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  • 1
    Electronic Resource
    Electronic Resource
    Water cleavage by solar radiation—an inspiring challenge of photosynthesis research (1993)
    Springer
    Photosynthesis research 38 (1993), S. 229-247 
    Details
    ISSN: 1573-5079
    Keywords: absorption spectroscopy ; ENDOR ; EPR ; EXAFS ; manganese ; P680 ; Photosystem II ; S-states ; water oxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states Si (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states Si, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S3 with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with YZ ox and simultaneous internal ligand to metal charge transfer. Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00046749
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  • 2
    Electronic Resource
    Electronic Resource
    Inhibition of photosynthetic oxygen evolution by protonophoric uncouplers (1995)
    Springer
    Photosynthesis research 46 (1995), S. 455-465 
    Details
    ISSN: 1573-5079
    Keywords: Photosystem II ; water oxidation ; ADRY-agents ; uncouplers ; CCCP ; PCP ; TTFB
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The protonophoric uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP), 2,3,4,5,6-pentachlorophenol (PCP) and 4,5,6,7-tetrachloro-2-trifluoromethylbenzimidazole (TTFB) inhibited the Hill reaction with K3[Fe(CN)6] (but not with SiMo) in chloroplast and cyanobacterial membranes (the I50 values were approx. 1–2, 4–6 and 0.04–0.10 μM, respectively). The inhibition is due to oxidation of the uncouplers on the Photosystem II donor side (ADRY effect) and their subsequent reduction on the acceptor side, ie. to the formation of a cyclic electron transfer chain around Photosystem II involving the uncouplers as redox carriers. The relative amplitude of nanosecond chlorophyll fluorescence in chloroplasts was increased by DCMU or HQNO and did not change upon addition of uncouplers, DBMIB or DNP-INT; the HQNO effect was not removed by the uncouplers. The uncouplers did not inhibit the electron transfer from reduced TMPD or duroquinol to methylviologen which is driven by Photosystem I. These data show that CCCP, PCP and TTFB oxidized on the Photosystem II donor side are reduced by the membrane pool of plastoquinone (Qp) which is also the electron donor for K3 [Fe(CN)6] in the Hill reaction as deduced from the data obtained in the presence of inhibitors. Inhibition of the Hill reaction by the uncouplers was maximum at the pH values corresponding to the pK of these compounds. It is suggested that the tested uncouplers serve as proton donors, and not merely as electron donors on the oxidizing side of Photosystem II.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00032300
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  • 3
    Electronic Resource
    Electronic Resource
    Studies on the protolytic reactions coupled with water cleavage in photosystem II membrane fragments from spinach (1987)
    Springer
    Photosynthesis research 13 (1987), S. 167-184 
    Details
    ISSN: 1573-5079
    Keywords: H+-oscillation ; photosystem II ; protolytic reactions ; Q-400(Fe2+) ; water oxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The protolytic reactions of PSII membrane fragments were analyzed by measurements of absorption changes of the water soluble indicator dye bromocresol purple induced by a train of 10 μs flashes in dark-adapted samples. It was found that: a) in the first flash a rapid H+-release takes place followed by a slower H+-uptake. The deprotonation is insensitive to DCMU but is completely eliminated by linolenic acid treatment of the samples; b) the extent of the H+-uptake in the first flash depends on the redox potential of the suspension. In this time domain no H+-uptake is observed in the subsequent flashes; c) the extent of the H+-release as a function of the flash number in the sequence exhibits a characteristic oscillation pattern. Multiphasic release kinetics are observed. The oscillation pattern can be satisfactorily described by a 1, 0, 1, 2 stoichiometry for the redox transitions Si → Si+1 (i=0, 1, 2, 3) in the water oxidizing enzyme system Y. The H+-uptake after the first flash is assumed to be a consequence of the very fast reduction of oxidized Q400(Fe3+) formed due to dark incubation with K3[Fe(CN)6]. The possible participation of component Z in the deprotonation reactions at the PSII donor side is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00035239
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  • 4
    Electronic Resource
    Electronic Resource
    Photoproduction of hydrogen peroxide in Photosystem II membrane fragments: A comparison of four signals (1993)
    Springer
    Photosynthesis research 38 (1993), S. 409-416 
    Details
    ISSN: 1573-5079
    Keywords: peroxide formation ; oxygen evolution ; water oxidation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The present study describes the formation of different forms of peroxide in Photosystem II (PS II) by using a chemiluminescence detection technique. Four chemiluminescence signals (A, B, C and D) of the luminolperoxidase (Lu-Per) system, which detects peroxide, are found in illuminated O2-evolving Photosystem II (PS II) membrane fragments isolated from spinach. Signal A (‘free peroxide’) peaking around 0.2–0.3 s after mixing PS II membrane fragments with Lu-Per is eliminated by catalase or removal of oxygen from the suspension and ascribed to O2 interaction with reduced PS II electron acceptors. In contrast, signal B peaking around 1.5 min remains largely unaffected under anaerobic conditions, as well as in the presence of catalase (20 μg/ml). Under flash illumination the extent of this signal exhibits a weak period four oscillation (maximum at third and 7th flash). Its yield increases up to the third flash, but is close to zero in the fourth flash. An analogous behaviour is observed in flashes 5 to 8. Signal B is ascribed to Lu-Per interaction with the water-oxidizing system being in S2 and/or S3-state. Signal C (‘bound peroxide’) detected as free peroxide after acid decomposition of illuminated PS II particles is observed on the 1 st flash and oscillates with period 2 with superposition of period 4. It is evidently related to peroxide either released from S2 or formed at S2 upon acid shock treatment. Signal D (‘slowly released peroxide’) peaking around 2–3 s after mixing is observed in samples after various treatments (LCC-incubation, washing with 1 M NaCl at pH 8 or with 1 M CaCl2, Cl--depletion) that lead to at least partial removal of the extrinsic proteins of 18, 24 and 33 kDa without Mn extraction. The average amplitude of this signal corresponds with a yield of about 0.2 H2O2 molecules per RC and flash. In a flash train, the extent of signal D exhibits an oscillation pattern with a minimum at the 3rd flash. We assume that these treatments increase the release of ‘bound’ peroxide (upon injection into the Lu-Per assay) either formed in the normal oxidative pathway of the water oxidase in the S2 or the S3-state or give rise to peroxide formation due to higher accessibility of the Mn-cluster to water molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00046768
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  • 5
    Electronic Resource
    Electronic Resource
    Studies on the deconvolution of flash-induced absorption changes into the difference spectra of individual redox steps within the water-oxidizing enzyme system (1988)
    Springer
    Photosynthesis research 16 (1988), S. 243-259 
    Details
    ISSN: 1573-5079
    Keywords: photosystem II ; water oxidation ; difference spectra of Si-oxidation ; QAFe2+ oxidation ; pH-dependence
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The possibility to determine the difference spectra Δεi+1jλ of each univalent redox step Si→Si+1(i=0,...3) of the water-oxidizing enzyme system was analyzed by theoretical calculations and by measurements of 320 nm absorption changes induced by a train of saturating laser flashes (FWHM:7 ns) in PS II membrane fragments. It was found: a) Lipophilic quinones complicate the experimental determination of optical changes due the Si-state transitions because they lead to an additional binary oscillation probably caused by a reductant-induced oxidation of the Fe2+ at the PS II acceptor side. b) In principle, a proper separation can be achieved at sufficiently high K3[Fe(CN)6] concentrations. c) An unequivocal deconvolution into the difference spectra Δεi+1jλ of flash train-induced optical changes which are exclusively due to Si-state transitions is impossible unless the Kok parameters α, β and [Si]0 can be determined by an independent method. Measurements of the oxygen yield induced by a flash train reveals, that in thylakoids and PS II membrane fragments Si is the stable state of dark adapted samples even at alkaline pH (up to pH=9). However, in PS II membrane fragments at pH〉7.7 the misses probability α markedly increases, in contrast to the properties of intact thylakoids. Based on these data the possibility is discussed that an equilibrium exists of two types of S2-states with different properties.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00028843
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