ELECTRON TRANSPORT IN R. RUBRUM 



291 



0.215- 



-h 0.220 

 (V) 



0.225 



10 20 

 TIMECMIN.) 



30 



40 



Fig. 11. Effectof light on oxidation-reduction potential of bacterio- 

 chlorophyll. A crude sample of bacteriochlorophyll from/?, nibnim 

 was dissolved in a small volume of acetone, and then diluted with a 

 large volume of ethylene glycol. The resulting solution was further 

 diluted with one-half volume of 0.01 M Tris buffer (pH 8) containing 

 0.01 M ascorbate. Eh was measured at 24°C in helium gas in alter- 

 nate periods of darkness and light (approx. 1,500 ft-candles) by a 

 Radiometer, type pH 4. 



Excited BChl + X 



oxidized BChl (bacterioviridin?) 



+ XH (photosynthetic reductant) 



oxidized BChl + reduced cyt + proton (s) ^BChl + oxidized cyt 



One of the simplest explanations might be that bacteriochlorophyll, 

 when illuminated by actinic light, is converted into its new form which 

 has an Eq value low enough (around -0.32 V, perhaps with the aid of a 

 nonheme iron protein such as photosynthetic pyridine nucleotide reduc- 

 tase (65,66,67) to reduce the opposite terminus of the electron transport 

 system and thus to complete a cyclic electron transport system. 



