scholarly journals Photosystem II: Thermodynamics and Kinetics of Electron Transport from QA- to QB(QB- ) and Deleterious Effects of Copper(II)

1993 ◽  
Vol 48 (3-4) ◽  
pp. 234-240 ◽  
Author(s):  
G. Renger ◽  
H. M. Gleiter ◽  
E. Haag ◽  
F. Reifarth
Keyword(s):  
Oxygen Evolution ◽  
Manganese Content ◽  
Fluorescence Emission ◽  
Laser Flash ◽  
Fluorescence Induction ◽  
Oxygen Evolving Complex ◽  
Ps Ii ◽  
Thermodynamics And Kinetics ◽  
Redox Active ◽  
Kinetics Of

Studies on thermodynamics and kinetics of electron transfer from QA- to QB(QB-) were performed by monitoring laser flash induced changes of the relative fluorescence emission as a function of temperature (220 K < T < 310 K) in isolated thylakoids and PS II membrane fragments.In addition, effects of bivalent metal ions on PS II were investigated by measuring conventional fluorescence induction curves, oxygen evolution, manganese content and atrazine binding mostly in PS II membrane fragments. It was found: a) the normalized level of the fluorescence remaining 10 s after the actinic flash (Ft/F0) steeply increases at temperatures below -10 to - 20 °C, b) the fast phase of the transient fluorescence change becomes markedly retarded with decreasing temperatures, c) among different cations (Cu2+, Zn2+, Cd2+, Ni2+, Co2+) only Cu2+ exhibits marked effects in the concentration range below 100 μᴍ and d) Cu2+ decreases the normalized variable fluorescence, inhibits oxygen evolution and diminishes the affinity to atrazine binding without affecting the number of binding sites. The content of about four manganeses per functionally competent oxygen evolving complex is not changed by [Cu2+] < 70 μᴍ.Based on these findings it is concluded: i) a temperature dependent equilibrium between an inactive (I) and active (A) state of QA- reoxidation by QB(QB- ) is characterized by standard enthalpies ΔH° of 95 kJ mol-1 and 60 kJ mol-1 and standard entropies ΔS° of 370 kJ K-1 mol-1 and 240 kJ K-1 mol-1 in isolated thylakoids and PS II membrane fragments, respectively, ii) the activation energies of QA- reoxidation by plastoquinone bound to the QB site are about 30 kJ mol-1 (thylakoids) and 40 kJ mol-1 (PS II membrane fragments) in 220 K < T < 300 K, and iii) Cu2+ causes at least a two-fold effect on PS II by modifying the atrazine binding affinity at lower concentrations ( ~ 5 μᴍ) and interference with the redox active tyrosine Yz at slightly higher concentration ( ~ 10 μᴍ) leading to blockage of oxygen evolution.

Antagonistic Effects of α-Tocopherol and α-Tocoquinone in the Regulation of Cyclic Electron Transport around Photosystem II

1997 ◽  
Vol 52 (11-12) ◽  
pp. 766-774 ◽  
Author(s):  
J. Kruk ◽  
K. Burda ◽  
A. Radunz ◽  
K. Strzałka ◽  
G. H. Schmid
Keyword(s):  
Electron Transport ◽  
Photosystem Ii ◽  
Oxygen Evolution ◽  
Transition Probability ◽  
Continuous Illumination ◽  
Oxygen Evolving Complex ◽  
Cyclic Electron Transport ◽  
State Distribution ◽  
Ps Ii ◽  
Oxygen Evolving

Abstract α-Tocoquinone (α-TQ ) and α-tocopherol (α-TOC) which cannot substitute for plastoquinone-9 (PQ-A) as an electron acceptor from photosystem II (PS II), influence the oxygen evolution activity of thylakoid membranes under continuous illumination. In the presence of the herbicide DCMU and the protonophore FCCP which stimulate cyclic electron transport around PS II, α-TQ decreased oxygen evolution whereas α-TOC enhanced it. The effects are attributed to a stimulation or an inhibition of cyclic electron transport around PS II by α-TQ and α-TOC, respectively. Results of flash light experiments on PS II preparations show that both α-TQ and α-TOC increased the d-parameter which describes the transition probability from the S3- to the S0-state of the oxygen-evolving complex, although to a smaller extent when PQ-A is added alone to the preparations. The initial S-state distribution in darkadapted samples was changed only upon PQ-A addition and influenced neither by α-TQ nor by α-TO C supplementation. These effects indicate different kinds of interaction of PQ-A, α-TQ and α-TOC with the PS II components. α-TQ increased and α-TOC decreased the “total miss” parameter both in the presence or absence of PQ-A. A possible site of interaction of α-TQ and α-TO C with the cyclic electron transport around PS II is suggested.

scholarly journals Effects of Heavy Metals on the Fast Chlorophyll Fluorescence Induction Kinetics of Photosystem II: a Comparative Study

1999 ◽  
Vol 54 (9-10) ◽  
pp. 735-739 ◽  
Author(s):  
M. Ciscato ◽  
J. Vangronsveld ◽  
R. Valcke
Keyword(s):  
Time Course ◽  
Fluorescence Induction ◽  
Chlorophyll Fluorescence Induction ◽  
Ps Ii ◽  
Induction Kinetics ◽  
Fluorescence Induction Kinetics ◽  
Bean Plants ◽  
Chlorophyll Fluorescence Induction Kinetics ◽  
Toxic Concentrations ◽  
Kinetics Of

The effects of toxic concentrations of Cu, Zn and Cd on the fast induction kinetics of fluorescence from photosystem(PS)II were investigated in a comparative way. The fast fluorescence transient from primary leaves of metal-treated bean plants was studied. During several days after metal application, the time course of the changes induced by the different metals was monitored. The results evidenced not only a different time course of the changes in fluorescence related parameters for the three metals, but also different effects on the fluorescence induction kinetics, which could possibly be linked to different mechanisms of action of the metals

Fluorometric Detection of Photosystem II Herbicide Penetration and Detoxification in Whole Leaves

Weed Science ◽  
1984 ◽  
Vol 32 (5) ◽  
pp. 675-680 ◽  
Author(s):  
Manfred Voss ◽  
Gernot Renger ◽  
Clemens Kötter ◽  
Peter Gräber
Keyword(s):  
Photosystem Ii ◽  
Foliar Application ◽  
Fluorescence Induction ◽  
Fluorometric Detection ◽  
Variable Fluorescence ◽  
Ps Ii ◽  
Fluorescence Measurements ◽  
Detoxification Mechanism ◽  
Herbicide Effects ◽  
Kinetics Of

The applicability of fluorescence measurements for the detection of herbicide effects in whole leaves was analyzed. Based on the results known for isolated chloroplasts, normalized variable fluorescence of the initial rise was shown to be an appropriate tool for monitoring effects of photosystem II (PS II) herbicides. Equipment is described for monitoring the degree of inhibition by fluorescence induction measurements and microcomputer data analysis. The method is used to study the effect of pyrazon [5-amino-4-chloro-2-phenyl-3(2H)-pyridazinone], BAY DRW 1139 [4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one], and phenmedipham {3-[(methoxycarbonyl)amino] phenyl (3-methyl-phenyl)carbamate} after foliar application to different species. A rapid decrease of normalized variable fluorescence indicates penetration into leaf cells of all species tested. During a 5- to 7-day experiment, the apparent variable fluorescence decreased continuously in herbicide-susceptible plants, while it recovered in resistant plants due to an internal detoxification mechanism. The described method provides a rapid, simple, and nondestructive tool for analyzing the kinetics of penetration and detoxification of PS II herbicides in whole leaves.

Photosynthetic Electron Transport Inhibitors: Some Problems Related to an Accurate Determination of the Molecular Site of Action

1984 ◽  
Vol 39 (5) ◽  
pp. 338-341 ◽  
Author(s):  
C. J. van Assche
Keyword(s):  
Accurate Determination ◽  
Photosynthetic Electron Transport ◽  
Threshold Value ◽  
Fluorescence Induction ◽  
Back Reaction ◽  
Ps Ii ◽  
Chlorophyll Fluorescence Parameters ◽  
Transport Inhibitors ◽  
Kinetics Of

When DCMU and DCMU-type inhibitor concentrations vary over a large range, different effects may be observed on chlorophyll fluorescence parameters such as fluorescence transients, areas over fluorescence induction curves as well as kinetics of area accumulation. These results would indicate a heterogeneity of PS II electron acceptors, or an inhibitor partitioning behavior dependent on the concentration. Also, a threshold value can be reached at supra-optimal concentrations, beyond which the back reaction is almost completely blocked. Such an approach makes it possible to find out inconsistencies in the results, brought about by dual effects of some inhibitors such as phenol-type herbicides, as previously proposed.

scholarly journals Tuning the Catalytic Water Oxidation Activity through Structural Modifications of High-Nuclearity Mn-oxo Clusters [Mn18M] (M = Sr2+, Mn2+)

Water ◽  
2021 ◽  
Vol 13 (15) ◽  
pp. 2042
Author(s):  
Joaquín Soriano-López ◽  
Rory Elliott ◽  
Amal C. Kathalikkattil ◽  
Ayuk M. Ako ◽  
Wolfgang Schmitt
Keyword(s):  
Catalytic Activity ◽  
Water Oxidation ◽  
Fossil Fuels ◽  
Structural Changes ◽  
Mixed Valence ◽  
O2 Evolution ◽  
Oxygen Evolving Complex ◽  
Oxidation Activity ◽  
Ps Ii ◽  
Redox Active

The water oxidation half-reaction is considered the bottleneck in the development of technological advances to replace fossil fuels with sustainable and economically affordable energy sources. In natural photosynthesis, water oxidation occurs in the oxygen evolving complex (OEC), a manganese-oxo cluster {Mn4CaO5} with a cubane-like topology that is embedded within a redox-active protein environment located in photosystem II (PS II). Therefore, the preparation of biomimetic manganese-based compounds is appealing for the development of efficient and inexpensive water oxidation catalysts. Here, we present the water oxidation catalytic activity of a high-nuclearity mixed-metal manganese-strontium cluster, [MnIII12MnII6Sr(μ4-O8)(μ3-Cl)8(HLMe)12(MeCN)6]Cl2∙15MeOH (Mn18Sr) (HLMe = 2,6-bis(hydroxymethyl)-p-cresol), in neutral media. This biomimetic mixed-valence cluster features different cubane-like motifs and it is stabilized by redox-active, quinone-like organic ligands. The complex displays a low onset overpotential of 192 mV and overpotentials of 284 and 550 mV at current densities of 1 mA cm−2 and 10 mA cm−2, respectively. Direct O2 evolution measurements under visible light-driven water oxidation conditions demonstrate the catalytic capabilities of this cluster, which exhibits a turnover frequency of 0.48 s−1 and a turnover number of 21.6. This result allows for a direct comparison to be made with the structurally analogous Mn-oxo cluster [MnIII12MnII7(µ4-O)8(µ3-OCH3)2(µ3-Br)6(HLMe)12(MeOH)5(MeCN)]Br2·9MeCN·MeOH (Mn19), the water oxidation catalytic activity of which was recently reported by us. This work highlights the potential of this series of compounds towards the water oxidation reaction and their amenability to induce structural changes that modify their reactivity.

Irradiation behavior of pyrolytic silicon carbide

1983 ◽  
Vol 41 ◽  
pp. 72-73
Author(s):  
R. J. Lauf
Keyword(s):  
Silicon Carbide ◽  
Fission Products ◽  
Thermodynamics And Kinetics ◽  
Neutron Fluences ◽  
Fuel Particles ◽  
Sic Coatings ◽  
Irradiation Behavior ◽  
Kinetics Of ◽  
Htgr Fuel

Fuel particles for the High-Temperature Gas-Cooled Reactor (HTGR) contain a layer of pyrolytic silicon carbide to act as a miniature pressure vessel and primary fission product barrier. Optimization of the SiC with respect to fuel performance involves four areas of study: (a) characterization of as-deposited SiC coatings; (b) thermodynamics and kinetics of chemical reactions between SiC and fission products; (c) irradiation behavior of SiC in the absence of fission products; and (d) combined effects of irradiation and fission products. This paper reports the behavior of SiC deposited on inert microspheres and irradiated to fast neutron fluences typical of HTGR fuel at end-of-life.

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