Feature 19-1 Biological Redox Systems

The use of reversible redox systems in indirect electrolytic reactions may be of significance, especially in biological systems. Many of these systems comprise large molecules which diffuse slowly to the electrode and may lose some of their structural features on contact with the electrode, thereby becoming biologically inactive. In a number of cases this difficulty may be circumvented by using a suitable mediator to transfer the electrons between the biological redox system and the electrode. 

Electronic absorption spectroscopy is also a useful technique for the characterization of Cr intermediates. When coupled with global kinetic analysis, ligand stoichiometries, and spectra of transient species in the redox chemistry of Cr can be determined. The first applications of this global analysis to biologically relevant systems involved studies of the Cr(Vl) reactions with the main biological reductants Cys, GSH, and ascorbate (70, 97). The distinct electronic absorption features of the Cr(VI), Cr(V), Cr(lV), and Cr(lll) oxidation states... 

In this chapter, the unique features of transition metals in biological systems are discussed from the point of view of structural roles, spectroscopic properties, electron transfer, hydrolytic and redox catalysis, and metal-responsive gene expression. The following chapters provide more detail on these subjects. Several important examples not discussed elsewhere in this volume will be presented. The goal of this chapter (and this volume) is to acquaint the reader with the wide range of roles played by metal ions in biological systems and thereby to demonstrate why metals are such useful cofactors and why scientists from such broad disciplines are drawn to study their properties. 

Biological cell membranes are multi-component systems consisting of a fluid bilayer lipid membrane (BLM) and integrated membrane proteins. The main structural features of the BLMs are determined by a wide variety of amphiphilic lipids whose polar head groups are exposed to water while hydrocarbon tails form the nonpolar interior. The BLMs act as the medium for biochemical vectorial membrane processes such as photosynthesis, respiration and active ion transport. However, they do not participate in the corresponding chemical reactions which occur in membrane-dissolved proteins and often need redox-active cofactors. BLMs were therefore mostly investigated by physical chemists who studied their thermodynamics and kinetic behaviour . ... 

It is also of significance to incorporate complex molecules into microporous crystals to form photochemically or photophysically active centers. Because of the separation by the host framework, the complexes located in the channels or cages of microporous crystals are isolated. If the isolated centers with oxidation or reduction features are loaded in the connected and adjacent cages of a microporous crystal, redox pairs may be formed. Electron transfer may occur on these redox pairs under the excitation of light, and therefore photochemical reactions may proceed effectively. This is important for the utilization of solar energy. In addition, this type of assembly system may also be used to simulate the electron transfer process of oxidation-reduction in biological systems. 

So far, we have shown that redox-modulated recognition, a prevalent feature in biological systems, can be employed in the design of functional devices. As mentioned earher, one challenge to the efficient application of such systems is the ability to immobilize, order and thus individually address them. One way to provide the desired anisotropy is through the use of colloids functionalized with self-assembled monolayers (SAMs). " In a recent model smdy, a diacyl diaminopyri-dine-functionahzed gold colloid (DAP-Au), capable of binding flavin, has been... 

Although complex, the cytochromes c3 provide the opportunity to obtain information that will greatly extend our knowledge of biological electron transfer and the interaction of redox centers in multiheme proteins. Moreover, because of unique electrochemistry and electrical properties, the cytochromes c3 provide the opportunity to develop a system useful as a model for bioelectronic devices. Much research remains to be done to understand fully the redox properties of the cytochromes c3. However, the data discussed clearly define interesting and important issues, which include (1) the paths by which electrons move between hemes (2) how electrons enter and exit the cytochrome c3 molecule during physiological electron transfer (3) the nature of the factors that control the interaction potentials between hemes (4) the factors responsible for the observed behavior on metal surfaces and, importantly, (5) the specific molecular features responsible for the behavior of... 

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