Protein-electrode interface

The Marcus Theory can also be applied for heterogeneous electron transfer reaction at electrode surfaces [24 and references therein]. The electronic coupling between the protein and the electrode can be varied using different self-assembled monolayers controlling the orientation of the redox active protein on the surface and the distance between the redox active site of the protein and the electrode. The driving force is related to the appHed potential and the redox potential of the protein. In many cases the rate of electron transfer across the protein-electrode interface is limited by conformational reorganization. This has focussed the efforts of many groups on tailored interaction between proteins and enzymes and electrode surfaces. 

Fig. 8. Some possible arrangements of negatively charged proteins, cations, and the deprotonated PGE electrode surface. Upper. Cations bound in potential cavities at interface of protein and electrode. Middle Layer stabilized by cations binding at the protein-electrode interface and between adjacent protein molecules. Lower. Promotion of protein-electrode interaction by complex formation with a positively charged protein molecule...

Fe "-OOH (ES) complex, 43 95-97 heme-bound CO, 43 115 lock-and-key model, 43 106-107 mutation in proximal heme cavity, 43 98 residue location, 43 101-102 van der Waals surfaces, 43 112-113 Velcro model, 43 107 zinc-substituted, 43 110-111 plastocyanin, cross-linked, cyclic voltammogram, 36 357-358 promoters, 36 345-346 protein-electrode complex, 36 345, 347 redox potential, 36 349 self-exchange rate constants, 36 402 stability at electrode/electrolyle interface, 36 349-350... 

The site-specific modification of enzymes with a single electron-relay group located near to the redox cofactor and providing efficient electrical contact with the conductive support has been achieved by the reconstitution of enzymes with cofactors covalently linked to redox groups. Affinity interactions between enzymes and their cofactors at the electrode interface can allow the efficient electrical contacting of aligned proteins. 

The photoregulated electrical interactions between the various redox proteins and the electrode interfaces provide a means for the amperometric transduction and amplification of recorded optical signals. These integrated assemblies reveal the fundamental potential for future bioelectronic devices. 

The protein environment also provides a challenge in investigating redox processes since it acts as a barrier to electron flux in and out of the iron centre. This is due to the steric bulk of the folded polypeptide chain that can also act as an electronic insulator. As there is distance dependence in the outer-sphere electron transfer at the protein/solid electrode interface, these heterogeneous electron-transfer rates tend to be slow. This problem is overcome by the use of a small-molecule mediator as discussed in Section 2.2.3. 

Membrane structures that contain the visual receptor protein rhodopsin were formed by detergent dialysis on platinum, silicon oxide, titanium oxide, and indium—tin oxide electrodes. Electrochemical impedance spectroscopy was used to evaluate the biomembrane structures and their electrical properties. A model equivalent circuit is proposed to describe the membrane-electrode interface. The data suggest that the surface structure is a relatively complete single-membrane bilayer with a coverage of 0.97 and with long-term stability/... 

Galit Zilberman joined the research group at Drexel in November 2003. Her previous research at Tel Aviv University with QCM measurements at the solution/electrode interface [59] led us to e q)lore the use of the QCM/HCC as a detector of protein-ligand interactions in aqueous solution [60], and as a detector of the growth of E Coli bacteria on thin film of nutrient medium deposited on the QCM [61]. Zilberman also studied the growth kinetics of alkyl- and carboxylic acid self-assembled monolayers (SAMs) on gold and the EDC-catalyzed amide bond formation on a carboxylic acid-tenninated SAM. 

This indeed is not an easy task to examine because it may well be that it is only at certain sites in a membrane that there is sufficient electronic conductivity for the electrode to function. It may well be that our model of a biological electrode (say a membrane) is a model of an insulating layer in which are insulated a number of wires, and this would mean that the proteins which are part of biomembranes, and which stick through them, may be the source of the transport between the two sides of the membrane and an origin of an electron and proton transfer site at the protein-solution interface (Figure 19). 

Drawing a simple comparison with bare complexes Uke ferrocene, we would expect that the electron-transfer activity of a metal centre enclosed or buried within a protein molecule should be considerably suppressed. Several investigators have addressed the problem of how electrons may move rapidly between fixed remote sites in proteins, and it is certain that both distance and the nature of the intervening medium are important [12-16]. At an electrode interface the electron may have to traverse some depth of polypeptide matrix and may also encounter strongly bound ions and solvent molecules. How much of a restriction might this impose We may reason that two limiting situations will occur. 

These early discoveries spawned a number of investigations largely devoted to understanding the protein/functionalized electrode interface. Questions to be answered included the following. 

Interfacial electrostatics pose a far more critical determinant for the electrochemistry of proteins than is usually the case for simple redox complexes. This is apparent even from the simplest of experiments in which one seeks to examine the electrochemistry of a protein other than cytochrome c at one of the electrode interfaces described above. The result is generally poor even with a high concentration of supporting (1 1) electrolyte to screen adverse coulombic interactions. Often, no voltammetric response is observed at all. A major factor in remedying this... 

In this chapter, electrochemical properties of ET proteins at electrode interfaces studied by spectroelectrochem-ical techniques are described. In situ spectroelectrochemical techniques at well-defined electrode surfaces are sufficiently selective and sensitive to distinguish not only steady state structures and oxidation states of adsorbed species but also dynamics of reactants, products, and intermediates at electrode surfaces on a monolayer level. The spectroelectrochemical techniques used in studies of ET proteins include IR reflection-absorption, potential-modulated UV-vis reflectance (electroreflectance), surface-enhanced Raman scattering (SERS) and surface plasmon resonance, total internal reflection fluorescence, (TIRE) and absorbance linear dichroism spectroscopies. 

---