Electron-transfer processes in biological systems

This expression constitutes the basis of current interpretations of electron transfer processes in biological systems. From Eq. (9), the functions Hg, (Q) and Hbb (Q) represent potential energy surfaces for the nuclear motion described by Xav and Xbw respectively, if the weak diagonal corrections Taa and T b are neglected. Then, the region Q Q where Xav and Xbw overlap significantly corresponds to the minimum of the intersection hypersurface between Hga (Q) and Hbb (Q)- Referring to definition (5), this implies ... 

The construction of an artificial protein-protein complex is an attractive subject to elucidate the electron-transfer process in biological systems. To convert Mb into an electron-transfer protein such as cytochromes, Hayashi and Ogoshi (101) prepared a new zinc Mb having a unique interface on the protein surface by the reconstitutional method as shown in Fig. 27. The modified zinc protoporphyrin has multiple functional groups, carboxylates, or ammonium groups, at the terminal of the two propionates. Thus, the incorporation of the... 

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. 

The prospect of advancing chemical sensor technology, modelling electron-transfer processes in biological systems and producing new redox catalysts has led to considerable interest in the design and syntheses of redox-active macrocyclic receptor molecules that contain a redox centre in close proximity to a host binding site. ... 

The study of homogeneous electron transfer processes in biological systems by means of electrochemical methods is now receiving increasing attention. For a number of small-sized redox proteins (e.g. cytochrome c, ferredoxines and azurine) a direct electron transfer at chemically modified electrodes has been achieved [26, 27]. These systems have been investigated by standard electrochemical techniques such as CV, RDEV, and impedance measurements (see chapter 1 of this volume). Information on direct electron transfer between a redox enzyme and the electrode can be found in papers [28-33]. 

The prospect of developing new materials of relevance to the emerging field of molecular electronics, modelling electron-transfer processes in biological systems and producing new electroactive and photoactive catalysts has led in recent years to considerable interest in transition metal polypyridyl complexes. Two recent examples of the application of the OTTLE spectroelectrochemical technique to the study of these fascinating systems are described here. 

Dust, J., Charge-transfer and electron transfer processes in biologically significant systems. Can. J. Chem., 70, 151-157, 1992. 

A number of publications in recent years have demonstrated an active interest in the theoretical aspects of electron transfer (ET) processes in biological systems (1.-9). This interest was stimulated by the extensive experimental information regarding the temperature dependence of ET rates measured over a broad range of temperatures (10-16). The unimolecular rate of cyto-chrome-c oxidation in Chromatium (10-12), for example, exhibits the Arrhenius type dependence and changes by three orders of... 

There is currently much interest in electron transfer processes in metal complexes and biological material (1-16, 35). Experimental data for electron transfer rates over long distances in proteins are scarce, however, and the semi-metheme-rythrin disproportionation system appears to be a rare authentic example of slow electron transfer over distances of about 2.8 nm. Iron site and conformational changes may also attend this process and the tunneling distances from iron-coordinated histidine edges to similar positions in the adjacent irons may be reduced from the 3.0 nm value. The first-order rate constant is some 5-8 orders of magnitude smaller than those for electron transfer involving some heme proteins for which reaction distances of 1.5-2.0 nm appear established (35). 

Exhaustive reviews dealing with the applications of electron transfer theories to biological systems have been published recently [4,22], and should be consulted for a general presentation of electron transfer processes as well as detailed mathematical developments. Shorter reviews are also available [23, 24]. In this section, we review the physical basis of the formalism generally used in the case of... 

Since the initial observation of flavin radical species by Michaelis and coworkers the involvement of flavins in one-electron oxidation-reduction processes in biological systems has occupied the attention of workers in the field of redox enzymology up to the present time. Flavin coenzymes occupy a unique role in biological oxidations in that they are capable of functioning in either one-electron or two-electron transfer reactions. Due to this amphibolic reactivity, they have been termed in a recent review to be at the crossroads of biological redox processes. 

The approach for this system is the mimicry of the highly efficient photosynthesis process in biological systems, by which an antenna device collects the light energy before a series of exciton, energy, and electron transfers, which lead to the synthesis of the plant s fuel.70-73... 

In order for the color state to persist for an extended time, a charge separation of the photogenerated ions must be produced such as that in a photosynthetic process in biological systems. Also, the rate of reverse electron transfer can be retarded by the presence of an external field. 

Most LRET processes in biological systems are nonadiabatic. In quantum-mechanical electron-transfer theory, the rate constant for nonadiabatic ET from a donor to acceptor can be expressed as the product of the square of an electronic coupling matrix element (Hp ) and a nuclear Franck-Condon factor (FC) kpy = (27t/h)[Hpj2(FC).The [HpJ is a measure ofthe... 

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