ELECTRON TRANSPORT IN R. RUBRUM 



293 



planations have been offered, one of which is that there are optimal 

 values for the oxidation- reduction potentials of intrachromatophore 

 components for maximal coupling of photoactivated electron transport 

 to phosphorylation processes (71,72,26). As expected, washed chroma- 

 tophores of R. nibnim photosynthesize ATP from ADP and Pi in the 

 presence of appropriate concentrations of ascorbate (ascorbate- 

 induced photophosphorylation) (Fig. 12). Under aerobic conditions, 

 photophosphorylation reached its maximal rate in the presence of 

 approximately 5 x 10"2 M ascorbate. 



cr 1.0 



Z) 



o 

 ^0.8 



SO® 



0.4 



^0-2 

 o 



E 



0.0 



W 



2,000 Ft -candles ^.-Os 



-co 



^—^ .t 



-5 



-4 -3 



LO G ( P M S , M ) 



Fig. 13. Effect of phenazine methosulfate on photophosphorylation in R. rubnim 

 chromatophores. Experimental conditions were the same as for Fig. 12, except 

 that PMS was substituted for ascorbate. 



Photophosphorylation was induced in the presence of PMS (Fig, 13). 

 The concentration of PMS capable of inducing the maximal rate of 

 photophosphorylation was notably influenced by the intensity of illumi- 

 nation, which was not true of ascorbate. When anaerobic conditions 

 were maintained, the rate of phosphorylation was maximal in the pres- 

 ence of a much lower concentration (10-2 m) of ascorbate. In spite of 

 the difference in effective ascorbate concentrations, the maximal rates 

 of photophosphorylation were the sameunderbothconditions.lt follows 

 that the presence of oxygen influences the photophosphorylation rate by 

 lowering the reducing capacity of ascorbate through oxidation. Although 



