ENERGY TRANSFER IN GREEN BACTERIA 421 



(At wavelengths below 774 m^ both the absorption curve and the initial 

 rate measurements should be taken with some reservation.) The ratio 

 of action to absorption at 812, 825, and 837 m/v is about 50 per cent 

 higher than the ratio at 774 and 786 mju. This indicates that light 

 absorbed directly by chlorophy 11-770 is about 50 per cent more efficient 

 than light absorbed by chlorobium chlorophyll in sensitizing cytochrome 

 oxidation and corresponds to an efficiency of 60 to 70 per cent for 

 transfer of excitation energy from chlorobium chlorophyll to chloro- 

 phyll- 7 70. 



DISCUSSION 



These results show that chlorophyll- 770 can sensitize the oxidation 

 of c-type cytochrome (s) in green bacteria with good efficiency. Energy 

 absorbed by chlorobium chlorophyll is transferred to chlorophyll- 770 

 with an efficiency of at least 30 per cent in Cps. ethylictim. The true 

 value is probably between 50 and 80 per cent. Although the possibility 

 that chlorobium chlorophyll may participate directly in some photo- 

 chemical reactions is not completely ruled out, the evidence favors its 

 function solely as a light-gathering pigment which transfers energy to 

 chlorophyll-770. 



Energy transfer by inductive resonance (15) is probably as good a 

 mechanism for green bacteria as for other photosynthetic organisms 

 (1). Based on the fluorescence and absorption characteristics of Cps. 

 ethylicum, the critical distance Rq for 50 per cent probability of energy 

 transfer either between chlorobium chlorophyll-660 molecules or from 

 chlorobium chlorophyll-660 to chlorophyll-770 was estimated to be 

 about 35 A (16). If the efficiency of energy transfer from chlorophyll- 

 660 to chlorophyll-770 is assumed to be between 50 and 80 per cent, 

 the ratio of chlorophyll-660 to chlorophyll-770 should be between 80 

 and 20 according to an approximate equation given by Duysens (Ref. 1, 

 p. 88). This is not too far from Fuller's (p. 71, this vol.) estimate of 

 100, in view of the uncertainties involved in each estimation. 



The light-induced cytochrome reactions in the two green bacteria 

 studied are similar to those observed in purple bacteria, especially 

 Chromatium (5). The close relationship of chlorophyll-770 to bacter- 

 iochlorophyll further implies a basically similar type of chlorophyll- 

 cytochrome interaction in the green and purple bacteria. From a phy- 

 logenetic point of view the similarities between these two groups of 

 photosynthetic bacteria are seen to be more numerous and fundamental 

 than previously realized. 



