478 THE ACCESSORY PIGMENTS CHAP. 17 



porphyrins and bile pigments in the animal organisms are not only- 

 possible but probable, and one may suspect that a similar relationship 

 exists between chlorophyll and the phycobilins in the algae. 



The proteins associated with the phycobilins are probably globulins, 

 with isoelectric points at pH 4 to pH 5. Roche (1933) found 4.30 for 

 the phycoerythrin from Ceramium rubrum, and 4.75 for the phycocyanin 

 from Aphanizomenon flos aquae. Because of the convenience with which 

 the sedimentation of colored proteins can be observed, phycoerythrin 

 and phycocyanin were among the first substances investigated by means 

 of Svedberg's ultracentrifuge. The papers by Svedberg and Lewis 

 (1928), Svedberg and Katsurai (1929), Svedberg and Eriksson (1932), 

 and Eriksson-Quensel (1938) brought much information as to the 

 molecular weights and other properties of these chromoproteids under 

 different conditions. At first, Svedberg and Lewis (1928), working at 

 pH 6.8-7.0, found that the phycoerythrin protein was twice as large as 

 the phycocyanin protein (molecular weights, 208,000 and 106,000, 

 respectively). They calculated from the sedimentation constant of 

 phycoerythrin a diffusion constant of 7.0 X 10~^ cm.^ (at 30° C), a 

 value consistent with the assumption that the phycoerythrin molecules 

 are spheres obeying Einstein's diffusion law, having a density of 1.33 and 

 a radius of 39.5 A. 



Later, experiments by Svedberg and Katsurai (1929) showed that, 

 if measured near the isoelectric point (pH 4-5), both phycoerythrin and 

 phycocyanin have molecular weights of about 208,000. The phycocyanin 

 molecule is, however, more easily split at the higher pH values than the 

 phycoerythrin molecule. At pH 6.8, the phycoerythrin molecules are 

 still intact, whereas most of the phycocyanin molecules from Porphyra 

 are split into halves, and those from Aphanizomenon consist of 65% 

 "full-size" molecules and 35% "half -size" molecules. At pH 11, phy- 

 coerythrin is also partly split into smaller molecules (75% 208,000 and 

 25% 34,700), and phycocyanin (from Aphanizomenon) is completely 

 divided into molecules with a molecular weight of 34,700. Svedberg 

 and Eriksson (1932) made experiments with fresh algal extracts and 

 showed that their molecular weight was the same as that of products 

 purified by crystallization. The molecular weight of phycocyanin from 

 A. flos aquae was again found to be 208,000, that of the phycoerythrin 

 from Polysiphonia urceolata, 196,000. The phycocyanin breaks into 

 half-size molecules both on the alkaline and on the acid side of the stable 

 region (pH 2.5-5.0). Below pH 1.5 and above pH 8.0, it breaks into 

 inhomogeneous products. 



The observation of the sedimentation equilibrium gives the molecular 

 weight, M) the measurement of the sedimentation velocity gives the 

 coefficient of viscosity, /; by combination with M, it is possible to calculate 



