Mediators, electron transfer bridges

It is now believed that the MoFe-protein s P-cluster contains a [4Fe-3S] cuboid joined to a [4Fe-4S] cuboid, although, as discussed below, it was first reported crystallographically as two [4Fe-4S] clusters.8 Uncertainty existed for sometime as to exact nature of bridging disulfide or sulfide ligand joining the two Fe S clusters but it is now known that the P-cluster does NOT contain a disulfide bond. This is important because the all-ferrous structure [4Fe-4S]° proposed from Mossbauer studies then becomes more possible for the P-cluster s [4Fe-4S] cube. In 1993 Bolin et al.1 proposed a six-coordinate S for the P-cluster s center as in Figure la,b of Thorneley s article.8 This is now believed to be the correct conformation. A central six-coordinate S makes this cluster much harder to synthesize in the laboratory, and this feat has not been accomplished as of the date of this text s publication. Whatever its oxidation state or structure, the P-cluster mediates electron transfer from Fe-protein to the M center of MoFe-protein, and it must be reduced at some point to allow transfer of its electron(s). 

In complex (81), the electron-donating phenothiazine moiety is separated from the Ru(bpy)2 " unit by a triazole bridge that carries a formal negative charge. An investigation of this system shows that such anionic bridges can mediate electron transfer between chromophore and quencher. ... 

The phenomenon of ligand bridging between redox centres in homogeneous electron transfer reactions is well established. It has now been shown that such ligands may adsorb on to the electrode surface and mediate electron transfer to or from the electrode from some oxidizable or reducible species in solution. 

An area currently very active in electrochemical research deals with the design, fabrication and applications of chemically modified electrodes (CME s). The attractiveness of CME s stems from their potential to replace precious metals such as Pt in electrocatalysis for energy production (1-9), energy storage (10-13), electrosynthesis (14-19), electroanalysis (20-28), and other purposes (29-31). One approach has been to "immobilize", either by covalent attachment, strong adsorption or incorporation into polymeric structures, electrochemically active molecules, called mediators, which act as electron transfer bridges between the electrode surface and the solution species. It has been... 

A possible explanation for the lack of electron-transfer characteristics in the trimer 9d is derived when extrapolating the linear relationship in Fig. 9.8 to the distance of the trimer. As a matter of fact, the charge-separation would not be able to compete with the intrinsic singlet lifetime of C6o (i.e. dashed line). This, in turn, explains the lack of fullerene emission quenching in 9d. Nevertheless, the photophysical assays clearly established that oPPE bridges effectively mediate electron-transfer processes over distances up to 20 A. These findings were further corroborated by quantum mechanical calculations. 

Upon photoexcitation fast intramolecular charge-separation is observed similar in rate and in temperature dependence to the early steps of photosynthesis. It is proposed that the bridges play an active role in mediating electron transfer via through-bond interaction, thereby enhancing the rate of charge separation significantly as compared to other model systems. 

Scourotis, S.S. and Beratan, D.N. (1999) Theories of structure-function relationships for bridge-mediated electron transfer reaction, in Jortner, J., Bixon, M. (eds.), in Advances in Chemical Physics 107, Part 1, John Wiley Sons. NY., 377- 452. 

Nitrogenase catalyzes the ATP-driven eight-electron and eight-proton reduction of N2 to 2 NH3 and H2 and offers the most complex and fascinating example of Fe-S cluster mediated electron transfer. The electron transport chain involves transfer of electrons from the subunit-bridging [4Fe-4S] center on the homodimeric Fe-protein, to the subunit-bridging double-cubane [8Fe-7S] P-cluster (see... 

The 7i-stacked bases of ds DNA might be expected to provide a better medium for bridge-mediated electron transfer than the sigma bonds of proteins or hydrocarbons. It has in fact been proposed by Turro and Barton [18d] that ultrafast photoinduced electron transfer processes involving intercalated donors and acceptors can occur with little or no distance dependence. According to this paradigm, duplex DNA can function as a molecular wire or r-way . 

Solvent-mediated electron-transfer reactions involves the transient transfer of an electron to the solvent acting as a bridge between the donor and the acceptor [1]. Clusters offer the possibility to split this sequential process, the first step of which is the transfer on an electron from the donor to the solvent. In the absence of an acceptor, this step also occurs in solution, but cluster experiments allow a more direct characterization as follows. 

Figure 4.17 Schematic representation of bridge-mediated electron transfer (ET) and hole transfer (HT) in a linear supermolecule. The dominant route (ET or HT) involves the bridge orbitals lying...

S. S. Skourtis and D. Beratan, Theories for structure-function relationships for bridge-mediated electron transfer reactions, in Electron Transfer—From Isolated Molecules to Biomolecules, edited by M. Bixon and, 1. Jortner, Advances in Chemical Physics, Vol. 106 (Wiley, New York, 1999), Part I, p. 377. 

Skourtis S S and Beratan D N 1999 Theories of structure-function relationships for bridge-mediated electron transfer reactions Adv. Chem. Phys. 106 377-452... 

The variety of electron transfer reactions described above for PQ2+ and PQ + obviously provide some (model compound) information related to the possible mechanism of action of PQ2+ in herbicidal applications. An even bigger and more applicable range of electron transfer processes involving PQ2+ and PQ + is provided by studies of the action of paraquat as a mediator (i.e. electron-transfer bridge), or terminal electron acceptor, in redox processes involving biologically active compounds (see for example Steckham and Kuwana, 1974 Krasnovsky, 1972). Whilst such studies properly reside outside the scope of this review, there arc two aspects of bipyridylium ion... 

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