INTERACTION BETWEEN CYTOCHROME-C AND THE PHOTOSYNTHETIC REACTION-CENTER OF PURPLE BACTERIA - BEHAVIOR AT LOW-TEMPERATURE

In purple photosynthetic bacteria the electron donor to the special pa ir, after its oxidation by a light-induced reaction, is a c-type cytoc hrome: either a soluble monoheme cytochrome which forms a transitory c omplex with the reaction center, or a tetraheme cytochrome which remai ns permanently bound to the reaction center. The effects of low temper atures on electron transfer in the complex are presented and discussed . They provide estimates for the reorganization energy. The most promi nent effect of low temperature is that a dominant fast phase of electr on transfer becomes impossible at a temperature of around 250 K (monoh eme cytochrome) or located between 250 K and 80 K according to the red ox state (tetraheme cytochrome). This inhibition is attributed to a fr eezing-like transition of pools of water molecules which blocks struct ural changes of the protein which are normally associated with the cyt ochrome oxidation.