Electron transfer in biology

Huber, R. Nobel lecture. A structural basis of light energy and electron transfer in biology. EMBO. ... 

Hellner EE (1979) The Frameworks (Bauverbande) of the Cubic Structure Types. 37 61-140 Hemmerich P, Michel H, Schug C, Massey V (1982) Scope and Limitation of Single Electron Transfer in Biology. 48 93-124... 

Part A. Quantum-Mechanical Theory of Diffusion Independent Electron Transfer in Biological Systems by Ephraim Buhks (University of Delaware)... 

Although electron transfers in biological systems are generally expected to be non-adiabatic, it is possible for some intramolecular transfers to be close to the adiabatic limit, particularly in proteins where several redox centers are held in a very compact arrangement. This situation is found for example in cytochromes C3 of sulfate-reducing bacteria which contain four hemes in a 13 kDa molecule [10, 11], or in Escherichia coli sulfite reductase where the distance between the siroheme iron and the closest iron of a 4Fe-4S cluster is only 4.4 A [12]. It is interesting to note that a very fast intramolecular transfer rate of about 10 s was inferred from resonance Raman experiments performed in Desulfovibrio vulgaris Miyazaki cytochrome Cj [13]. 

Sutin N, Brunschwig BS (1990) In Johnson MK et al (eds) Electron transfer in biology and the sohd state, American Chemical Society, Washington DC, p 65 (Advances in Chemistry)... 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

Intermolecular electron transfer plays an important role in the operation of biological systems. For example, electron transfer from one biological molecule to another is the primary act of energy conversion in the processes of respiration and photosynthesis. Despite a large number of works dedicated to the study of intermolecular electron transfer in biological systems, the mechanisms of these reactions have been insufficiently elucidated. This is due to great difficulties in the interpretation of experimental results which are in their turn explained by the very intricate structure of biological systems. 

EVOLUTION OF THE IDEAS ABOUT THE MECHANISMS OF ELECTRON TRANSFER IN BIOLOGICAL SYSTEMS... 

Chance and Williams [11] advanced a hypothesis that electron transfer in biological systems should be performed via rotation of electron carriers, cytochromes, between the donor and the acceptor. In this case, the electron transfer is ensured by successive approaches of the active centre of the cytochrome to the active centres of the donor and the acceptor of electrons. 

Electron-transfer in biological systems takes place through the mediation of a number of proteins, which contain a variety of active sites such as heme, Fe—S, Cu, and flavin. These active sites are protected from the solvent by a hydrophobic environment created by the peptide chain 48). The redox potential of a biological redox couple in vivo lies, for the most part, between —0.5 and +0.85 V. The former and latter potentials correspond to the redox potentials of H20/H2 and H20/02 respectively 49). 

Characteristic features of the electron-transfer in biological systems are longdistance and directional electron transfer, and regulation of the rate of electron-transfer. 

Direct evidence for long range electron-transfer in biological systems was first observed by Gray et al.50,51) and Isied et al.481 using [Ru(NH3)5]3+ substituted metallo protein. Histidine-83 of blue copper (azurin) was labeled with Ru(III)(NH3)5 50). Flash photolysis reduction of the His-83 bound Ru(III) followed by electron-transfer from the Ru(II) to Cu2+ was observed with a rate constant of 1.9 s 1. The result shows that intramolecular long distance (approx. 1 nm) electron-transfer from the Ru(II) to the Cu2 + of the azurin takes place rapidly. 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

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