Critical distance electron transfer

In summary, there are two classes of amino acid residues one involved in critical alignment of docking for electron transfers and the other involved in crude orientation or transient complex formation. Those involved in critical docking aUgnment tend to be different for the two substrates, Fd and those bound to opposite sides of FNR. Finally, hydrophobic clusters usually line the binding cavity and are critical for electron transfer. The situation with plastocyanin and cytochrome Cg to be presented next is similar the two independent sets of data demonstrate a common design scheme for redox partners of long-distance electron transfers. 

The pathway model makes a number of key predictions, including (a) a substantial role for hydrogen bond mediation of tunnelling, (b) a difference in mediation characteristics as a function of secondary and tertiary stmcture, (c) an intrinsically nonexponential decay of rate witlr distance, and (d) patlrway specific Trot and cold spots for electron transfer. These predictions have been tested extensively. The most systematic and critical tests are provided witlr mtlrenium-modified proteins, where a syntlretic ET active group cair be attached to the protein aird tire rate of ET via a specific medium stmcture cair be probed (figure C3.2.5). 

In addition, the determination of metal-ligand bond distances in solution and their oxidation state dependence is critical to the application of electron transfer theories since such changes can contribute significantly to the energy of activation through the so-called inner-sphere reorganizational energy term. 

The mechanism of electron transfer over the long distances (of the order of 1000 pm or more) necessitated by the large size of redox enzymes is one that is not completely clear despite much current study. These transfers are critical whether one is considering the photosynthetic center (page 917) or electron carriers such as the... 

Next the difficulties in obtaining a good description of the particle electrode interaction are noticed. For non-electrochemical systems several particle surface interaction models exist of which the perfect sink , that is all particles arriving within a critical distance of the electrode are captured, is the simplest one. However, the perfect sink condition can not be used, because it predicts a continuous increase in particle codeposition with increasing current density, which contradicts experimental observations. Therefore, an interaction model based on the assumption that the reduction of adsorbed ions is the determining factor for particle deposition is proposed. This electrode-ion-particle electron transfer (EIPET) model leads to a Butler-Volmer like expression for the particle deposition rate ... 

Consider a J-electron system, such as a transition metal compound. The valence d atomic orbitals do not range far from the nucleus, so COs comprised of Bloch sums of d orbitals and, say, O 2p orbitals, tend to be narrow. As the interatomic distance increases, the bandwidth of the CO decreases because of poorer overlap between the d and p Bloch SUMS. In general, when the interatomic distance is greater than a critical value, the bandwidth is so small that the electron transfer energy becomes prohibitively large. Thus, the condition for metallic behavior is not met insulating behavior is observed. 

Alkanethiol-based or alkylsiloxane-based SAMs have been profitably employed in all these instances to probe the distance effect in electron-transfer dynamics. The thiol-based SAMs have the virtue that the spacer length can be predictably altered simply by varying the number of methylene units in the chain. The distance dependence of is embodied in the parameter f, the decay coefficient (for a critical discussion of the subtleties involved in the extraction and interpretation of this parameter, see Ref. [399]). A value of 0.49 0.07 has been reported for f for n-InP-alkanethiol-ferrocyanide interfaces [403]. This value is smaller than its counterpart for corresponding films on gold surfaces which range from 0.6 to 1.1 per methylene unit. The reason for this difference is not entirely clear, although several hypotheses were advanced by the authors [403]. 

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