical occurs in the plant and that this free radical is responsible for phytotoxic action. The redox potentials of -0.446 and 0.346 mV of paraquat and diquat, respectively, are ensured by the reduction potential of light reaction I of photosynthesis (Calderbank, 1968). 

One of the most interesting differences that is known between the primary electron donors of bacteria and photosystem II of plants is the dramatically higher oxidation potential for P in the plant system. The difference in redox potentials of these two donors in vivo is much larger than the difference in the redox potentials of bacteriochlorophyll a and chlorophyll a in organic solvents. 

From ferredoxin the electron pair returns to chlorophyll over a chain of redox catalysts (see left half of the diagram. Fig. 41). One of these redox catalysts— perhaps the first one—is the system plastoquinone/plastohydroquinone with a redox potential of 0.00 volt furthermore, cytochrome f is interposed here. The transport of electrons from plastoquinone to chlorophyll a E = H-0.45 volt) is coupled to a phosphorylation step just as in the respiratory chain, one inorganic phosphate is taken up and stored as ATP. 

By plotting the redox potentials of the electron carriers (Table 14.1) the pathway of the water-derived electrons may be followed. This is referred to as the Z-scheme (Figure 14.4), first proposed by Hill and Bendall in 1960. Although alternative models have been proffered the Z-scheme remains fundamental to our understanding of photosynthetic electron transport. The interaction of a photon with the reaction centre of PS II initiates a chain of redox reactions. The first recognized electron acceptor of PS II is a bound molecule of metastable phaeophytin a (unchelated chlorophyll a) which instantly donates the electrons to one of two protein-bound plastoquinone (Figure 8.7c) molecules which has been identified as the primary... 

P700, the primary electron donor of PSI, and Ao, the electron acceptor, are chlorophylls, P700 having a redox potential of around -490 mV. Ai is possibly a quinone, and FeSx, FeSA, and FeSe are iron-sulfur centers with potentials of -705, -530, and -580 mV, respectively. FeSA and FeSe probably operate in parallel and denote electrons to ferredoxin (redox potential, -420 mV) (Figure 1.7). 

This raises the electrochemical potential, of these molecules and alters the formal value of electrode potential in redox reactions involving the chlorophyll (see Section 29.4). 

If a scale of electrochemical redox potentials is considered, it is obvious that water is not an easily oxidizable species nor is carbon dioxide easily reduced. The energy requirement of photosynthesis in green plants can be met only by the cooperation of two excited chlorophyll molecules. The first one gives a part of its excitation energy to the second one (Figure 5.4). 

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