34 PHOTOSYNTHESIS 



cases the fluorescence appears stronger than that of chloro- 

 phyll in organic solutions, but this may be because the 

 colour of the fluorescence is nearer to the maximum colour 

 sensitivity of the eye. There appear to be no measurements of 

 the fluorescent yield. The fluorescence of the pigments is 

 completely lost by denaturation of the protein component, 

 even under conditions where precipitation does not take 

 place. (This may be compared to the loss of fluorescence of 



300 ^00 500 600 



Wai/elength of light 



m/' 



FIG. 3.5. Absorption curve for phycocyanin. (Data of Svedberg 

 and Katsurai.) 



chlorophyll in the chloroplasts under mild conditions — such 

 as heating to 55° C. — which may also be presumed as due to 

 denaturation of a protein-pigment complex.) With the de- 

 natured pigments the fluorescence may be restored by com- 

 bining the prosthetic groups with zinc. Indeed the spectra 

 of the chromoproteins seem to resemble those of the zinc 

 compounds rather than that of the free prosthetic groups; 

 yet, so far, the purified chromoproteins have been found to 

 be metal free. 



