COUPLED REDUCTION OF UBIQUINONE 273 



data taken at 275 m^, which presumably corresponds to ubiquinone 

 photoreduction, show that the presence of DPIPH2 results in a more 

 extensive decrease in the absorbancy which can be correlated with 

 increased reduction of ubiquinone in the presence of DPIPH2. 



It is apparent that the changes at 275 m/U resulting from illumination 

 are slower than those at 430 m/i, which are faster than the recorder 

 can follow. This would relate the 430 m/n changes to a substance in- 

 volved in the primary photochemistry resulting from light absorption, 

 while the 275 mfi changes are consistent with a subsequent chemical 

 reaction leading to ubiquinone reduction. The nature of the chromato- 

 phore component responsible for the increased absorption at 430 m/j. 

 upon illumination is not known. From a detailed analysis of the kinetics 

 of the 430 mil change under the influence of light and oxidizing agents, 

 Clayton has concluded that this change is associated with one of the 

 primary photochemical reactions taking place in the bacterial chroma- 

 tophore (6). Since DPIPH2 does affect the 430 mju band, showing a 

 marked effect upon thedecay of this absorption band, this indicates that 

 DPIPH2 effectively supplies electrons to the oxidized component which 

 produces the 430 m/i band upon illumination. 



SUMMARY 



The absorption changes occurring upon illumination of R. nibrum 

 chromatophores in the absence and presence of DPIPH2 have been 

 investigated. The presence of DPIPH2 resulted in the appearance of a 

 minimum in the difference spectrum at about 280 m/i, which is thought 

 to be due to a coupled reduction of the ubiquinone contained in the 

 chromatophore and the oxidation of added DPIPH2. The effect of 

 DPIPH2 upon the kinetics of the absorption changes was also studied. 



REFERENCES 



1. Duysens, L. N. M., Reversible photooxidation of a cytochrome pigment in 

 photosynihes\7.\ng Rhodospirillum nibrum. Nature, 173, 692-694 (1954). 



2. Chance, B., and Smith, L., Respiratory pigments of Rhodospirillum nibrum. 

 Nature, 175, 803-810 (1955). 



3. Smith, L., Baltscheffsky, M., and Olson, J. M., Absorption spectrum 

 changes observed on illumination of aerobic suspensions of photosynthetic 

 bacteria. J. Biol. Chem., 235, 213-218 (1960). 



4. Smith, L., and Ramirez, J., Reactions of carotenoid pigments in photo- 

 s.vTithctic bacteria. J. Biol. Chem., 235, 219-225 (1960). 



5. Smith, L.,and Baltscheffsky, M., Respiration and light-induced phosphory- 

 lation in extracts of Rhodospirillum nibrum. J. Biol. Chem., 234, 1575- 

 1579 (1959). 



