Photosystem model studies

Fig. 7. Model for the native photosystem-l complex (PSI-200) constructed from the reaction-center core (CC I) and two copies of each ofthe four light-harvesting chlorophyll-protein complexes. Figure adapted from Boekema, Wynn and Malkin (1990) The structure of spinach photosystem I studied by eiectron microscopy. Biochim Biophys Acta 1017 55.

Fig. 8. (A) A composite model constructed for the native photosystem I composed of the CCI with the new model for a PS-1 reaction-center monomer containing both the subunit polypeptides and the core-antenna Chl-a molecules plus the peripheral LHC I proteins. (B) The Jansson model. (C) The Fromme model. (A) adapted from Boekema, Wynn and Malkin (1990) The structure of spinach photosystem I studied by electron microscopy. Biochim Biophys Acta 1017 55 and Schubert, Klukas, Kraud, Saenger, Fromme and Witt (1997) Photosystem I of Synechococcus elongates at 4 A resolution Comprehensive structure analysis. J Mol Biol 272 756 (B) from Jansson (1994) The light-harvesting chtorophyll alb-binding proteins. Biochim Biophys Acta 1184 15. (C) from Fromme (1996) Structure and function of photosystem I. Current Opinion in Structural Biology 6 474.

Bowyer J, Hilton M, Whitelegge J et al. Molecular modelling studies on the binding of phenylurea inhibitors to the D1 protein of photosystem II. Z Naturforsch 1990 45c 379-387. 

The objectives of the study presented in this paper were to observe and characterize phototransients produced by laser excitation of natural waters and humic substance (HS) solutions. The photosystems were studied on two scales. On the laboratory scale pulsed laser flash photolysis was used to study the time resolved and spectral behavior of the photochemical transients. Studies to Identify and quantify the transients Included adding energy and electron acceptors and model compounds to the solution and varying parameters such as pH, metal, and oxygen content. On the field scale laboratory data taken at solar actinic wavelengths Is extrapolated using published solar photon fluxes to predict environmental effects of the phototransients studied In this work. This paper thus contains an overview of many experiments performed over four years (16). 

Yang M, Damjanovid A Vaswani HM, Fleming GR. Energy transfer in photosystem I of cyanobacteria Synechococcus elongatus model study with structure-based semi-empirical Hamiltonian and experimental spectral Density. Biophys. J. 2003 85 140-158. 

I have presented an overview of the current state-of-the-art in studies of the Mn complex in photosystem II. There are many unresolved questions and a clear picture of the structure and function of Mn in photosynthetic water oxidation is still not available. One useful approach to help determine the structure of the Mn complex in photosystem II involves the synthesis and characterization of Mn model complexes for comparison with the properties of the Mn complex in photosystem II. Recently, several tetrameric high-valent Mn-oxo complexes have been reported (see the chapter in this volume by G. Christou). Further characterization of existing and new high-valent tetrameric Mn-oxo model complexes, especially EPR and EXAFS measurements, will no doubt help clarify the present uncertain picture of the structure of the Mn complex in photosystem II. 

Photoinduced electron transfer from eosin and ethyl eosin to Fe(CN)g in AOT/heptane-RMs was studied and the Hfe time of the redox products in reverse micellar system was found to increase by about 300-fold compared to conventional photosystem [335]. The authors have presented a kinetic model for overall photochemical process. Kang et al. [336] reported photoinduced electron transfer from (alkoxyphenyl) triphenylporphyrines to water pool in RMs. Sarkar et al. [337] demonstrated the intramolecular excited state proton transfer and dual luminescence behavior of 3-hydroxyflavone in RMs. In combination with chemiluminescence, RMs were employed to determine gold in aqueous solutions of industrial samples containing silver alloy [338, 339]. Xie et al. [340] studied the a-naphthyl acetic acid sensitized room temperature phosphorescence of biacetyl in AOT-RMs. The intensity of phosphorescence was observed to be about 13 times higher than that seen in aqueous SDS micelles. 

Chl-coated semiconductor (n-type) electrodes have thus far been studied using ZnO, CdS, and Sn02, all of which act as efficient photoanodes for converting visible light. Such Chl-sensi-tized photoanodes could be regarded as in vitro models for the photosystem II (oxygen evolution) function in photosynthesis, p-type semiconductor electrodes have not been utilized successfully to produce cathodic Chl-sensitized photocurrents with satisfactory efficiencies. On the other hand, Chl-coated metal electrode systems seem to overcome this problem. 

Several approaches to artificial photosynthesis involve the mimicking of membranes to effect charge separation. An easy extension of the micellar effects described above to systems amenable to study as photosynthetic models can be encountered in the charge separation derived on synthetic vesicles or membranes (275). Sonic dispersal of long chain ammonium halides, phosphates, sulfonate, or carboxylates produces prolate ellipsoidal vesicles with long term stabilities which can entrain and trap molecules in their compartments. With donor-acceptor photosystems, four physical arrangements about the vesicle are important, Fig. 6. 

Vrettos JS, Limburg J, Brudvig GW. Mechanism of water oxidation combining biophysical studies of photosystem II with inorganic model chemistry. Biochim Biophys Acta 2001 1503 229-45. 

The reactions of the several manganese gluconate complexes with molecular oxygen and hydrogen peroxide have been studied in terms of stoichiometry and reaction kinetics. Reaction mechanisms are proposed on the basis of the kinetic data. In addition, the thermodynamic and mechanistic characteristics of an ideal model system for photosystem-II are analyzed and evaluated. 

Mn(III) complex for the solution represented by curve C. Apparently, the excess K3Fe(CN)6 oxidizes the free ligand in the solution, and this in turn is catalytically reduced by the reduced form of the Mn(III) complex. Because of the presence of quinone-like materials in photosystem II (2), this represents a model that warrants additional study. 

In a classical flash study, Kok et al. showed that the 02-evolving complex of photosystem II is oxidized sequentially in a four-quantum, four one-electron oxidative process, the steps being named S0-S4, with S0 representing the totally reduced form (186). In the S state model (Scheme 1), S4 is only transiently stable, rapidly converting to S0 with concomitant 02 evolution. 

FI Michel and J Deisenhofer (1986) X-ray diffraction studies on a crystalline bacterial photosynthetic reaction center A progress report and conclusion on the structure of photosystem II reaction centers. In LA Staehelin and CJ Arntzen (eds) Encyclopedia of Plant Physiology, New Ser, Vol 19 pp 371-381. Springer B Svensson, I Vass, E Cedergren and S Styring (1990) Structure of donor side components in photosystem II predicted by computer modeling. The EMBO J 9 2051-2059... 

This chapter deals with another electron carrier on the reducing side of the electron-transfer chain of photosystem I, namely, the iron-sulfur center FeS-X (also abbreviated as Fx in the literature). As described below, studies indicate that it is a [4Fe 4S] cluster that is uniquely in being coordinated to both the two major polypeptide subunits PsaA and PsaB that make up the protein heterodimer of the PS-I reaction center. For reference, we show in Fig. 1 a model of the location of this electron carrier in the photosystem-I reaction center, in terms of both its physical locale (A) and its position in the electron-transport chain (B). 

The mode of their action consists in the acetolactate synthase (ALS) inhibition and, as a result, in the inhibition of the branched-chain amino acid biosynthesis from acetolactate (90MI1, 92MI7, 98MI4). Results of QSAR studies were published (03MI3). Another approach was used to analyze a model of inhibition of photosystem II (96MI1). 

Current ideas on photosystem II in green plant photosynthesis suggest that the resting system contains Mn and that in conversion of two molecules of water into oxygen, Mn and/or Mn are involved. A study of [Mn (GH3)2] as a possible model for this system yielded electrochemical evidence for Scheme 1. ... 

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