416 



PHOTOMECHANICAL CONSIDERATIONS 



of 33 per cent at 294°K is a minimum estimate, since the fluorescence 

 yield of chlorophyll- 770 in vivo may be lower than assumed. 



Evidence that photochemistry can compete with fluorescence emis- 

 sion from chlorophyll- 770 in Cps . ethylicum is given in Fig. 3. When 

 the intensity of the 436 m// exciting light is decreased from 10l8 to 

 10l5 quanta sec-1 cm-2, the intensity of the chlorophyll- 770 emission 

 relative to the chlorobium chlorophyll emission drops about 30 per 

 cent. The fluorescence yield of the chlorobium chlorophyll appears to 

 remain about the same over this range of intensities. These ob- 

 servations, extending to very low levels of excitation, suggest that 

 chlorophyll- 770 is involved in some photochemical reaction, but that 

 chlorobium chlorophyll is not. 



700 



750 



800 



850 



900 



Fig. 3. Uncorrected fluorescence emission spectra of Cps. ethyl iciiDi at 294° K 

 caused by 436 m^ excitation at intensities of 1018 and 1015 quanta cm-2 sec-1. 

 The spectra have been made to coincide at 769 m//. The dashed curves represent 

 the two components assumed to account for the overall emission spectrum at 

 the lower intensity of excitation. 



Light -induced absorbancy changes. 



In the spectrophotometer the most easily discernible chemical 

 reaction caused by light is the oxidation of cytochrome. Irradiation of 

 either C. thiosulfatophilum L or Cps. ethylicum with red light under 

 anaerobic conditions causes the oxidation of mainly c-type cyto- 

 chrome (s). Fig. 4 shows the disappearance of bands at 423 m/i and 553 

 m^. The asymmetry of the oe trough suggests the oxidation of some 

 6-type cytochrome (s) as well. 



