MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 117

mechanisms of electron transfer in photosynthetic systems were originally worked out in
the classical works of Chance and De Vault (1967) for the electron-transfer reaction
between oxidized chlorophyll and reduced cytochrome c in photosynthetic bacteria. But
the new basic idea underlying the suggested mechanism is an assumption that the donor
(D) and several acceptor centers compose a cascade in an ordered structure, in
which all these centers are placed at an optimum distance from each other and are
separated by a nonconducting protein medium. Such a separation slows down the
forward electron transfer between adjacent and pairs as compared to
electron transfer in a system with close contacts between the centers. Nevertheless, the
transfer can be sufficiently fast, if the optimum distances do not exceed 6-10 Å (see
Section 2.1). What is important is that the recombination of each pair becomes
slower and slower as moves away from the donor. In the system of tightly packed
centers the recombination rate is expected to be very fast.
From this analysis, the main two conclusions are: (1) an effective fast conversion of
light energy to energy of a chemical compound of high quantum yield can take place
only in biological and model cascade photochemical systems in which photo- and
chemically active centers (aromatic photochromes, transition metal clusters) are
separated by “insulated” zones of 6-10 Å width, consisting of nonsaturated molecules
and bonds, and (2) the electron transfer between the donor and acceptor centers has to
occur by a long-range, most probably nonadiabatic mechanism.
The tunneling hypothesis has been supported in subsequent experiments. By the
electron paramagnetic resonance measurements (Kulikov et al., 1979) the distances