1306 THE PIGMENT FACTOR CHAP. 32 



For energy transfer from phycocyanin to chlorophyll a in blue-green 

 algae, <p was assumed to be as high as 0.7 (i. e., 70% yield of phycocyanin 

 fluorescence in vivo, if no transfer were to occur). This gave for the "criti- 

 cal concentration" of chlorophyll a, [/c]o = 0.006 mole/1. Actually, the 

 concentration of chlorophyll in the cell is much higher; if one assumes 

 [fc]o = 0.03 mole/1, (concentration of chlorophyll a in Chroococcus cells 

 according to Arnold and Oppenheimer 1950), the probability of transfer 

 becomes >90%, and the yield of fluorescence of phycocyanin would thus be 

 reduced from 70% to 0.1%. Observations of Arnold and Oppenheimer 

 indicated, however, a yield of the order of 5%; Duysens saw in this an 

 indication that in blue-green algae chlorophyll and phycocyanin are sepa- 

 rated in different structural units. This view is supported by the fact 

 that phycocyanin can be separated from chlorophyll by fractional precipita- 

 tion or centrifugation. Chlorophyll seems to be contained in (grana-like) 

 "chromophores" (cf. p. 1741), while phycocyanin seems to be located in the 

 extragranular plasma. This separation would increase the average dis- 

 tance between a phycobilin and a chlorophyll molecule to something like 

 1000 A. ; offhand, this seems much too far to permit the observed, about 

 90% effective, energy transfer. Perhaps the difficulty can be over- 

 come by assuming a migration of energy through numerous phycocyanin 

 molecules in the cytoplasm, to the granum boundary, where it can be 

 transferred to chlorophyll. Another possibility is that phycocyanin is 

 located, in the main, not outside, but inside the grana in the protein layers, 

 while chlorophyll occupies the interfaces between protein and the lipoidic 

 layers. This assumption could bring the theoretical rate of energy transfer 

 from phycocyanin to chlorophyll in line with the observed 5% residual 

 fluorescence of phycocyanin ; but it calls for the further hypothesis that in 

 the smashing of the cells, phycocyanin is leached out of the grana, while 

 chlorophyll remains bound to their "skeleton." More recently this hypoth- 

 esis has found support in the observations of McClendon and Blinks 

 (1952) that the loss of phycobilins can be prevented by the addition of 

 high-molecular substances to the medium {cf. p. 1754). 



Duysens estimated the probability of the transfer of energy from phycoerythrin to 

 phycocyanin and chlorophyll in Porphyridinm cruentum, assuming all thi-ee pigments to 

 be present in a common phase, and concluded that because of the greater band overlap 

 the transfer to phycocyanin should be about five times faster, and thus occur to a prac- 

 tical exclusion of that to chlorophyll. The transfer from phycocyanin to chlorophyll 

 would than follow as a second step, with the efficiency estimated in the discussion of 

 OsciUatoria. The overlap integral is negligible for the reverse transfers — from chloro- 

 phyll to any one of the phycobilins. 



