TEMPERATURE-DEPENDENCE OF THE REORGANIZATION ENERGY FOR CHARGE RECOMBINATION IN THE REACTION-CENTER FROM RHODOBACTER-SPHAEROIDES

The rate of charge recombination from the primary quinone to the bacte riochlorophyll dimer of the reaction center from the photosynthetic pu rple bacterium Rhodobacter sphaeroides has been investigated using tim e-resolved optical spectroscopy. Measurements were performed at temper atures from 293 to 10 K on reaction centers that have specific mutatio ns that result in a range of 425-780 meV for the free energy differenc e of charge recombination compared to 520 meV for wild type [Lin, X., Murchison, H. A., Nagarajan, V., Parson, W. W., Alien, J. P., & Willia ms, J. C. (1994) Proc. Natl. Acad. Sci. U.S.A, 91, 10265-10269]. In al l cases the rate increased as the temperature decreased, although the details of the dependence were different for each mutant. The observed dependence of the rate upon temperature is modeled as arising princip ally from a several hundred meV change in reorganization energy. The r elationships among the rate, temperature, and free energy differences can be well fit by a Marcus surface using two modes centered near 150 and 1600 cm(-1) with a total reorganization energy that decreases from 930 to 650 meV as the temperature decreases from 293 to 10 K. In the inverted region, where the driving force is greater than the reorganiz ation energy, the rate is found to be approximately independent of the free energy difference. This is modeled as due to the additional coup ling of high frequency modes to the reaction. An alternative model is also considered in which a 140 meV increase in the reorganization ener gy is matched by a 140 meV increase in the free energy difference as t he temperature decreases. The possible role of solvent dipoles in dete rmining this temperature dependence of the reorganization energy and t he implications for other electron transfer reactions are discussed.