TIME-DEPENDENT THERMODYNAMICS DURING EARLY ELECTRON-TRANSFER IN REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES

The temperature dependence of fluorescence on the picosecond to nanose cond time scale from the reaction centers of Rhodobacter sphaeroides s train R-26 and two mutants with elevated P/P+ midpoint potentials has been measured with picosecond time resolution. In all three samples, t he kinetics of the fluorescence decay is complex and can only be well described with four or more exponential decay terms spanning the picos econd to nanosecond time range. Multiexponential fits are needed at al l temperatures between 295 and 20 K. The complex decay kinetics are ex plained in terms of a dynamic solvation model in which the charge-sepa rated state is stabilized after formation by protein conformational ch anges. Many of these motions have not had time to occur on the time sc ale of initial electron transfer and/or are frozen out at low temperat ure. 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 stat es. Long-lived fluorescence is still observed even at 20 K, particular ly 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 sm aller on the picosecond time scale or in the high-potential mutants. T he 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, oc curring near the strong coupling limit, rather than as a nonadiabatic reaction between vibronically equilibrated states.