PRIMARY CHARGE SEPARATION IN MUTANT REACTION CENTERS OF RHODOBACTER-CAPSULATUS

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 no nexponential, 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 grou nd state of the special pair. The qualitative form of the decays led u s to explore a model based on a small Gaussian distribution of free en ergy gaps for the electron-transfer process. The model is used to esti mate the reorganization energy arising from low-frequency protein and intramolecular modes coupled to the estimate the electron transfer. Ou r fitting procedure gives values of less-than-or-equal-to 250 and 500 cm-1 at 292 and 22 K, respectively. A consideration of possible errors leads us to conclude that the room temperature value could be no larg er than twice the fitted value (i.e., less-than-or-equal-to 500 cm-1). The implications of our results for the mechanism of primary charge s eparation are