Superexchange tunneling

Fig. 16.10 The electron transfer time (inverse rate) in a chemically modified photoreaction center of bacteriochlorophyll, showing a crossover from thermally activated sequential hopping behavior at high temperature to a superexchange tunneling behavior at low temperature. (Open circles are experimental data from M. E. Michel-Beyerle et al., unpublished fall and dashed lines are theoretical fits from the articles by M. Bixon and J. Jortner cited at the end of this chapter.)...

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. 

Stuchebrukhov AA (1996) Tunneling currents in electron transfer reaction in proteins. II. Calculation of electronic superexchange matrix element and tunneling currents using nonorthogonal basis sets. J Chem Phys 105(24) 10819-10829... 

Fig. 2 (a) Schematic representation of the energy levels diagrams for a DBA system and a MBM junction in which the electron transfer process is dominated (b) by superexchange or non-resonant tunnelling, (c) by resonant tunnelling or (d) by hopping ... 

In the superexchange regime, the tunneling may be mediated by the DNA bases, by the backbone, or by the solvent. Theoretical (INDO) studies by Cave and coworkers predicted a /I of about 2.0 for water, and they estimated (using a limited set of water configurations) that the ab initio value would fall in the range of 1.5-1.8 [91]. The experiments of Gray and... 

Direct and superexchange electron tunneling between adjacent and remote sites of plastocyanins were considered in Ref. [326], Exchange matrix elements for direct... 

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