To calculate the coupling, a specific pathway must be chosen and then
all of the individual contributions are evaluated and the coupling is deter-
mined. In practice, computer algorithms sample many different possible
pathways to determine the optimal path.
Different sets of experimental data were considered in evaluating these
different models. In the laboratory of Harry Gray and coworkers (Gray
& Winkler 2005), ruthenium compounds, serving as electron donors, were
attached at different locations on a cytochrome and the electron transfer
to the heme was measured. The variation of the measured rate was
well described in terms of the coupling for the optimal path coupling
in each case. Alternatively, Leslie Dutton and coworkers (Page et al.
2003) considered the dependence of many different electron-transfer
rates and found that the rate could be described using only the separa-
tion distance RDA. In both cases, electron transfer is expected to decrease
exponentially with distance, but the distance is either the closest distance
or the pathway distance. The appropriateness of each model to describe
the transfer of electrons in proteins is still under discussion, although
the distinction between the models is lessened if electrons are considered
to sample several different comparable pathways (Page et al. 2003; Gray
& Winkler 2005; Lin et al. 2005; Miyashita et al. 2005). While the nature
of electron transfer in proteins was being investigated, experiments of
electron transfer were being performed under more artificial circumstances,
namely in DNA. The initial data indicated that electrons could travel large
distances at rates that indicated much larger coupling values than in pro-
teins, but later experiments showed that the electron transfer did not
involve long-range transfer but, rather, a series of short steps that yielded
parameters in keeping with those observed for proteins (Murphy et al.
1993; Giese 2002).
All of these models are for electron transfer within a protein complex,
but in cells electrons are often transferred between two different proteins,
representing a second-order process rather than a first-order process
(Chapter 7). In such cases, the observed rate of electron transfer is usu-
ally limited by the rate of diffusion and formation of the protein–protein
complex prior to the electron transfer. The formation of the protein–
protein complex is often driven by electrostatic interactions involving a
primarily negatively charged surface on one protein and a largely positively
charged surface on the second. Once the proteins are spatially close, other
interactions, such as hydrophobic interactions and steric considerations,
stabilize the position of one protein relative to the other. Investigations
continue into understanding the contributions of different factors that
influence formation of the complex, such as reorganization of the solvent
surrounding the surface of the proteins (Lin et al. 2005; Miyashita et al.
2005) and protein dynamics (Wang et al. 2007).
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