Biological systems electron-transfer processes

In many biological systems, electron transfer takes place between redox couples present in media with different dielectric properties. Electrochemical studies at the ITIES enable one to address systematically the effect of polarization and specific properties of the electrolyte medium on the dynamics of electron transfer. This knowledge has particular relevance in processes involving redox phase transfer catalysis. 

Along with hydrogen transfers (in the form of protons, hydrogens, or hydrides), electron transfer is one of the most fundamental chemical processes in biological systems. Electron transfers are found in the respiratory chain, in photosynthesis, and in uncountable metabolic pathways. Enzymes such as the family of oxidoreductases have evolved to catalyze chemical reactions such as the functionalization of C-H bonds. Inner-sphere or outer-sphere (Long Range) electron... 

Phosphino-metallaborate complexes in Section 12.13.6.7 can be applied to N2 fixation and modeling systems for electron-transfer processes occurring in biological systems Fe(l)-Fe(m)/Fe(m)-Fe(l), such as the reducing FeS clusters of certain metaloenzymes <2003JA322>. 

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... 

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 ... 

One may wonder whether a purely harmonic model is always realistic in biological systems, since strongly unharmonic motions are expected at room temperature in proteins [30,31,32] and in the solvent. Marcus has demonstrated that it is possible to go beyond the harmonic approximation for the nuclear motions if the temperature is high enough so that they can be treated classically. More specifically, he has examined the situation in which the motions coupled to the electron transfer process include quantum modes, as well as classical modes which describe the reorientations of the medium dipoles. Marcus has shown that the rate expression is then identical to that obtained when these reorientations are represented by harmonic oscillators in the high temperature limit, provided that AU° is replaced by the free energy variation AG [33]. In practice, tractable expressions can be derived only in special cases, and we will summarize below the formulae that are more commonly used in the applications. 

Although their conceptual basis is now firmly established, non-adiabatic electron transfer processes are still the subject of intensive theoretical studies. Nevertheless, the framework provided by the standard formalism presented in this section seems sufficiently general to be used for the interpretation of kinetic data obtained in biological systems. Owing to the great number of parameters involved in the theoretical expressions, attainment of useful information requires obtaining numerous data by elaborate experiments. The next section is devoted to a review of the different approaches that have been developed over the last few years. 

These two lines of investigation illustrate the important advances made in this field since the early theoretical interpretations of biological electron transfer processes. Hence, they allow one to envisage the understanding of these processes at the molecular level, and the synthesis of efficient model systems [86, 194]. 

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... 

Electron transfer in biological systems can be thought of as a two-step process involving formation of a complex between the donor and acceptor molecule followed by an electron transfer event. The overall electron transfer rate will depend on the rate of complex formation and stability of the complex and on the intrinsic electron transfer rate within the complex. Since the flavin electron transfer systems discussed in this chapter are all stable inter- or intramolecular complexes, the discussion will be limited to considerations of a first order intra-complex electron transfer process. 

These stereoselectivity experiments appear to indicate that the reactants approach each other quite closely during the electron-transfer process. It should be possible by judicious choice of reactants to increase substantially the stereoselectivity observed in the above systems. This may in the future make a significant contribution to our understanding of the mechanisms of electron transfer reactions both in chemical systems and in biology, where electron-transfer reactions occur in a very asymmetric environment. 

In general, whilst the chapters of this Handbook focus attention on the developments of recent years, they also place earlier studies in their proper perspective. Therefore the Handbook is intended to serve a joint purpose, both as a reference resource and as an introductory overview, covering practically all kinds of electron transfer processes that occur in Chemistry, from biological systems to materials science. Although the Handbook is not aimed at being a specific lecture course, several sections or chapters can profitably be used as the basis for both advanced graduate and postgraduate courses. 

In 1967, the first three-stage electron-transfer process examined by pulse radiolysis was reported [67], Such a cascade process is of relevance to electron transport in biological systems. By irradiating an aqueous solution containing acetone (0.82 mol dm ), acetophenone (3.34 mmol dm ), and benzophenone (72 pmol dm ) at pH 13, Adams et al. [67] were able to observe, at 2 ps after the pulse, the spectrum of the acetophenone radical anion (2max 445 nm, max 260 m mol ) [68] originating from the reduction of acetophenone by the (CH3)2CO radical. In the following 50 ps, the acetophenone radical anion reacted with benzophenone k — 7.8 X 10 dm mol" s ) so that the absorption band at 445 nm disappeared... 

---