RELATIONSHIP BETWEEN THERMODYNAMICS AND MECHANISM DURING PHOTOINDUCEDCHARGE SEPARATION IN REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES

Detailed fast transient absorption measurements have been performed at low temperature on reaction centers from Rhodobacter sphaeroides stra in R-26 and on a double mutant, [LH(L131)+LH-(M160)], in which the P/P + oxidation potential is roughly 140 mV (1100 cm(-1)) above that of wi ld-type reaction centers. In both samples, the decay of the excited si nglet state of the initial electron donor is not well described by a s ingle-exponential decay term. This is particularly true for reaction c enters from the double mutant where at least three exponential kinetic components are required to describe the decay, with time constants ra nging from a few picoseconds to hundreds of picoseconds. However, sing ular value decomposition analysis of the time-dependent absorption cha nge spectra indicates the presence of only two spectrally distinct sta tes in reaction centers from both R-26 and the double mutant. Thus, th e complex decay of P at low temperature does not appear to be due to formation of either the state P+BA- as a distinct intermediate in elec tron transfer or P+BB- as an equilibrated side product of electron tra nsfer. Instead, the decay kinetics are modeled by assuming dynamic sol vation of the charge-separated state, as was done for the long-lived f luorescence decay in the accompanying paper [Peloquin, J. M., Williams , J. C., Lin, X., Alden, R. G., Taguchi, A. K. W., Alien, J. P., & Woo dbury, N. W. (1994) Biochemistry 33, 8089-8100]. The results of assumi ng a static distribution of electron-transfer rates at early times fol lowed by dynamic solvation of the charge-separated states on longer ti me scales are also presented. Regardless of which model is used to des cribe the early time kinetics of excited-state decay, the time-depende nt excited-state population on the 100-ps or longer time scale is best described in terms of thermal repopulation of P from the charge-sepa rated state, even at 20 K. This results in a time- and temperature-dep endent driving force estimated for initial electron transfer of less t han 200 cm(-1) on all time scales from picoseconds to nanoseconds. Ass uming a nonzero internal reorganization energy associated with charge separation, the small driving force does not appear to be consistent w ith the lack of temperature dependence of electron transfer and the fa ct that a mutant with a P/P+ oxidation potential 140 mV (1100 cm(-1)) higher than wild type is still able to undergo electron transfer, even at low temperature. These observations are more in line with an essen tially adiabatic electron-transfer reaction near the strong coupling l imit.