Electron transfer distance

Figure C3.2.5. Strongest tunnelling patliways between surface histidines and tire iron atom in cytochrome c. Steps in patliways are denoted by solid lines (covalent bonds), dashed lines (hydrogen bonds), and tlirough-space contacts (dotted lines). Electron transfer distance to His 72 is 5 A shorter tlian in His 66, yet tire two rates are approximately...

Incorporation of an artificial flavin nucleobase and of a cyclobutane pyrimidine dimer building block into DNA DNA double strands, DNArPNA hybrid duplexes, and DNA-hairpins, provided compelling evidence that an excess electron can hop through DNA to initiate dimer repair even at a remote site. The maximum excess electron transfer distance realised so far in these defined Donor-DNA-Acceptor systems is 24 A. New experiments are now in progress to clarify whether even larger transfer distances can be achieved. 

We next focus on the use of fixed-site cofactors and coenzymes. We note that much of this coenzyme chemistry is now linked to very local two-electron chemistry (H, CH3", CH3CO-, -NH2,0 transfer) in enzymes. Additionally, one-electron changes of coenzymes, quinones, flavins and metal ions especially in membranes are used very much in very fast intermediates of twice the one-electron switches over considerable electron transfer distances. At certain points, the chains of catalysis revert to a two-electron reaction (see Figure 5.2), and the whole complex linkage of diffusion and carriers is part of energy transduction (see also proton transfer and Williams in Further Reading). There is a variety of additional coenzymes which are fixed and which we believe came later in evolution, and there are the very important metal ion cofactors which are separately considered below. 

An interesting approach to measuring rates of electron transfer reactions at electrodes is through the study of surface bound molecules (43-451. Molecules can be attached to electrode surfaces by irreversible adsorption or the formation of chemical bonds (461. Electron transfer kinetics to and from surface bound species is simplified because there is no mass transport and because the electron transfer distance is controlled to some degree. 

Figure 13.16 (a) Polypeptide fold and (b) electron transfer distances in E. coli quinol-fumarate reductase, (c) intercofactor distances in the Wolinella succinogenes enzyme. (From Iverson et al., 2002. Reproduced by permission of the Journal of Biological Chemistry.)... 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

Analysis of Electron-Transfer Distances and Tunneling Constants 112... 

Table 1 Summary of best estimates of electron- transfer distances, tunneling constants, and calculated electron affinities [7aj. rReprinted with permission from the J. Phys. Chem. Copyright (2000) American Chemical Society...

Using conformational searching/quench dynamics and 7) relaxation measurements, each back-folded isomer was determined to be smaller than its extended counterpart. Thus, the effective distance of electron transfer was not reflected in the hydrodynamic radius of the molecule. Rather, the back-folded isomers were argued to be less mobile with the iron-sulfur core buried more deeply within them. The extended isomers were more mobile with the iron-sulfur core more able to come closer to the molecular surface. By this reasoning, the back-folded isomers had a larger effective electron transfer distance than the extended isomers. 

At loadings lower than 1 MX per 20 DNA bp, the fraction of the electrons captured by the intercalator was found to follow the relation D(t) = l/pln/c0t this assumes random MX intercalation in the DNA and an increase electron transfer to intercalator with ln(f) as expected for a single-step tunnelling process. The electron-transfer distances, after 1 min at 77 K, were about 8-10 bps for the most electron affinic intercalators MX and NPa. For these intercalators tunnelling decay constants p of 0.8 - 0.9 A, were reported with a /c0 = 1 x 10 s 1 in the standard relation for fall off of p with tunnelling distance, k = k0e bD. These... 

Using mutant proteins as well as a variety of redox pairs and electron-transfer distances the validity of the Marcus equation with respect to the thermodynamic driving force and distance dependence has been verified.153 This is even true for cytochrome c mutants functioning in living yeast cells.146... 

In Figure 1 are shown potential energy-normal coordinate curves for the vaig mode for site A as Fe111 in the reactants (R) and Fe11 in the products (P). The coordinate is a normal coordinate of the molecule composed of an equally weighted linear combination of the displacement coordinates (q) for the six Fe—O local coordinates, Q = l/V6[q(Fe—0)i + g(Fe—0)2 +. .. + g(Fe—0)6] = V6[q(Fe—O)]. It has no connection with an electron transfer distance nor with the intersite separation between reactants. 

The electron transfer from the photoexcited Ru(bpy)32+ to MV2+ confined in a polysiloxane film showed a complete static quenching following the Perrin model,32) and the electron transfer distance rc was 1.4 nm, which is comparable to a conventional electron transfer distance in biological systems. The presence of a tryptophan residue model, 3-methyindole (IND) enhanced much the quenching efficiency (Fig. 19.6) by lengthening the electron transfer distance, and the electron transfer distance was estimated to be 2.7 nm, almost twice that without the mediator.32 ... 

The same electron transfer was investigated in a polyethyleneoxide film known to be a polymer electrolyte capable of transporting ions. The electron transfer mechanism was analyzed to take place by both static and dynamic mechanisms,33) the electron transfer distance was estimated to be 1.7 nm, and the dynamic rate constant was 4.6 x 106 M 1s 1, which is two orders of magnitude lower than that in an aqueous solution. 

For a complete static quenching following the Perrin mechanism, Eq. (19.9) is derived, where r0 (nm) is the electron transfer distance between donor and acceptor molecules (molecular center to center). 

Figure 3.32. A change in the initial distribution of electron transfer distances Y(r) = cmo(r) with the encounter diffusion coefficient D. Ions are produced in 9,10-dicyanoanthracene (DCA, fluorescer) + p-anisidine (ANS, quencher) encounters at c = [ANS] = 0.3 M. The value of D is 4.6 x 10-5 cm2/s in acetonitrile and 0.094 x 10 5cm2/s in ethylene glycol. The parameters of Wi(r) were determined experimentally [90] and used in Ref. 27 to obtain these distributions.

As discussed in Chapter 4, a wide variety of functionalized alkane thiols, HS(CH2) -2, where 5 < n < 16, form highly ordered self-assembled monolayers. As illustrated in Figure 5.1, redox-active species can be covalently bound to these bridges. The seminal work of Chidsey [2], Acevedo and Abruna [3] and Finklea and Hanshew [4] has demonstrated that electroactive adsorbed monolayers can exhibit close to ideal reversible electrochemical behavior under a wide variety of experimental conditions of time-scale, temperature, solvent and electrolyte. These studies have elucidated the effects of electron transfer distance, tunneling... 

Willner et al. [52] have created some elegant interfacial supramolecular assemblies to address this issue by removing the non-covalently bound flavin adenine dinucleotide (FAD) redox center from glucose oxidase and immobilizing the enzyme on a tether consisting of cystamine chemisorbed on a gold surface, a pyrroloquinoline quinone (PQQ) link and FAD. The mediator potential and electron transfer distances of this assembly were carefully chosen so that transfer of electrons from the FAD to the PQQ and to the electrode is very fast. A maximum rate of 900 150 s-1 for the enzymatic reaction within this monolayer assembly was obtained, which is indistinguishable from the value of about 1000 s-1 obtained for the enzyme in solution. While monolayers can offer molecular-level control of the interfacial structure, the... 

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