Electron tunneling 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. 

The present chapter discusses briefly modern ideas on the mechanisms of electron transfer during photosynthesis and the experimental data pointing to the important part played by electron tunneling reactions in the operation of the reaction centres of photosynthesizing systems. 

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Experimental research on the role of electron tunneling in biological systems is typically carried out on subchromatophore and subchloroplast fragments at low temperatures. The operation of photosynthesizing objects at low temperatures was first discovered by Chance and Nishimura [28] who reported the oxidation of cytochrome c under the action of light in photosyn-... 

The particular role of electron tunneling in biological systems has been repeatedly discussed in the literature [15-18,40,41,44,45,109-1151. Let us summarize briefly the main considerations. 

Henry F. Schaefer III, Quantum Chemistry The Development of Ab Initio Methods in Molecular Electronic Structure Theory, Oxford University Press, Fair Lawn, NJ, 1984. Don DeVault, Quantum-Mechanical Tunneling in Biological Systems, 2nd ed., Cambridge University Press, New York, 1984. 

The theme here is electron transfer, in inner- and outer-sphere reactions and, to a lesser degree, in related processes like optically induced charge transfer or excited state decay. Three books have been written on electron transfer, by Reynolds and Luniry, Cannon and Ulstrup, the last of which emphasizes theoretical aspects. In addition, a series of theoretical and experimental articles appear in the book Tunneling in Biological Systems , edited by Chance et and in volume 74 (1982) of the Faraday Discussions of the Chemical Society. A number of reviews have appeared, dealing both with general aspects - and more specialized themes, and many will be referred to in the sections that follow. 

Therefore, it appears reasonable to assume the existence of the tunneling mechanism of electron transfer between the electrode and the enzyme active center. The possibility of a sub-barrier electron transfer at large distances (10-20 A) has been shown, at least theoretically, for electrochemical systems and is widely used to discuss electron transport in biological systems. ... 

In a number of works, it was supposed that in biological systems, due to certain structures of the molecules of the medium with vacant electron levels, tunnel reactions are more effective than in non-ordered media. But until recently, no direct experimental evidences supporting this assumption had been presented. 

Tunneling is a ubiquitous phenomenon. It is observed in biological systems (1), and in electrochemical cells (2). Alpha particle disintegration (3), the Stark effect (4), superconductivity in thin films (5), field-electron emission (6), and field-ionization (7) are tunneling phenomena. Even the disappearance of a black hole (or the fate of a multi-dimensional universe) may depend on tunneling, but on a cosmological scale (S-9). 

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