Biological electron-transfer reactions, intramolecular

Simple pyridinyl radicals, Py, like those derived from 1-ethyl-4-carbomethoxy-pyridinium ion through l reduction, have proven useful for the investigation of properties of radicals, including complexes, and the study of the mechanisms of their reactions. Discoveries include (a) distinction between atom-transfer and electron-transfer reactions by means of solvent effect on rate constants, (b) intramolecular radical complexes (ir-mers) within pyridinyl diradicals and pyridinyl diradical metal halide complexes, (c) occurrence of either electron-transfer or dimerization between pyridinyl radicals in water according to structure and (d) con5)lexation between pyridinium ions and pyridinyl radicals. Consideration of actual and potential biological roles for pyridinyl radicals is aided by these discoveries. 

The interest of electron transfer reactions in many areas of science was recently remarked by Rudolph A. Marcus in his lecture on the occasion of the presentation of the Nobel Prize in Chemistry in 1992 [140]. In particular, electron transfer played an important role in chemical and biological reactions and thus, different types of bridge donor-acceptor molecules have been prepared in order to study the intramolecular electron-transfer mechanisms [141,142], The design of novel organic molecules containing electron donor (D) and electron acceptor (A) moieties constitutes a promising field of study due to the interesting optical and... 

Carbohydrates Carbon Cycle Chromatin Structure and Modification Electron Transfer Reactions Energy Flows in Ecology and IN THE Economy Energy Transfer, Intramolecular Ion Transport Across Biological Membranes Lipoprotein/Cholesterol Mp.tabot.tsm Protein Synthesis Thermodynamics... 

In biological systems, electron transfer kinetics are determined by many factors of different physical origin. This is especially true in the case of a bimolecular reaction, since the rate expression then involves the formation constant Kf of the transient bimolecular complex as well as the rate of the intracomplex transfer [4]. The elucidation of the factors that influence the value of Kf in redox reactions between two proteins, or between a protein and organic or inorganic complexes, has been the subject of many experimental studies, and some of them are presented in this volume. The complexation step is essential in ensuring specific recognition between physiological partners. However, it is not considered in the present chapter, which deals with the intramolecular or intracomplex steps which are the direct concern of electron transfer theories. 

Two intramolecular electron transfer systems of current interest, ruthenated proteins and photosynthetic reaction centers, are now discussed. These examples serve to illustrate the types of experimental approaches and information that may be obtained concerning the details of electron transfer processes in biological systems. 

Phys. 74 6746 (1981) b) G. L. Gloss, L. T. Calcaterra, N. J. Green, K. W. Penfield, and J. R. Miller, Distance, stereoelectronic effects, and the Marcus inverted region in intramolecular electron transfer in organic radical anions, J. Phys. Chem. 90 3673 (1986). a) S. Larsson, Electron transfer in chemical and biological systems. Orbital rules for nonadiabatic transfer, J. Am. Chem. Soc. 103 4034 (1981) b) S. Larsson, n Systems as bridges for electron transfer between transition metal ions, Chem. Phys. Lett. 90 136 (1982) c) S. Larsson, Electron transfer in proteins, J. Chem. Soc., Faraday Trans. 2 79 1375 (1983) d) S. Larsson, Electron-exchange reaction in aqueous solution, J. Phys. Chem. 88 1321 (1984) e) S. Larsson,... 

Intramolecular electron transfer is currently the subject of much experimental and theoretical studies, due to its wide occurence in diverse fields of chemistry, physics and biology. From a fundamental point of view, it is one of the simplest conceivable chemical reactions. Its study in discrete molecular species is a very active research topic and could lead to a better understanding of the process of charge migration in condensed matter. This studies are performed on chemical systems containing two redox sites linked by some kind of bridge. In addition to their interest for fundamental research, such systems can be considered as precursors of molecular" electronic devices. ... 

---