236 Biological Chemistry. 



wheat, is obtained by extraction of wheat gluten that is, 

 wheat flour freed from starch by lixiviation with water 

 with 70 per cent, alcohol. From the solution in this 

 solvent, the protein can be precipitated by addition of 

 absolute alcohol. It separates as a thick glutinous mass, 

 which solidifies on repeated trituration with absolute 

 alcohol ; the powder is then rapidly washed with absolute 

 alcohol and ether and dried in vacuo. After the separa- 

 tion of the gliadin from wheat gluten a residue remains, 

 from which the glutenin can be extracted by very weak 

 solutions of caustic alkalis ; from this solution it can be 

 precipitated by neutralization with acids; too much acid 

 must not be added, as the protein is soluble in excess. 



The sclero-proteins, such as hair, horn, silk, etc., are, 

 as a rule, insoluble in any solvents. They are usually 

 prepared by separating all the other constituents of the 

 tissues. Thus the fats can be extracted by organic sol- 

 vents. Gelatin and collagen are usually included amongst 

 the sclero-proteins. These are extracted from bones. The 

 conjugated and derived proteins will be considered later.- 



GENERAL PROPERTIES OF THE PROTEINS. 



Owing to the fact that they are very complex chemical 

 substances which cannot, except in very few cases, be 

 obtained in crystalline form, and do not melt or boil with- 

 out decomposition, the criteria of purity which are avail- 

 able in the case of the simpler organic compounds are 

 wanting in the case of the proteins. It is consequently 

 often difficult to determine always whether a given pro- 

 duct is a pure chemical entity. As a class, however, they 

 can be recognized with tolerable facility. The following 

 are some of the chief reactions : 



(1) As they contain nitrogen, the proteins evolve 

 ammonia when heated with soda-lime. They also contain 



