Quinone-chlorophyll electron transfer

Natural photosynthesis applies electron transfer systems, where a relay of electron-transfer reactions evolves among chlorophyll and quinone moieties embed-... 

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

Figure 5.7 Electron transfer processes in the first stages of photosynthesis. The energy of light E absorbed by the antenna chlorophylls is transferred to the special pair (BChl)FC is the ferrocytochrome, BPh the bacteriopheophytin and QFe, Q are quinones...

Components of the electron transport chain in bacteria have been shown to include b- and c-type cytochromes, ubiquinone (fat-soluble substitute quinone, also found in mitochondria), ferredox (an enzyme containing nonheme iron, bound to sulfide, and having the lowest potential of any known electron-canying enzyme) and one or more flavin enzymes. Of these a cytochrome (in some bacteria, with absorption maximum at 423.5 micrometers, probably Cj) has been shown to be closely associated with the initial photoact. Some investigators were able to demonstrate, in chromatium, the oxidation of the cytochrome at liquid nitrogen temperatures, due to illumination of the chlorophyll. At the very least this implies that the two are bound very closely and no collisions are needed for electron transfers to occur. 

A simple system for modelling the intermediate step of the charge separation process during photosynthesis [the stage of electron transfer from the reduced pheophytin (i.e. chlorophyll deprived of the Mg atom) to quinone] has been advanced and studied [67]. In this work the charge photoseparation process was studied in solutions of P-L-Q compounds in electron-donor, Et3N, solvent at 77 K. The structure of one of the P-L-Q compounds studied is given in Fig. 13. We will consider briefly the main results of ref. 67. 

Photooxidation of P700 in photosystem I reduces a chlorophyll, which transfers electrons to a series of membrane-bound iron-sulfur centers, probably by way of a quinone. From the iron-sulfur centers, electrons move to the soluble iron-sulfur protein, ferre-... 

A simple system for modelling the intermediate step of the charge separation process during photosynthesis (the stage of electron transfer from the reduced pheophytin, i.e. chlorophyll deprived of the Mg atom, to quinone) has been... 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

From the quinones, the electron is transferred to plastocyanin and then to cytochrome bf. The two H+ ions (protons) left behind remain in the thylakoid lumen. As the electrons move down this electron transport chain, protons are pumped into the thylakoid lumen. Eventually the transported electron is given up to the oxidized P700 chlorophyll of Photosystem I. 

Figure 2. Paths of electron transfer in PSII P680, reaction-center chlorophyll that functions as the primary electron donor P680, first excited singlet state ofP680 Pheo, pheophytin QA, primary quinone electron acceptor QB, secondary quinone electron acceptor cyt b559, cytochrome b559 Chlz, redox-active chlorophyll that mediates electron transfer between cytochrome b559 and P680 YD, redox-active tyrosine that gives rise to the dark-stable tyrosine radical Yz, redox-active tyrosine that mediates electron transfer from the Mn complex to P680.

Chlorophyll studies of adducts with various biological molecules are also known (bovine plasma albumin and (3-carotene [195], quinone riboflavin [196], and NADH [173]. Mitsui et al. [196] have shown that in porphyrin complexes of viologen the counterion (I-, C1-, Br-) affects the electron transfer process by reduction of the electron-accepting properties of viol-... 

Like chlorophyll, plastoquinone A has a nonpolar terpenoid or isoprenoid tail, which can stabilize the molecule at the proper location in the lamellar membranes of chloroplasts via hydrophobic reactions with other membrane components. When donating or accepting electrons, plastoquinones have characteristic absorption changes in the UV near 250 to 260, 290, and 320 nm that can be monitored to study their electron transfer reactions. (Plastoquinone refers to a quinone found in a plastid such as a chloroplast these quinones have various numbers of isoprenoid residues, such as nine for plastoquinone A, the most common plastoquinone in higher plants see above.) The plastoquinones involved in photosynthetic electron transport are divided into two categories (1) the two plastoquinones that rapidly receive single electrons from Peso (Qa and Qb) and (2) a mobile group or pool of about 10 plastoquinones that subsequently receives two electrons (plus two H+ s) from QB (all of these quinones occur in the lamellar membranes see Table 5-3). From the plastoquinone pool, electrons move to the cytochrome b f complex. 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

Figure 19.20. Electron Flow Through Photosystem I to Ferredoxin. Light absorption induces electron transfer from P700 down an electron-transfer pathway that includes a chlorophyll molecule, a quinone molecule, and three 4Fe-4S clusters to reach ferredoxin. The positive charge left on P700 is neutralized by electron transfer from reduced plastocyanin.

Reaction 1 represents formation of the geminate reaction products [the chlorophyll r-cation radical ( Chl+) and quinone radical anion ( Q )] within the bilayer by electron-transfer quenching of the photoexcited chlorophyll triplet state reaction... 

Figure 3. Fiincvional organisation oFpholosyslem I (in protein complexes contained in the thylakoid membrane. Excitation energy is harvested by chlorophyll (Chi) and carotenoids (Car) molecules and transfered to the special pair (Chlj). Vectorial electron transfer across the membrane takes place front excited Chi to plastoqutnone (pQ) via phcopliitin (Ph) and quinone (Q) electron mediators.

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

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