PHYCOERYTHRIN 259 



coloured solution which exhibits a well-marked orange fluor- 

 esence ; the spectrum shows that the chief absorption is that 

 of the blue-green rays. 



Preparation. 



To prepare a solution of phycoerythrin the red sea-weed, 

 Cerainium rubrum, which is one of the best to use, is washed 

 in ordinary water to free it from sea salts and adhering sand. 

 It is then soaked in distilled water ; in two days most of the 

 pigment will have diffused out. The solution is filtered through 

 glass wool and a few drops of eucalyptus oil added as an anti- 

 septic, for putrefaction soon sets in. 



It is a matter of great difficulty to obtain a pure sample of 

 phycoerythrin, for, in an aqueous solution, it passes over into 

 an irreversible gel,* even when kept at 0° C. This, of course, 

 renders ordinary filtration extraordinarily slow, and thus in- 

 creases the difficulty of purification. 



The solid phycoerythrin may be prepared from the aqueous 

 solution by concentrating it under reduced pressure at a 

 temperature not higher than 38° C. ; any precipitate which 

 comes down during this process must be filtered off. Methyl- 

 ated spirit is then added to the concentrated solution until 

 the fluorescence disappears. The precipitated phycoerythrin 

 is allowed to settle and the more or less clear supernatant 

 fluid is filtered off, again treated with alcohol, and filtered. 

 The operation is repeated until the red colour has entirely 

 disappeared from the solution. The precipitates are washed 

 by decantation with 70 per cent alcohol ; the pigment, in a 

 pasty mass, is placed in a clock glass and dried in a vacuum. 



Reactions. 



The following reactions are among those recorded by 

 Hanson : — 



1. Phycoerythrin is precipitated from its solution by 

 alcohol, by small quantities of mercuric chloride, and by 

 saturation with ammonium sulphate and magnesium sulphate. 



2. When dilute acids are added gradually, the fluorescence 

 first disappears, leaving a somewhat opalescent solution of 



* See Section viii, on Colloids. 

 17* 



