The temperature dependence of fluorescence on the picosecond to nanosecond time scale from the reaction centers of Rhodobacter sphaeroides strain R-26 and two mutants with elevated P/P+ midpoint potentials has been measured with picosecond time resolution. In all three samples, the kinetics of the fluorescence decay is complex and can only be well described with four or more exponential decay terms spanning the picosecond to nanosecond time range. Multiexponential fits are needed at all temperatures between 295 and 20 K. The complex decay kinetics are explained in terms of a dynamic solvation model in which the charge-separated state is stabilized after formation by protein conformational changes. Many of these motions have not had time to occur on the time scale of initial electron transfer and/or are frozen out at low temperature. This results in a time- and temperature-dependent enthalpy change between the excited singlet state and the charge-separated state that is the dominant term in the free energy difference between these states. Long-lived fluorescence is still observed even at 20 K, particularly for the high-potential mutants. This implies that the driving force for electron transfer on the nanosecond time scale at low temperature is less than 200 cm−1 (25 meV) in R-26 reaction centers and even smaller on the picosecond time scale or in the high-potential mutants. The mechanistic implications of this surprising result are considered, and it is suggested that, at least under certain conditions, electron transfer in the reaction center may be best described as adiabatic, occurring near the strong coupling limit, rather than as a nonadiabatic reaction between vibronically equilibrated states.
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