Superexchange mechanism distance dependence

Fig. 2b. Both kcs and kcr are seen to decrease as the distance R between Sa and G C increases. In accord with a superexchange mechanism for photoin-duced electron transfer, the distance dependence can be described by Eq. (2) ... 

As with conductivity measurements, methods and results of theoretical treatments of CT in DNA have varied significantly. Mechanisms invoking hopping, tunneling, superexchange, or even band delocalization have been proposed to explain CT processes in DNA (please refer to other reviews in this text). Significantly, many calculations predicted that the distance dependence of CT in DNA should be comparable to that observed in the a-systems of proteins [26]. This prediction has not been realized experimentally. The dichotomy between theory and experiment may be related to the fact that many early studies gave insufficient consideration to the unique properties of the DNA molecule. Consequently, CT models derived for typical conductors, or even those based on other biomolecules such as proteins, were not adequate for DNA. 

Figure 15. A schematic illustrating the difference between the superexchange mechanism and molecular wire behavior in a D-B-A dyad. Superexchange the bridge states lie above the D level consequently the electron is transferred in one coherent jump and is never localized within the bridge. The distance dependence behavior is exponential decay.

A number of distance dependence studies have been carried out using synthetic DNA duplexes. In most of these investigations, the data were found to be consistent with the superexchange mechanism since the ET and HT rates followed exponential decays with increasing distance, with /J ranging from 0.1 A 1 to 1.4 A A1461... 

When the donors and acceptors lack spherical symmetry, there will also be an orientation dependence. In cases such as those to be discussed below, where the donor and acceptor moieties are linked by covalent bonds, there is considerable evidence that in certain situations the electron transfer occurs through the linkage bonds [22]. Although such linkages are not present in photosynthetic reaction centers, it has been proposed that the accessory Bchl or other intervening material may still take part in electron transfer through a superexchange mechanism [8, 26]. The distance dependence of photoinitiated electron transfer has recently been reviewed [13]. 

Through-space distance dependence 9 The influence of the medium 11 Electron transfer and electron transport 12 The superexchange mechanism 15... 

From these extensive studies, we may conclude that the distance dependence of ET rates, mediated by a superexchange mechanism involving saturated hydrocarbon bridges, is largely independent of the nature of the hydrocarbon and has an average /3 value of 0.99 0.2 bond-1. 

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