390 PHOTOMECHANICAL CONSIDERATIONS 



fluorescence increases during illumination in a way that parallels the 

 bleaching of P870; energy transfer from BPhto P870 is thus indicated. 

 In contrast, the yield of 900 m/i fluorescence is perfectly constant 

 during illumination. If this fluorescence comes from P870, it should 

 decline as the P870 becomes bleached. A reasonable interpretation is 

 that, in this material, small amounts of P870 and BChl are isolated 

 from each other in an ocean of BPh. The fluorescent BChl can receive 

 excitation energy directly, or from BPh, but cannot transfer it to the 

 nonfluorescent P870. 



Excitation spectra have been measured for the fluorescence of BPh 

 and BChl and for the light reactions of P870 and Cyt. For these experi- 

 ments, cells and chromatophores were used in which about half of the 

 BChl had been converted to BPh. For fluorescence and for the bleach- 

 ing of P870, excitation spectra covered the region 450-650 m/i and thus 

 encompassed the 535 m/i absorption band of BPh and the 590 m/n band 

 of BChl. The excitation spectrum for Cyt oxidation, measured from 

 700 to 950 m/Li, embraced the 760 and 800 m/j. bands of BPh and the 870 

 mu band of BChl. Comparison of these excitation spectra with absorp- 

 tion spectra showed that (a) no energy was transferred from BChl to 

 BPh, (b) energy was transferred from BPh to BChl with approximately 

 25% efficiency, and (c) energy transfer from BPh to P870 proceeded 

 with an efficiency of about 50%. These results suggest that energy can 

 be transferred from BPh directly to P870, as well as by way of BChl. 



In summary, P870 (or P890) is a distinct component of the chroma- 

 tophore that functions as part of a photochemical reaction center. This 

 reaction center is held provisionally to be the "bacterial" counterpart 

 of the green plant reaction center as outlined in Fig. 1. P870 is prob- 

 ably BChl in a specialized environment that renders it nonfluorescent. 



The occurrence of the reversible light reactions of P870, UQ, and 

 Cyt, in chromatophores that contain almost no light-harvesting BChl, 

 raises a question as to the obligatory role of chlorophyll in photosyn- 

 thesis. Suppose that R. spheroides cells can be pheophytinized to the 

 extent that no light-harvesting BChl (other than P870 itself) remains. 

 If such cells can be made to perform complete photosynthesis it will 

 follow that the requirement for BChl is restricted to its functioning 

 in the capacity of P870. 



MECHANISMS IN THE INITIATION OF PRIMARY 

 PHOTOCHEMICAL REACTIONS 



The functioning of a photosynthetic unit involves a distinct sequence 

 of events. Energy, once absorbed by the light-harvesting system, mast 

 find its way to a reaction center. There it must be detained and utilized 

 in a photochemical process. 



