Electron donor photooxidation

The large difference in the susceptibility of Z vs. D to photodamage suggests that the relative contributions of each to processes such as photoactivation and electron donor photooxidations can be determined. Fig. 3... 

The discovery by Knaff and Amon(32) of a light-induced photooxidation at — 189°C requiring short-wavelength light has provided information as to a possible primary electron donor for Sn. The photooxidized substance has been identified as a form of cytochrome b absorbing at 559 nm (cytochrome 559)- Pretreatment of the spinach chloroplasts with ferricyanide to oxidize... 

Photooxidation and photoreduction of an electron donor and an electron acceptor, respectively, as illustrated schematically with a one-electron molecular orbital scheme. 

Other carboxylate-dye interactions have been reported. Ethylenediamine tetracarboxylic acid (EDTA) and its salts are well known reductants for a variety of dyes (54,55). The amino-acid N-phenylglycine can be photooxidized and induce polymer formation (26,56,57). Studies of the efficiency of photopolymerization of acrylate monomers by MB/N-phenylglycine combinations as a function of the pH of the medium suggest that either the amino group or the free carboxylate can act as an electron donor for the dye excited state, but that the amine functional-lity is the more efficient coinitiator (10). Davidson and coworkers (58) have shown that ketocarboxylic acids are photode-carboxylated by electron transfer quenching of dye triplet states under anaerobic conditions. Superoxide formation can occur when oxygen is present. 

Triazine (e.g., atrazine, simazine) and substituted urea (e.g., diuron, monuron) herbicides bind to the plastoquinone (PQ)-binding site on the D1 protein in the PS II reaction center of the photosynthetic electron transport chain. This blocks the transfer of electrons from the electron donor, QA, to the mobile electron carrier, QB. The resultant inhibition of electron transport has two major consequences (i) a shortage of reduced nicotinamide adenine dinucleotide phosphate (NADP+), which is required for C02 fixation and (ii) the formation of oxygen radicals (H202, OH, etc.), which cause photooxidation of important molecules in the chloroplast (e.g., chlorophylls, unsaturated lipids, etc.). The latter is the major herbicidal consequence of the inhibition of photosynthetic electron transport. 

As is clear from the above discussion, reduction of surface-located Fe(III) (which may or may not lead to oxide dissolution) is associated in most instances with oxidation of the electron donor at the particle surface and many of the same factors that influence the rate of reductive dissolution will also affect the rate of donor oxidation. Leland and Bard [138] found that the rate constants of photooxidation of oxalate and sulfite varied by about two orders of magnitude with different Fe(III) oxides and concluded that this appears to be due to differences in crystal and surface structure rather than to differences in surface area, hydrodynamic diameter or band gap . 

Photoinitiated electron transfer reactions are among the earliest photochemical reactions documented in the chemical literature and (ground state) electron donor-acceptor interactions have been known for over one hundred years. Some aspects of plant photosynthesis were already known to Priestly in the eighteenth century. The photooxidation of oxalic acid by metal ions in aqueous solution was discovered by Seekamp (UVI) in 1803 and by Dobereiner (Fe,n) in 1830. The electron donor-acceptor interactions between aromatic hydrocarbons and picric acid were noticed by Fritzsche in the 1850s the quinhydrones are even older,... 

The redox partners of these proteins have yet to be identified, although it has been shown that auracyanins can donate electrons to the membrane-bound cytochrome c-554, which is the direct electron donor for the photooxidized bacterial reaction center P870+ (McManus et al., 1992). However, whether it is their proper in vivo function remains uncertain. The sulfocyanin gene is in the same operon with the components of the respiratory electron transfer chain and, since Su. acidocaladar-ius completely lack c-type cytochromes, it is implicated as a substrate for the CuA-containing terminal oxidase. Interestingly, the occurrence of... 

Once formed, the primary redox products are converted in subsequent thermal reactions steps to the final compormds Area and Dox- When oxygen is the electron acceptor and a pollutant like phenol is the electron donor, carbon dioxide and water are the final redox products (Scheme 2). The primary reductive redox product is superoxide which can be converted to the strongly oxidizing OH radical via protonation, disproportionation of HO2 and reductive photocleavage of the produced H2O2. Instead of water oxidation, the oxidative primary step may consist of the oxidation of the pollutant producing a phenoxy radical and a proton. Such complete photooxidation reactions are often termed as mineralization and in general titania is employed as the photocatalyst 4-7). 

Hybrid systems have been constructed in which a metal complex is covalently linked to an organic species so as to produce a donor-acceptor dyad, with either subunit functioning as the chromophore. Thus, ruthenium(II) tris(2,2 -bipyridyl) complexes have been synthesized bearing appended anthraquinone or tyrosine functions. Both systems enter into intramolecular electron-transfer reactions. With an appended anthraquinone moiety, direct electron transfer occurs from the triplet excited state of the metal complex to the quinoid acceptor. This is not the case with tyrosine, which is an electron donor, but the metal complex can be photooxidized by illumination in the presence of an added acceptor. The bound tyrosine residue reduces the resultant ruthenium(III) tris(2,2 -bipyridyl) complex... 

IPET has also been demonstrated to occur in some reductive processes (Serpone, 1994a) and in the photooxidation of phenolic substrates on CdS, ZnO and Ti02 suspensions. If these semiconductors are photoactivated when coupled to certain other metal oxides as acceptors, they become the electron donors (Serpone et al, 1995c). 

Although the question ofthe role ofBA in electron transfer has been controversial for sometime, there have been some new developments, which will be discussed in Chapter 7. The question ofthe nature of the currently recognized reaction partner of photooxidized P870, i.e., the primary electron acceptor BOa, and of how P870 is re-reduced by the secondary electron donor will be dealt with in Chapters 7 and 10, respectively. In the remainder of this chapter we will discuss the physical and chemical properties ofthe primary electron donor of photosynthetic bacteria. 

Fig. 8. Absorption changes during photooxidation and dark re-reduction of the primary electron donor P (A) and oxidation of cytochrome c (B) following a brief, intense flash. Figure source Parson and Clayton (Straley, Parson, Mauzerall and Clayton) (1973) Pigment content and molar extinction coefficients of photochemical reaction centers from Rhodopseudomonas sphaeroides. Biochim Biophys Acta. 305 606.

Fig. 3. Absorbance change due to quinone reduction in a reaction-center complex isolated from the carotenoidless mutant of Rb. sphaeroides. The sample contained reduced cytochrome and excess ascorbate as the secondary electron-donor system so that photooxidized P does not accumulate. The presence of excess ascorbate kept the oxidized cytochrome reduced, the net quinone reduction spectrum was obtained. Figure source Clayton (1980) Photosynthesis Physical Mechanisms and Chemical Patterns, p 95. Cambridge University Press.

Note the data points marked by x in the near infrared region between 1000 and 1300 nm, as reported by Dutton, Kaufmann, Chance and Rentzepis". This combination of extinction coefficients is almost certainly due to the oxidation of the primary electron donor, P870, i.e., the bacteriochlorophyll special-pair [BChl]2, since it is not found for species such as BChF, BChl", or B (generated electrochemi-cally). Picosecond measurements have revealed the maximum absorbance increase at 1250 nm in Rb. sphaeraides and C. vinosum and at 1300 nm mRp. viridis. Picosecond measurements in this wavelength region can thus provide direct information on the reaction course of P870 photooxidation. 

Reduction of the Photooxidized Primary Electron Donor by Cytochromes.182... 

Based on the nature of the cytochromes, there are two kinds of photosynthetic bacterial reaction centers. The first kind, represented by that of Rhodobacter sphaeroides, has no tightly bound cytochromes. For these reaction centers, as shown schematically in Fig. 2, left, the soluble cytochrome C2 serves as the secondary electron donor to the reaction center the RC also accepts electrons from the cytochrome bc complex by way ofCytc2- The rate of electron transfer from cytochrome to the reaction center is sensitive to the ionic strength of the medium. Functionally, cytochrome C2 is positioned in a cyclic electron-transport loop. In Rb. sphaeroides, Rs. rubrum and Rp. capsulata cells, the two molecules of cytochromes C2 per RC are located in the periplasmic space between the cell wall and the cell membrane. When chromatophores are isolated from the cell the otherwise soluble cytochrome C2 become trapped and held by electrostatic forces to the membrane surface at the interface with the inner aqueous phase. These cytochromes electrostatically bound to the membrane can donate electrons to the photooxidized P870 in tens of microseconds at ambient temperatures, but are unable to transfer electrons to P870 at low temperatures. 

The photosynthetic reaction center of Rb. sphaeroides and Cyt Ci have recently been co-crystallized by Adir, Axelrod, Beroza, Isaacson, Rongey, Okamura and Feher with an RC-to- Cyt C2 ratio of 4, as determined by light-induced absorption-change measurements. X-ray diffraction studies of these crystals have shown two molecules of Cyt C2 to be adjacent to the periplasmic surface of the M-subunit polypeptide containing the reaction center. This co-crystallized Cyt C2 has also been found to donate electrons to the photooxidized primary electron donor (PSTO" ) at the same rapid rate as in vivo. 

The second kind of reaction center, as represented by that of Chromatium vinosum or Rhodopseudo-monas viridis, has a tightly bound c-type cytochrome [see Fig. 2, right]. This so-called reaction center-associated cytochrome is a tetraheme of molecular mass of 40 kDa and structurally quite different from the other known, c-type cytochromes. One of the hemes in this RC-associated, c-type cytochrome also serves as the immediate electron donor to the photooxidized primary donor of the photosynthetic bacteria (either P870 in C. vinosum or P960 in Rp. viridis). The oxidized cytochrome in the tetraheme is in turn reduced by the soluble cytochrome C2. The RC-associated cytochromes are not easily dissociated from the RC, even at high ionic strength. 

The subject matter of this chapter is confined to the role of cytochrome as a secondary electron donor, D, i.e., the interaction with the photooxidized primary electron donor formed during the photochemical charge-separation process in photosynthetic bacteria. Another cytochrome, present essentially as a ubiquinone-cytochrome c oxidoreductase in the cytochrome-6ci complex, is particularly important in energy conservation and the creation of a proton gradient for ATP synthesis in of photosynthetic bacteria. This cytochrome fee, complex, is discussed in Chapter 35 dealing with proton transport. 

We now discuss kinetic evidence that supports the notion that a reduced cytochrome is the direct electron donor to the photooxidized P870. . In subsequent sections we discuss properties and reactions of the RC-associated cytochromes, i.e., those cytochromes that are firmly associated with the reaction centers. The topics to be discussed include the temperature-insensitive electron transfer from the cytochrome to the reaction center and the spatial arrangement of the hemes in the tetraheme cytochrome subunit. 

We have seen from the foregoing discussion that c-type cytochromes serve an important function as secondary electron donors in photosynthetic bacteria. The high rate of electron transfer from c-type cytochromes to the photooxidized primary donor and the ability of the cytochrome to donate an electron even at low temperature indicate that the reaction center and the cytochrome are closely linked together. 

Fig. 12. Kinetics of flash-induced absorbance-change transients at 870 and 550 nm, indicative of photooxidation and rereduction of the primary electron donor P870 in Rb. sphaeroides R-26 reaction centers in the absence (upper row) and presence (lower row) of mammalian cytochrome c. Figure source Ke, Chaney and Reed (1970) The electrostatic interaction between the reaction-center bacteriochlorophyll derived from Rhodopseudomonas sphaeroides and mammalian cytochrome c and its effect on light-activated electron transport. Biochim Biophys Acta 216 377.

By flash excitation of this reaction system, it is not only possible to measure the absorbance change at 870 nm produced by photooxidation of the primary electron donor but also that at 550 nm produced by the oxidation of cytochrome c, as shown in Fig. 12, lower right trace. Since the differential extinction coefficient of cytochrome c was precisely known, the authors were able to use the differential extinction coefficient of P870 commonly accepted at the time, and the measured amplitudes of absorbance changes at 870 and 550 nm, to calculate a stoichiometry of 1 P870 1 cyt c for this reaction. Parson and Clayton " later utilized this model system to obtain conversely a more accurate differential extinction coefficient of 128 mM -cm" forP870 (see Section II. of Chapterd for further details). 

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