Unidirectional electron transfer

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

In the net sense, the bilayers provide a means for directed or unidirectional electron transfer from an external film to the electrode and in that sense they impart a rectifying character to the electrode-film interface. 

Ringsdorf and co-workers have shown that triphenylenes can form alternating donor—acceptor supramolecular polymers in solution by doping them with equimolar amounts of electron acceptors.128129 Supramolecular polymers formed in this manner allow for electron transfer perpendicular to the molecular planes upon excitation of the chromophores, i.e., unidirectional charge-transport through the column. 130 The formation of these donor—acceptor pairs is favored in apolar solvents. In more polar solvents the triphenylenes alone do not polymerize and consequently donor—acceptor polymers with low DP are formed. 

Tab. 17.5 Rates for unimolecular and bimolecular electron transfer for porphyrin-based DeDp-ApAe complexes with amidinium-carboxylate interfaces. 7 7.3 Unidirectional PCET... 

The Bchl-b molecules of the special pair are arranged with a nearly perfect two-fold symmetry. The Bchl-b rings of the special pair are nearly parallel to this symmetry axis. A more subtle deviation from symmetry is the different degree of nonplanarity of the two Bchl-b ring systems of the special pair. The tetrapyrrole ring of BCmp (M = branch, P = special pair) is considerably more deformed than that of BCLP. This can cause an unequal charge distribution between the two Bchl-b systems of the special pair, which in turn can be part of the reason for unidirectional electron transfer, essential for the photosynthesis process [38]. 

Many chromophore molecules, such as bacteriochlorophylls, bacteriopheophytins and quinones, are arranged in Reaction Centers with a relevant distance and enei status such as to ensure unidirectional electron transfer. Therefore even a single Reaction Center is a sophisticated molecular device suitable for technological approaches. 

The principle of "mediated" electron transfer, whereby electrons are passed from the reduced form of a relatively negative redox couple to the oxidized form of a relatively positive couple, has been demonstrated to occur between two polymer layers of slightly different Ru (bpy)3 complex polymers by Murray and coworkers (18), This kind of stepwise, unidirectional electron transfer may be very significant in future polymer coated PEC cells which seek to separate charge, and of additional Interest, Ru (bpy)3 complexes are frequently used as cyclic PEC catalysts in water splitting experiments. Some details of this experiment are thus Informative. 

These preliminary calculations do not immediately provide an explanation for the unidirectionally of electron transfer in the RC. However, there are several potentially important factors which have been omitted. It is possible that residues that have been assumed to be charged in this analysis are actually neutral because they are buried in the protein or low dieletric material around the protein. Also, no partial charges have been placed on the bacteriochlorophyll and bacteriopheophytin acetyls, keto groups or esters. These are often close to neighboring cofactors. Further work on this problem is in progress. 

Figure 3.2 Uni-directional (A) and bi-directional (B) PCET. Synonyms for unidirectional PCET are collinear PCET, concerted electron-proton transfer (CEP, ref. 236), electron-proton transfer (EPT, 120 and 237), concerted proton-electron transfer (CPET, 238 and 239) and concerted electron transfer proton transfer (ETPT, ref. 240). Bi-directional PCET is also termed orthogonal PCET, bi-directional concerted electron-proton transfer (CEP, ref. 236) and multisite electron proton transfer (MS-EPT, ref. 237). Adapted from ref. 21.

A central issue discussed at this symposium is nature of the initial electron acceptor I is it the monomer bacteriochlorophyll on the L side, Bj, or the monomer bacteriopheophytin on the L side, Hj At some level, an answer to this question must also provide significant information on the origin of unidirectional electron transfer. If an intermediate such as P B exists, then there are two electron transfer steps, P P Bj " and P Bj which is expected to depend on electric field. If there is only one step and the state P B participates to mediate the interaction between P and P Hj via superexchange, then the electric field is expected to also affect the energy of this virtual intermediate and may affect the electronic coupling matrix element [1-3]. 

The observed asymmetry of the spin density distribution in favor of Dl has been explained using a model that assumes an energetic difference between the dimer halves Dl and Dj of a magnitude comparable to that of the interdimer interaction energy. Significant shifts of spin density between Dl and Df have been observed, when RC s of different native bacteria and mutants were compared, indicating differences in the energetics of the primary donors in the different bacteria. The orbital asymmetry of the primary donor is obviously a common feature in many bacterial photosystems and may play a functional role for the unidirectional electron transfer in bacterial photosynthesis. 

Reaction Centers (RC s) from phototrophic bacteria catalyze light-driven transmembrane electron transfer as a first step in the (cyclic) electron transfer chain of such bacteria (for a review see Okamura et al., 1983 and Dutton et al., 1982). Many of the structural and functional features of RC s have already been elucidated the remaining questions mainly focus on (i) the effects of transmembrane gradients (of redox potential and electrochemical potential of protons) on the reactions catalyzed by the RC s and (ii) the interactions between RC s and physiological and artificial electron donors and acceptors. Many of the unsolved aspects can be optimally investigated under conditions, in which the RC s have been reconstituted into artificial membranes either in planar (Schonfeld et al., 1979) or vesicular form (Crofts et al., 1977 Pachence et al., 1979). Here I report on the structure of reconstituted RC vesicles and light-dependent unidirectional proton translocation catalyzed by these vesicles. 

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