Light induced charge separation

Clusters Fa tmd Fb can be (photo-)reduced by light-induced charge separation within the RC complex at cryogenic temperatures. Since, under these conditions, only one electron is injected into the Fa/Fb-protein, and since furthermore the EPR spectra of Fa and Fb are significantly different, a straightforward assignment of individual clusters to the cluster-binding motifs in the primary sequence was... 

Gerischer, H., and J. Katz (Eds), Light-Induced Charge Separation in Biology and Chemistry, Verlag Chemie, Weinheim, 1979. 

H.Kuhn, Light-Induced Charge Separation in Biology and Chemistry,... 

To account for the periodicity of four in the yield of O2 in a series of flashes (12-13), Kok and coworkers (14) proposed that photosystem II cycles through five states during flash illumination. These intermediate oxidation states are referred to as states (i = 0-4) with the subscript denoting the number of oxidizing equivalents accumulated. The sequential advancement of the S states occurs via the light-induced charge separation in photosystem II. 

Lewis NS (1990) Mechanistic studies of light-induced charge separation at semiconductor/ liquid interfaces. Acc Chem Res 23 176-183... 

Finally, solute radical ions can be generated by light-induced, one-photon or multiphoton ionization of their parent compounds (Chaps. 5 and 16). This approach is particularly useful in the ultrafast studies of short-lived, unstable radical ions that aim to unravel their solvation, recombination, reaction, and vibrational relaxation dynamics of the primary charges (see, e.g., Chap. 10). Whereas the time scale of radiolytic production of secondary ions is always limited by the rate with which the primary species reacts with the dispersed parent molecules, light-induced charge separation can occur in <100 fsec. There are many studies on photoionization of solute molecules in liquid solutions we do not intend to review these works. 

EPR is the method of choice to study the paramagnetic intermediates occurring in the light-induced charge separation process in the reaction centres of plants and photosynthetic bacteria2. Furthermore, other paramagnetic species in particular metal centres - that participate in the photosynthetic processes can be detected and characterized by EPR. Examples are various cytochromes,... 

Radical Pair States. - So far we have discussed single radical-ions that occur in the charge-separation process and can be trapped under certain condition in bRC or can even be chemically generated. The initial light-induced charge separation creates radical pairs (RPs) that can be studied by time-resolved transient or pulse EPR ... 

In plants, algae and cyanobacteria the light-induced charge separation of photosynthesis occurs in 2 large membrane proteins, called photosystem (PS) I and II. PS I catalyzes the ET from plastocyanin (or cytochrome c6) on the luminal side to ferrodoxin (or flavodoxin) on the stromal side of the membrane (for review see reference 177). PS I from the cyanobacterium Thermo(Y13)synechococcus (T.) elongatus was crystallized and an X-ray crystallographic structure at 2.5 A resolution has recently been obtained.18,178 Very recently, the structure from plant PS I has also been reported with a resolution of 4.4 A.179... 

Radical Pairs in PS II. - As in the case of bRCs and PS I light-induced charge separation in PS II creates a series of radical pairs that can, in principle, be studied by EPR techniques. The various methods used have been described by Stehlik and Mobius14, Lakshmi and Brudvig12,381 and Dzuba and Hoff11. These articles include references to the literature up to the year 2000. 

The first commercial application of photocatalysts has started to clean our environment by Ti02 powders and films. In order to utilize photocatalysts for solar energy conversion, sensitization of large bandgap semiconductors is important. The most difficult task for an artificial photosynthetic system is to establish visible light-induced charge separation with minimum back charge recombination. 

Gerischer, H., in "Light-Induced Charge Separation in Biology and Chemistry", Eds. H. Gerischer and J. J. Katz Dahlem Kon-ferenzen Berlin, 1979 p. 61. 

The TRMC measurements show that the light induced charge separation... 

For the series of the compounds with n = 1-4 the lifetime of D + -P-Q was found to decrease exponentially with the distance between Q and D+ with ae = 3.3 A [166]. This value considerably exceeds the value of ae 1.25 A found for P -L-Q [151] and may arise from the amide group insertion into the linking bridge in contrast to hydrocarbon spacers in the latter molecules. In Ref. [167], triad molecule with n = 1 has shown the capability of facilitating light-induced charge separation across a bilayer lipid membrane. 

The application of semiconductors as substrates in interfacial supramolecular assemblies has been dominated so far by films consisting of nanoparticles. In an attempt to understand the properties of such particles, and in particular, issues such as light-induced charge separation, their electronic properties will be discussed in some detail. Relevant issues in this respect are quantum effects, the size of the band gap, charge transport and band bending. 

O Regan, B. Moser, J. Anderson, M. Graetzel, M. Vectorial electron injection into transparent semiconductor membranes and electric field effects on the dynamics of light-induced charge separation, J. Phys. Chem. 1990, 94, 8720. 

Our research aims at elucidating the chemical nature of the redox centers involved in light-induced charge separation, the mechanisms of reaction and the role of the protein counterpart. During this last year our work has concerned mainly the primary radical pair and the secondary acceptor A,. 

Setif, P., Ikegami, I. and Biggins, J. 1987. Light-induced charge separation in Photosystem I at low temperature is not influenced by vitamin Kj. Biochim. Biophys. Acta (in press). 

The WOC is oxidized stepwise by a nearby tyrosine residue (Tyrz), which is itself oxidized by the chlorophyll cation radical P680+ (formed by light-induced charge separation). The electrons are eventually used by PSII for the reduction of plastoqui-none. After the WOC has lost four electrons, the accumulated oxidizing power drives the formation of molecular oxygen from two substrate water molecules, and the catalytic system is reset. The sequence of the four electron-transfer steps is summarized in the Kok cycle [32] of Figure 4.5.3, where the most probable spectroscopically derived oxidation states of the Mn ions [33] are shown for each of the five redox state intermediates S (n - 0-4). 

From a functional point of view, this important property can be readily built into low molecular weight chromophore assemblies acting as artificial reaction centers (coordination compounds, the population of CT states is directly related to the concept of light-induced charge separation in photosynthesis. Whenever such CT states are photoreactive and lead to the formation of the same kind of permanent redox products as observed in photosynthesis, the most essential features of the primary light reactions have been successfully duplicated. In a more strict sense, this is of course only true, if actinic red or NIR-light of comparable wavelength is absorbed by both the natural and artificial photosynthetic systems. 

The electron transfer pathway shown in Fig. 1 is driven by light-induced charge separation in the two photosystems. The first of these systems is PS II, shown on the left of Fig. 1. In PS II, the radical cation generated by the charge separation extracts... 

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