Aleksandr Ellervee1, Kou Timpmann1, Andrew Gall2,3, Bruno Robert2, and
Arvi Freiberg1
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1 Institute of Physics, University of Tartu, Riia 142, EE51014, Estonia;
2 Section de Biophysique des Protéines et des Membranes, DBCM/CEA & URA CNRS 2096, C. E. Saclay, 91191 Gif-sur-Yvette, Cedex, France; 3 Laboratoire Léon Brillouin (CEA-CNRS), CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France;
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A novel combination of different spectroscopies (Fourier-transform Raman, steady state and time-resolved absorption and emission spectroscopy) and protein engineering techniques with high pressures has been used to obtain new insights into the nature of couplings that govern the structure, spectral properties and functioning of the photochemical reaction centres of photosynthetic bacteria. It is shown that the reaction centre from Rhodobacter sphaeroides does not loose its three-dimensional structure at room temperature up to at least 0.8 GPa. However, Fourier-transform preresonance Raman spectra evidence a number of local reorganisations in the binding site of the primary electron donor between the atmospheric pressure and 0.2 GPa. Although no further structural reorganisation of the donor binding site could be observed at still higher pressures, in some cases, the excited electronic state of the primary electron donor gets dramatically perturbed. For example, in the carotenoid-less strain R26.1 of Rb. sphaeroides, differently from its wild type counterpart, a nearly one order of magnitude decrease of the oscillator strength of the Qy electronic transition is observed between 0.3 and 0.8 GPa 1. This effect is likely due to very small pressure-induced changes in the electron donor dimer structure, inducing an increase in the mixing between its singlet excited and optically forbidden charge-transfer states. In the wild type strain, where the oscillator strength is essentially conserved, the pressure leads to substantial acceleration of the primary electron transfer rate. This rate saturates at about (1.9 ps)-1 in the 0.4-0.5 GPa pressure range. We have also able to show that the observed accelation is a combined effect of enlargement of the reaction driving force and of reduction of the distance separating the electron donor and acceptor sites. Either of these mechanisms applied separately does not provide a satisfactory account of the experimental data. One may conclude, in contrast to the popular belief, that the wild type reaction centre is probably not fully optimised with respect to energetics of the primary electron transfer process.
1) Gall, A.; Ellervee, A.; Bellissent-Funel, M.-C.; Robert, B.; Freiberg, A. Biophys. J. 2001, 80, 1487-1497.
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