The whole cells of a number of purple photosynthetic bacteria representing both sulfur and nonsulfur species of these unique microorganisms were examined with respect to their ability to undergo the photooxidation of a cytochrome and other light-induced absorbance changes at 77°K. Out of a total of fifteen species thus far investigated, twelve have been found to possess a c-type cytochrome which is capable of undergoing photooxidation at temperatures of liquid nitrogen. Evidence is presented for the reversible oxidation-reduction reaction of cytochrome c in Rhodopseudomonas palustris at 77°K. In addition to the cytochrome reaction, absorbance changes caused by other spectral components were also observed in many species at 77°K. These changes were identified with a spectral shift of carotenoid peaks, P435, and tentatively with a minor band of bacteriochlorophyll in the light-minus-dark difference spectra. It is postulated that the difference in the electron transfer times of cytochrome c and the diversity of absorbance changes at 77°K noted in a variety of bacteria are due to species variation in the molecular orientation of spectral components within the photosynthetic apparatus. © 1969.
