MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 123

data estimates the free energy as the energy of reorganization as
and the coupling factor as of this non-adiabatic process.
The free energy difference between and is indirectly estimated by
measuring the rate of the charge recombination via the uphill route
It is suggested that the rate of charge recombination by this pathway is
proportional to the equilibrium constant between and This suggestion is
sustained by the fact that the recombination rate increases by a factor of 10 for each
0.060 eV increase of the difference of the redox potential of quinines replacing the
native ubiquinone-10. The calculated values of and increase monotonically
with the temperature increase from 40 K to 200 K, while enthalpy does not change
in this temperature range. Within 200 -318 K, and slightly decreased and
increased by a jump from 0.050 to 0.300 eV. The authors suggest that the
observation of large entropy at temperature lowers than 210 K (for example
at 100 K) is caused by a formation of the which is trapped before
media relaxation. The Gibbs energy of the trap state at 10 K is estimated as about 0.200
eV higher than relaxed form at room temperature
Another matter of recent interest is detailed mechanisms of electron transfers with
participation of primary and secondary acceptors and the role of the coupling
proton transfer in these processes. The chrystallographic structures of RC from R.
spheroides at cryogenic temperature (90 K) in the dark and under illumination, at
resolution 2.2 and 2.6 Å respectively, have been reported (Stowell et al., 1997). The
main difference in the two structures was the charge-separated state within an area of the
primary and secondary acceptor location. In the charge neutral state
the distance between two ubiquinones is approximately 5 . In the “light” structure

the has moved about 4.5 Å and undergone a 180° propeller twist. It was
proposed that a hydrogen bond of ubiquinone with HisL190 prompts the electron
transfer from to and These results give evidence in favor of the gating
model of the protein dynamic, which suggests that electron transfer occurs only in an
active conformational state of the medium, promoting electron transfer (Petrov et al.,
1977).
Recent theoretical studies have added important conclusions (Balabin and Onuchik
2000; Rabinshtein et al., 2000) and have confirmed above mentioned conclusions (that
electron transfer between the two quinines and in the bacterial photosynthetic
centers is coupled to conformational rearrangement. The pathway method (Beratan and
Onuchik, 1987; Beratan et al., 1990; Onuchik et al., 1992) for estimation of the
quantomechanical-coupling factor was assumes that the electron transfer involves
multiple pathway tubes of different the population of which is controlled by
conformational and nuclear dynamics. The MD simulation performed for both the “dark”
and the “light” structures indicates that dominant pathway tubes are similar for light and
dark RC structures, except the position of According to the calculation (Stowell et
al., 1997), the transition from “dark” to “light” states is accompanied by the flipping and
moving of which shortens the ET pathway by five covalent steps and replaces a
through-space jump by a hydrogen bond. As a result of this transition, the ET rate
increases by about three orders of magnitude.

Å