Biological tunnelling

Guokenberger R, Fleim M, Cevo G, Knapp FI F, Wiegrabe W and Flillebrand A 1994 Scanning tunnelling microscopy of insulators and biological specimens based on lateral conductivity of ultrathin water films Science 266 1538... 

Flansma P K, Elings V B, Marti O and Bracker C E 1988 Scanning tunnelling microscopy and atomic force microscopy application to biology and technology Science 242 209... 

Devault, D. Quantum mechanical tunnelling in biological systems. Quart. Rev. Biophys. 13 (1980) 387-564. 

Chemical and biological sensors (qv) are important appHcations of LB films. In field-effect devices, the tunneling current is a function of the dielectric constant of the organic film (85—90). For example, NO2, an electron acceptor, has been detected by a phthalocyanine (or a porphyrin) LB film. The mechanism of the reaction is a partial oxidation that introduces charge carriers into the film, thus changing its band gap and as a result, its dc-conductivity. Field-effect devices are very sensitive, but not selective. 

In the genuine low-temperature chemical conversion, which implies the incoherent tunneling regime, the time dependence of the reactant and product concentrations is detected in one way or another. From these kinetic data the rate constant is inferred. An example of such a case is the important in biology tautomerization of free-base porphyrines (H2P) and phtalocyanins (H2PC), involving transfer of two hydrogen atoms between equivalent positions in the square formed by four N atoms inside a planar 16-member heterocycle (fig. 42). 

DeVault, D., 1984, Quantum-Mechanical Tunneling in Biological Systems (Cambridge Univ. Press, Cambridge). 

Electrochemistry is the basis of many important and modem applications and scientific developments such as nanoscale machining (fabrication of miniature devices with three dimensional control in the nanometer scale), electrochemistry at the atomic scale, scanning tunneling microscopy, transformation of energy in biological cells, selective electrodes for the determination of ions, and new kinds of electrochemical cells, batteries and fuel cells. 

Factors that enhance tunnelling are a small particle mass and a narrow potential energy barrier. In biology, electron transfer is known to occur over large distances (up to about 25 X 10 m). Given the mass of protium is 1840 times that of the electron, the same probability for protium... 

Kohen, A.. Klinman, J. 1999 Hydrogen tunneling in biology. Chem. Biol. 6, R191-R198. 

It is getting more and more important to treat realistic large chemical and even biological systems theoretically by taking into account the quantum mechanical effects, such as nonadiabatic transition, tunneling, and intereference. The simplest method to treat nonadiabatic dynamics is the TSH method introduced... 

DeVault D (1984) Quantum-mechanical tunneling in biological systems, Cambridge Univ Press, Cambridge... 

In some cases, small biological redox partner proteins such as heme-containing cytochromes, ferredoxins comprising an iron-sulfur cluster, or azurin with a mononuclear Cu site have been used as natural mediators to facilitate fast electron exchange with enzymes. A specific surface site on the redox protein often complements a region on the enzyme surface, and enables selective docking with a short electron tunneling... 

Page CC, Moser CC, Chen X, Dutton PL. 1999. Natural engineering principles of electron tunneling in biological oxidation-reduction. Nature 402 47-52. 

Zamaraev, K.I., Kairutdinov, R.F. Photoinduced Electron Tunneling Reactions in Chemistry and Biology. 163, 1-94 (1992). 

A very brief introduction to the important topic of bioinorganic electron transfer mechanisms has been included in Section 1.8 (Electron Transfer) of Chapter 1. Discussions of Marcus theory for protein-protein electron transfer and electron or nuclear tunneling are included in the texts mentioned in Chapter 1 (references 3-7). A definitive explanation of the underlying theory is found in the article entitled Electron-Transfer in Chemistry and Biology, written by R. A. Marcus and N. Sutin and published in Biochem. Biophys. Acta, 1985, 811, 265-322. 

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