FLAVONES AND ANTHOCYANIN8 479 



the coefficient of diffusion D. On the other hand, diffusion can be 

 measured directly, as this was done by TiseHus and Gross (1934) for 

 the phycobilins. They obtained values for D at pH 5 of about 4.0 X 10~^ 

 cm.^/sec. — considerably lower than the values calculated from sedimen- 

 tation experiments. The agreement was restored by new measurements 

 of the sedimentation equilibrium by Eriksson-Quensel (1938), which gave 

 for the molecular weights values almost 50% larger than those reported 

 previously (and thus correspondingly reduced the diffusion coefficients). 

 The new molecular weights are: 



Compound pH M 



Phycoerythrin {Ceramium) 3-10 291.000 



Phycocyanin {Ceramium) 2.5-6 273.000 



7-8.5 138.000 



Assuming with Svedberg that protein molecules are built up of "units" 

 with a weight of about 17,600, the phycobilins must be classed with the 

 protein molecules consisting of 16 units (molecular weight, 282,000). 



The chromoproteids contain, according to Lemberg (1929), about 

 2% pigment and 98% protein. This corresponds to one molecule of 

 pigment per two Svedberg units of protein, a whole order of magnitude 

 less than what we found in chapter 14 for the hypothetical chlorophyll- 

 protein complex. We shall find in Volume II, Chapter 21, indications 

 that Lemberg's estimate of the ratio of pigment to protein may be too low. 



Light absorbed by the phycobilins undoubtedly is used for photo- 

 synthesis (c/. Vol. II, Chapter 30) ; and this may well be the main func- 

 tion of these pigments in algae (in contrast to what was said above about 

 the carotenoids). Whether the sensitization of photosynthesis by phy- 

 cobilins occurs directly, or by a preliminary energy transfer to chloro- 

 phyll, remains to be elucidated. 



C. Flavones and Anthocyanins * 



Carotenoids are the most lipophilic and least hydrophilic of the leaf 

 pigments; the chlorophylls, and even more so the phycobilins, are less 

 hydrophobic, particularly in association with proteins. Leaves also 

 contain, in addition to chlorophyll and the carotenoids, pigments which 

 form true aqueous solutions, and are therefore concentrated in the cell 

 sap rather than in the chloroplasts. These are yellow pigments of the 

 fiavone class; and since their distribution in the leaves makes a relation 

 to photosynthesis improbable, we shall be satisfied with only a few words 

 about them. 



Flavones are derivatives of benzopyrone. They occur in all parts of 

 plants, often in the form of glucosides. One of the most common of 



* 



Bibliography, page 482. 



