The initial electron transfer rate and protein dynamics in wild type and five mutant reaction centers from Rhodobacter sphaeroides have been studied as a function of temperature (10-295 K). Detailed kinetic measurements of initial electron transfer in Rhodobacter sphaeroides reaction centers can be quantitatively described by a reaction diffusion formalism at all temperatures from 10 to 295 K. In this model, the time course of electron transfer is determined by the ability of the protein to interconvert between conformations until one is found where the activation energy for electron transfer is near zero. In reaction centers with a free energy for electron transfer similar to wild type, the reaction proceeds at least as fast at cryogenic temperatures as at room temperature. This may be because interconversion between conformations at low temperature is restricted to conformations with near zero activation energy (it is not possible to diffuse away from this region of conformational space). In contrast, mutants with a decreased free energy initially find themselves in conformations unfavorable for electron transfer and require more extensive conformational diffusion to achieve a low activation energy conformation. They therefore undergo electron transfer more slowly at 10 K vs 295 K.