352 PHOTOMECHANICAL CONSIDERATIONS 



energy transfer from chlorobrium chlorophyll to this minor chlorophyll, 

 together with the fact that light absorbed by this minor chlorophyll 

 is more effective in oxidizing cytochrome (11), leads to the suggestion 

 that this pigment might be the unique terminal energy acceptor which 

 couples absorbed light energy to photosynthesis. In view of this, ob- 

 servations were made on the light-induced ESR signals in cells and 

 extracts of Cps. ethylicum, using light selectively absorbed by the 

 two pigments. 



MATERIALS AND METHODS 



The sample of whole cells was taken from a two-day-old culture of 

 Cps. ethylicum, grown as described previously (12). The subcellular 

 fraction was obtained by washing the cells with a 0,1 M Tris buffer 

 solution (pH 7,8) and rupturing at low temperature for about 4 min. 

 with a 10 kc/s Raytheon sonic oscillator. The sonicate was centrifuged 

 at 20,000 X g for 20 min, and the supernatant fluid was centrifuged at 

 140,000 X g for 3 hrs. The pellet was resuspended in a very small 

 amount of Tris buffer and transferred to an ESR sample vessel as a 

 thick suspension. This resulted in a sample which was totally light 

 absorbing. 



A Varian V-4500 spectrometer equipped with a 100 kc/s modulation 

 unit was operated with a modulation amplitude of 12 gauss which re- 

 sulted in a signal to noise ratio of ca. 6:1, The sample vessels used 

 were 0.03 cm. thick and contained a volume within the wave guide of 

 ca. 0.06 ml. Calculations of the g-values were made using the signals 

 from a standard radical of peroxylamine disulfonate in conjunction 

 with a dual sample accessory. Baird- Atomic interference filters were 

 used to select the wavelengths of the actinic light; these had a half- 

 value bandwidth of 9, 11.5 and 14.5 m/^ for the X max. of 703, 744 and 

 825 m^, respectively. The light source was a 1000- watt projection lamp 

 focused on a 50% T grid of the microwave cavity. The intensity of the 

 incident light at the three wavelengths was measured with an Eppley 

 thermopile and a Leeds and Northrup No. 2420B galvonometer. The 

 integrations were carried out numerically. 



RESULTS 



Both the whole cells and the subcellular fraction exhibited light- 

 induced ESR signals. Fig. 1 shows the signals as obtained by integra- 

 tion of the recordings from the spectrometer which were induced by 

 light of Amax. 703, 744 and 825 m/^ in whole cells of Cps. ethylicum. 

 Although a slight asymmetry on the low magnetic field position was 

 observed consistently, it was not of sufficient magnitude to infer the 



