The Intramolecular Wire

Electrons zip along an intramolecular wire between the eatalytie Cluster C and Cluster B at a rate of 3200s at 55 C under optimal eondi-tions. The mechanism of electron transfer from CO to Cluster C to Cluster B is relatively well understood, since it has been studied by a variety of rapid kinetics and electrochemical methods. 

The Intermoleclar Wires How Electrons Enter and Exit CO Dehydrogenase 

These results provide strong evidence for the existence of a CO channel between Cluster C in the CO dehydrogenase subunit and Cluster A in the acetyl-CoA synthase subunit. Such a channel would tightly couple CO production and utilization and help explain why high levels of this toxic gas do not escape into the environment. Instead, microbes sequester the CO as an energy rich carbon source. The channel would also protect aerobic organisms that harbor the microbes and help keep CO below toxic levels. 

Acetyl-CoA Synthase Another Catalytic NiFeS Cluster 

The chemistry of acetyl-CoA synthesis is thought to resemble the Monsanto process for acetate synthesis in that a metal center binds a methyl group and CO and the CO undergoes a carbonyl insertion into the methyl-metal bond. Elimination of the acetyl group is catalyzed by a strong nucleophile, iodide in the industrial process and CoA in the biochemical one. Currently, there are two views of the catalytic mechanism. 

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How eould this eatalytie bias be controlled One possibility is that the proton transfer pathway eould eontribute to specifieity (Peters et al., 1998). Another possibility is that differences in midpoint potential of the FeS clusters (or other redox sites) that constitute the intramolecular wire could be tuned to facilitate one of the two directions of the reaction. For example, these redox sites could best match the midpoint potentials of a particular oxidized or reduced electron carrier (Holm and Sander, 1999). Apparently, a conformational change in succinate dehydrogenase, coupled to the reduction of FAD, is responsible for its catalytic bias for fumarate reduction (Hirst et al., 1996). 

In contrast to the molecular wire of molecular interface, electron mediators are covalently bound to a redox enzyme in such a manner as an electron tunneling pathway is formed within the enzyme molecule. Therefore, enzyme-bound mediators work as molecular interface between an enzyme and an electrode. Degani et al. proposed the intramolecular electron pathway of ferrocene molecules which were covalently bound to glucose oxidase [ 4 ]. However, few fabrication methods have been developed to form a monolayer of mediator-modified enzymes on the electrode surface. We have succeeded in development of a novel preparation of the electron transfer system of mediator-modified enzyme by self-assembly in a porous gold-black electrode as schematically shown in Fig.12 [14]. 

Another way of arranging the intramolecular transmembrane electron transfer is to use the so called molecular wires, i.e. molecules with the electron conduction chain of conjugated bonds, redox active polar terminal groups and the length sufficient to span across the membrane. Such molecules can in principle provide for electron transfer from the externally added or photogenerated reductant across the membrane to the oxidant. This mechanism was suggested [41, 94] to explain the action of carotene-containing System 1 and 38 of Table 1. However, as it was shown later, the transmembrane PET in these systems proceeded also without carotene. 

Consider the case in which a DBA assembly acts as the molecular wire. There are two mechanisms for the current to pass from one electrode to the other. In the chemical mechanism D transfers an electron to A through the bridge. The newly formed D" " and A are then rapidly reduced and oxidized, respectively, at the electrodes, giving rise to a steady-state situation in which the current is determined by the rate of intramolecular electron transfer from D to A. The conductance via this pathway may be estimated from... 

An important potential application of electron transfer is the intramolecular electron transfer that could lead chemists to fabricate molecular wires. This rich field has been pioneered by Taube and Creuz who have discovered, in 1969, the first mixed-valence complex, the so-called Creuz-Taube ion, a diruthenium complex in which the Ru ions have oxidation states between 2 and 3 and are bridged by a pyrazine ligand ... 

Fig. 10 Simple circuit diagrams of the different series and parallel association of molecular wires Mi and M2 discussed in the text. The two molecular wires are (a) bonded in series, (b) connected in parallel on the metallic pads, (c) forming a single molecule with one intramolecular node, and (d) forming a single molecule with two intramolecular nodes...

Fig. 12 The Fig. 11 intramolecular circuit can be decomposed into four tunneling paths, to apply the parallel superposition rule, and predict the transmission coefficient through Fig. 11 molecule. Molecules 1 and 2 are for the contribution of two short tunnel paths and molecules 3 and 4 for the contribution of the two longer paths through the central perylene wire... 

One of the first applications of the new mesh and node intramolecular circuit rules discussed above is the well-known problem in electrical circuit theory of the balancing of a Wheatstone bridge. In Fig. 21, a molecular Wheatstone bridge is presented, made of loop-like 4 tolane molecular wires bonded via benzopyrene end-groups for nano-pads 1 and 3, and via pyrene end-groups for nano-pads 2 and 4. This four-electrode and four-branch molecule is connected to a battery and an ammeter. 

Fig. 21 The variation of the balancing tunneling current of the four branches four electrodes monomolecular Wheatstone bridge connected as presented in (a). In (b), the dashed line is for the current intensity 7W (in absolute value) measured by the ammeter A and deduced from the standard Kirchoff laws calculating each molecular wire tunneling junction resistance of the bridge one after the other from the EHMO-ESQC technique. In (b), Hie full line is the same tunnel current intensity but obtained with the new intramolecular circuit rules discussed in Sect. 2. (c) The resistance of the branch used to balance the bridge as a function of its rotation angle. The minimum accessible resistance by rotation is 78 MQ for the short tolane molecular wire used here...

Before examining the electrochemical properties of this class of compounds (we will limit the discussion to homonuclear derivatives), it must be clear that the technological application of molecular wires belongs to solid-state chemistry. Nevertheless, since the main target of such new molecules is to conduct electricity, it seems useful to ascertain preliminarily their intrinsic ability towards intramolecular electron mobility by electrochemical investigations in solution, i.e. in the absence of intermolecular interactions. 

The interpretation of the experimental data for the kinetics of photoacid-solvent clusters is complicated by the substantial fragmentation of the clusters after the excited-state reaction. The heat of reaction is often sufficient to allow the evaporation of one or several solvent molecules [14,16]. This difficulty does not arise when the H atom transfer or proton transfer occurs intramolecularly along a solvent wire attached to a bifunctional chromophore. 

Now, as we have defined wire-like transport by a motion that is assisted by molecular bridges, we may proceed to the mechanistic point of view. In particular, we will contrast theoretical results with experimental data for molecular wire-like behavior in regard to the transfer of electronic charge and/or energy. Intramolecular electron-transfer (ET) rate constants characterize the charge transport in DBA conjugates and in electronic transport junction we can apply the word conductance . 

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