ENERGY TRANSFER IN GREEN BACTERIA 



415 



chlorobium chlorophyll and carotenoid to chlorophyll- 770. The lesser 

 emission band at 769 m/u belongs to chlorobium chlorophyll. At liquid 

 nitrogen temperature (77°K) the chlorophyll- 770 emission sharpens and 

 shifts to 827 m/u, and two minor bands at 880 mi^ and 917 m/i can now be 

 resolved. The chlorobium chlorophyll emission also sharpens and shifts 

 to 781 m/i at 77 °K. Similar observations have also been made with C. 

 thiosulfatophilum L and another strain of Chlorobium (13). 



Two estimates for the efficiency of energy transfer to chlorophyll- 

 770 in Cps. ethylicum are given in Fig. 2. The intensity of the 436 m/i 

 exciting light (lO^^ quanta cm-2 sec-1) determined from the emission 



294 °K 



^00%^ 

 436m^ 



CHL0R.660 



a 



CAROTENOIDS 



CHL0R.770 



827 

 880 

 917 



77°K 



Fig. 2. Pathways for excitation energy in C[)s. ethyl icuni Symbols L and F de- 

 note nonradiative losses and fluorescence respectively. The percentages shown 

 are based on the data in Fig. 1 and assumptions stated in the text. 



of fluorescence standards (12) was well above the intensity required 

 for saturating the cytochrome oxidation reaction. The quantum emission 

 from each chlorophyll at each temperature was determined from Fig. 

 2. The nonradiative losses L from chlorophyll-770 in vivo were cal- 

 culated from the fluorescence yields at 77° and 294°K for the protein- 

 chlorophyll- 770 complex in vitro (9). The calculated transfer efficiency 



