Electron tunneling wires

Although ferryl intermediates of horseradish peroxidase and microperoxidase-8 have been produced in reactions with photogenerated [Ru(bpy)3]3+ [5], analogous experiments with P450s were unsuccessful, presumably due to the inefficiency of electron transfer from the buried heme active site through the protein backbone [6]. Photoactive molecular wires (sometimes referred to as metal-diimine wires, sensitizer-tethered substrates, or electron tunneling wires) were developed to circumvent this problem by providing a direct ET pathway between [Ru(bpy)3]3+ and the heme. These molecular wires, which combine the excellent photophysical properties of metal-diimine complexes... 

Fig-i Electron tunneling wires consist of a substrate or ligand tethered to a redox-active head group... 

Belliston Bittner W (2005) Ultrafast Photoreduction of Nitric Oxide Synthase by Electron Tunneling Wires. PhD, California Institute of Technology... 

Dunn AR, Dmochowski H, Winkler JR, Gray HB. Nanosecond photoreduction of cytochrome P450cam by channel-specific ru-diimine electron tunneling wires. J. Am. Chem. Soc. 2003 125 12450-12456. 

Troisi A, Ratner MA (2005) Modeling the inelastic electron tunneling spectra of molecular wire junctions. Phys Rev B 72(3) 033408... 

Apart from electron promoters a large number of electron mediators have long been investigated to make redox enzymes electrochemically active on the electrode surface. In the line of this research electron mediators such as ferrocene and its derivatives have successfully been incorporated into an enzyme sensor for glucose [3]. The mediator was easily accessible to both glucose oxidase and an electron tunnelling pathway could be formed within the enzyme molecule [4]. The present authors [5,6] and Lowe and Foulds [7] used a conducting polymer as a molecular wire to connect a redox enzyme molecule to the electrode surface. 

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]. 

A strong electric field (on the order of volts/angstrom) is created at the tip of a sharp, single - crystal wire. The electrons tunnel into the vacuum and are accelerated along radials trajectories by Coulomb repulsion. When the electrons impinge on a fluorescent... 

Figure 1-5. The flash-quench sequence for deliver mg electrons or holes to the active site of P450cam. The Ru-wire is excited with 470-nin light (Ru ), and intercepted witli either CofNHjjsCT or p-methoxy-WgV-dimethylaniline (PMDA) to generate the oxidized (Ru ) or reduced (Rif) Ru-wire. The photochemically generated hole or electron tunnels to the heme on the millisecond timescale, forming a heme cation radical or ferrous heme. See ref. 90.

The pruning procediure naturally leaves intact donor and acceptor complexes, and a number amino adds that make up the tunneling bridge which connects Ru and Cu ions. In this particular case, two stretches of the protein backbone provide the connection. The two stretches form "molecular wires along which electron tunnels between donor and acceptor. The two wires were identified because for each one the connection to the redox site is strong on one end and weak on the other the Met residue is more weakly coupled than the Cys residue to Cu ion, and the His residue of the Met wire is more strongly coupled to Ru than the Gin residue of the Cys wire to the Ru complex. The relative importance of these two paths can only be established in a more accurate calculation that can quantitatively correctly... 

The calculations shown in Fig. 2 were performed at a low level electronic structure description. We find, however, that the relevant part of the molecule is rather small and therefore more accurate, ab initio calculations are possible on the pruned molecule. The results of such calculations on a model system, which is derived from our pruned Hisl26 molecule, is the subject of the rest of the paper. Our goal is to find out how exactly electrons tunnel in molecular wires shown in Fig. 2. 

---