REACTION CENTERS IN PHOTOSYNTHESIS 385 



green plants as described in Fig, 1. For the bacterial system the fol- 

 lowing substitutions should be made: 



400 Chi a ^ 40 BChl 



P700 <- P870 or P890 



PQ *- UQ 



Cyt/ ^ a bacterial Cyt 



e~ from System II ^ e~ from substrate 



If the photochemical reaction centers exist as organized entities 

 it should be possible to separate them from the light-harvesting Chi or 

 BChl and study them in isolation. Some progress has been made in this 

 direction. 



In chloroplasts Kok achieved a sevenfold enrichment of P700 rela- 

 tive to Chi a, through extraction with acetone (16). The P700 reaction 

 remained intact in acetone- extracted chloroplasts, and Cyt / had not 

 been removed. The light-induced oxidation of Cyt f no longer occurred. 



In purple bacteria the P870-890 reaction appears as a slight bleach- 

 ing of the long- wave absorption band of BChl. Until recently it could 

 not be decided whether this reaction signals a slight change in most 

 of the BChl molecules or the gross alteration of a few molecules. It 

 has now been established that a distinct minor component (P870 or 

 P890) is bleached completely. This conclusion was reached through a 

 combination of techniques that eliminate the light-harvesting BChl 

 without attenuating the light reactions of Table 1,^ 



In chromatophores of blue-green (carotenoidless) mutant R. sphe- 

 roides suspended in water, the BChl is destroyed in the presence of 

 light and oxygen. The light reaction of P870is attenuated in proportion 

 to the loss of total BChl. Addition of 1% Triton X-100 (a nonionic de- 

 tergent) accelerates the photooxidative destruction of BChl, but not of 

 the P870 reaction. In the presence of the detergent. 957c of the BChl 

 can be destroyed without appreciable loss of the P870 reaction. Ab- 

 sorption spectra of blue-green mutant R. spheroides chromatophores, 

 before and after photooxidation in the presence of Triton X-100, are 

 shown in Fig. 3, In Fig. 3b ("after treatment") the bands at 535 and 

 760 m/i are due to the presence of a trace of bacteriopheophytin (BPh). 

 Of the BChl remaining after photooxidation, a component absorbing at 

 800 mn predominates. The dashed curves in Fig. 3 show absorption 

 spectra of the same chromatophores under illumination and thus 

 show the photobleaching of P870. In the BChl- depleted chromatophores 

 (Fig, 3b), the 870 m/U band undergoes a 60% reversible bleaching. The 

 ratio of P870 to BChl has been increased about twentyfold. 



6 The ensuing material is reported in detail elsewhere (38). 



