Light-induced oxidation of cytochromes in photosynthetic bacteria between 20 and -170°

In the photosynthetic bacteria Chromatium, Rhodopseudomonas spheroides and Rhodospirillum rubrum light-induced cytochrome oxidation was studied at temperatures between 20 and -220°. Of all cytochromes studied, only one, C 423.5 in Chromatium , was photooxidized at -220°. The photooxidation of C 420 in R. rubrum stops at about -30° and that of C 422 in Chromatium between −75 and -120°. The photooxidation of other cytochromes studied stops at temperatures between −30 and -120°. By recording, as a function of the wavelength, the difference in absorbancy of two cuvettes with bacteria, of which one could be illuminated, it was shown that, contrary to earlier suggestions, the cytochromes remain in the reduced state during cooling in the dark. The quantum requirement for C 423.5 and C 422 oxidation in Chromatium was calculated to be about 0.75 and 0.65, respectively, assuming a specific extinction change of 62 mM −1 cm −1 for both C 423.5 and C 422 at 422 mμ, which suggests that the quantum requirement is close to 1 and that the true specific extinction is somewhat higher. The rate of initial photooxidation as a function of intensity was found to be saturated at decreasing intensities, for the cytochromes studied, if the temperature was lowered sufficiently. This suggests that cytochrome photooxidation is not a primary, but a “dark” reaction. The light-induced increase in absorbancy around 435 mμ in R. rubrum , which also occurs at lower temperatures, had kinetics and saturating curves different from those occuring in the infrared, which showed that these absorbancy changes were caused by different reactions.