REACTION CENTERS IN PHOTOSYNTHESIS 389 



800 m/Li, with shoulders at 760 and 860 mju (Fig. 4). Chromatophores 

 from such "pheophytinized" cells contain very little BChl; their ab- 

 sorption spectrum (Fig. 5a) is predominantly that of the aggregated 

 BPh. 



Throughout the conversion of BChl to BPh, the light reactions of 

 P870, UQ, Cyt. and Types E and F (Table 1) remain unimpaired. In 

 pheophytinized material the light-induced bleaching at 870 m^ is a 

 large fraction of the residual absorption at this wave length (inset, Fig. 

 5a). 



When chromatophores from pheophytinized cells are treated with 

 Triton X-100 the remaining trace of light-harveting BChlis destroyed. 

 Also, the absorption spectrum is altered in a way that suggests dis- 

 persion of the aggregated BPh. The BPh bands at 860 and 800 m/i are 

 shifted to 750 m/i, and the band at 535 m// is intensified. In these chro- 

 matophores P870 has become isolated as a spectrophotometric entity 

 (inset, Fig. 5b), The absorption band remaining at 870 m/i is bleached 

 totally and reversibly by light. The P870, UQ, Cyt, Type E, and Type 

 F reactions all survive the combination of pheophytinization and de- 

 tergent treatment. 



These experiments establish P870 as a distinct entity inR.sphe- 

 roides chromatophores, and there is some evidence that P870 is sim- 

 ply BChl in a specialized environment. In extensively pheophytinized 

 cells the amount of residual light-harvesting BChl is one-half to one- 

 tenth the amount of P870. Extraction and chromatographic isolation of 

 the pigments from such cells yield a pigment that appears to be BChl, 

 having absorption bands at 375, 600, and 770 m/i in acetone-methanol. 

 The amount of this pigment corresponds to the sum of the P870 and the 

 smaller quantity of light-harvesting BChl.'^ 



In R. rubrum the existence of P890asa distinct substance has been 

 inferred from the kinetics of BChl fluorescence. Vredenberg and 

 Duysens (33) showed that the yield of BChl fluorescence increases 

 during illumination in a way that parallels the bleaching of P890, It was 

 assumed that P890 is nonfluorescent and that the transfer of energy 

 from BChl to P890 competes effectively with the emission of fluores- 

 cence by BChl. When the P890 becomes bleached it can no longer ac- 

 cept excitation energy from BChl; the fluorescence emitted by BChl 

 then increases. The expectations drawn from this model were cor- 

 roborated quantitatively in Vredenberg and Duysens' experiments. 



Experiments with R. spheroides chromatophores indicate that 

 P870 (and, by extension, P890 of R. rubrum) is indeed nonfluorescent. 

 In pheophytinized material the predominant fluorescence band, at 

 770 m/i, is due to BPh. A much smaller band at 890 m/i can be ascribed 

 to the residual light-harvesting BChl or to P870. The yield of 770 m/i 



7 In estimating the amounts of P870 and BChl in vivo it was assumed that the 

 extinction coefficients of these pigments are equal at 870 m/i. 



