Excited-state decays of the special pair of Rhodobacter capsulatus and 10 mutants as a function of temperature are presented. The mutations are at the symmetry-related sites L181 and M208. All the decays are nonexponential, although the degree of nonexponentially depends strongly on the mutant and the temperature. We correlated the changes in decay time with mutation-induced changes in the redox potential of the ground state of the special pair. The qualitative form of the decays led us to

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... re a model based on a small Gaussian distribution of free energy gaps for the electron-transfer process. The model is used to estimate the reorganization energy arising from low-frequency protein and intramolecular modes coupled to the electron transfer, Our fitting procedure gives values of 1250 and 500 cm-I at 292 and 22 K, respectively. A consideration of possible errors leads us to conclude that the room temperature value could be no larger than twice the fitted value (i.e., 1500 cm-l). The implications of our results for the mechanism of primary charge separation are discussed.