Electron transfer biological systems

There has been a resurgence of interest in proton-coupled redox reactions because of their importance in catalysis, molecular electronics and biological systems. For example, thin films of materials that undergo coupled electron and proton transfer reactions are attractive model systems for developing catalysts that function by hydrogen atom and hydride transfer mechanisms [4]. In the field of molecular electronics, protonation provides the possibility that electrons may be trapped in a particular redox site, thus giving rise to molecular switches [5]. In biological systems, the kinetics and thermodynamics of redox reactions are often controlled by enzyme-mediated acid-base reactions. 

When a compound is oxidized, it loses electrons. In biological systems, electrons from oxidation are generally transferred to electron-carrying molecules, such as NAD+ or FAD to form NADH and FADH2, respectively. Note that NAD+ and FAD are the oxidized forms of the molecules and NADH and FADH2 are the reduced forms. Thus, biological oxidations generate reduced electron carriers. Reduced electron carriers donate their electrons to acceptor molecules and become reoxidized in the process. The acceptor molecules are reduced because the oxidation of one species (e.g., the reduced electron carrier) cannot occur without the simultaneous reduction of another species (e.g., the acceptor molecule). 

Most reactions that occur in living cells are some form of oxidation-reduction reactions. Oxidation-reduction reactions must occur together, since no substances can lose electrons without another substance gaining electrons. In biological systems, oxidation-reduction reactions involve not only transfer of electrons but also transfer of hydrogen that has both one proton (H+) and one electron. Oxidation of H2 gas will result in the release of H+ and one electron ... 

The high overpotential of the oxygen reduction can be significantly lowered with electrocatalysts which also improve the speed of electron transfer." p-Quinones are recognized as such type of compounds which can transfer electrons in biological systems. Thus, among others, anthraquinones attract considerable interest in this respect because of their p-quinoid substructure which can be easily reduced. In addition, anthraquinones are also conductive and can be shaped into monolayers. Thus, anthraquinones can be used for the modification of electrodes for the reversible electron transfer which is attributed to the p-quinones group." However, these... 

Moser, C.C., Dutton, P.L. Biological electron transfer measurement, mechanism, engineering requirements. In Quantum mechanical simulation methods for studying biological systems, D. Bicout and M. Field, eds. Springer, Berlin (1996) 201-214. 

The electron transfer rates in biological systems differ from those between small transition metal complexes in solution because the electron transfer is generally long-range, often greater than 10 A [1]. For long-range transfer (the nonadiabatic limit), the rate constant is... 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

The field of modified electrodes spans a wide area of novel and promising research. The work dted in this article covers fundamental experimental aspects of electrochemistry such as the rate of electron transfer reactions and charge propagation within threedimensional arrays of redox centers and the distances over which electrons can be transferred in outer sphere redox reactions. Questions of polymer chemistry such as the study of permeability of membranes and the diffusion of ions and neutrals in solvent swollen polymers are accessible by new experimental techniques. There is hope of new solutions of macroscopic as well as microscopic electrochemical phenomena the selective and kinetically facile production of substances at square meters of modified electrodes and the detection of trace levels of substances in wastes or in biological material. Technical applications of electronic devices based on molecular chemistry, even those that mimic biological systems of impulse transmission appear feasible and the construction of organic polymer batteries and color displays is close to industrial use. 

Bertini I, Luchinat C, Scozzafava A (1982) Carbonic Anhydrase An Insight into the Zinc Binding Site and into the Active Cavity Through Metal Substitution. 48 45-91 Bertrand P (1991) Application of Electron Transfer Theories to Biological Systems. 75 1-48 Bill E, see Trautwein AX (1991) 78 1-96 Bino A, see Ardon M (1987) 65 1-28 Blanchard M, see Linares C (1977) 33 179-207 Blasse G, see Powell RC (1980) 42 43-96... 

The most extensive studies on the genetics and molecular biology of CODH have been performed with the coo system of R. rubrum. A coo gene cluster contains CODH (CooS), an Fe-S electron-transfer protein (CooF), metal cluster assembly proteins (CooCTJ) (126), and... 

Much effort has also been directed toward mimicking electron transfer on natural photosynthetic systems. Recently, the group of Harada has been able to prepare monoclonal antibodies against metallo porphyrins and show that the biological edifice can control photoinduced electron transfer from the porphyrin to organic acceptor molecules in solution. As it was important to design a biomolecule able to accommodate not only the metalloporphyrin unit but also organic substrates, Harada recently used a hexacoordinated phosphorus... 

Juris A (2001) In Balzani V (ed) Electron transfer in chemistry. Biological and artificial supramolecular systems. Wiley-VCH, Weinheim, Germany, vol 3, p 655... 

J. R. Bolton, N. Mataga, and G. MvLendon, Electron Transfer in Inorganic, Organic, and Biological Systems Advances in Chemistry Series 228, American Chemical Society, Washington, D.C., 1991. 

From an electrochemical viewpoint, biological systems are highly branched circuits consisting of ionic conductors of aqueous electrolyte solutions and highly selective membranes. These circuits lack metallic conductors, but it has been found relatively recently that they contain sections that behave like electronic conductors (i.e., sections in which electrons can be transferred over macroscopic distances, owing to a peculiar relay-type mechanism). 

The high catalytic activity of enzymes has a number of sources. Every enzyme has a particular active site configured so as to secure intimate contact with the substrate molecule (a strictly defined mutual orientation in space, a coordination of the electronic states, etc.). This results in the formation of highly reactive substrate-enzyme complexes. The influence of tfie individual enzymes also rests on the fact that they act as electron shuttles between adjacent redox systems. In biological systems one often sees multienzyme systems for chains of consecutive steps. These systems are usually built into the membranes, which secures geometric proximity of any two neighboring active sites and transfer of the product of one step to the enzyme catalyzing the next step. 

Bertrand, P. Apphcation of Electron Transfer Theories to Biological Systems. Vol. 75, pp. 1-48. 

Heterogeneous electron reactions at liquid liquid interfaces occur in many chemical and biological systems. The interfaces between two immiscible solutions in water-nitrobenzene and water 1,2-dichloroethane are broadly used for modeling studies of kinetics of electron transfer between redox couples present in both media. The basic scheme of such a reaction is... 

Electron transfer reactions constitute an ubiquitous class of chemical reactions. This is particularly true in biological systems where these reactions often occur at interfaces, in photosynthesis for instance. It is therefore challenging to use the surface specificity and the time resolution of the SHG technique to investigate these processes. At liquid-liquid interfaces, these processes are mimicked through the following scheme ... 

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