8 CHEMICAL STATICS 



3. The Connection between the Amino-Acid Content and the 

 Properties of the Proteins. — The various proteins are found to 

 be built up of the several units enumerated above together, 

 possibly, with units which we have not yet succeeded in isolating. 

 Some of the proteins are built up out of all of these units in vary- 

 ing proportions, others out of a lesser number, so that in some 

 cases amino-acids which are predominant among the products 

 of the hydrolysis of one protein are absent from among the prod- 

 ucts of the hydrolysis of another. Nevertheless the general 

 relationship of the various units and their occurrence in dif- 

 ferent proportions in the various proteins accounts not only 

 for the differences, but also for the similarity between 

 them. 



In many instances a definite parallelism can be traced between 

 the chemical and physical behavior of the proteins and their 

 amino-acid content. Thus the albumins, which are soluble in 

 distilled water and are not coagulated by half-saturation of their 

 solutions with ammonium sulphate, contain no glycin, while the 

 globulins which are (when uncombined with bases or acids) 

 insoluble in distilled water, and are coagulated by half-saturation 

 of their solutions with ammonium sulphate, do contain this 

 amino-acid. The alcohol-soluble vegetable proteins contain only 

 a trace (probably attributable to associated impurities) of glycin 

 and some of them contain no lysin, their content of diamino-acids 

 is very small, while their content of glutamic acid and of prolin 

 is very high. The phosphoproteins (casein, vitellin) are also 

 rather high in glutamic acid. Gelatin is characterized by its 

 high glycin content and keratin (the protein of hair and feathers) 

 by its high cystin content. The histones, which are predomi- 

 nantly basic substances, contain about 30 per cent of diamino- 

 acids, while the protamins, which are still more predominantly 

 basic, contain only small amounts of monoamino-acids, sal- 

 min containing over 80 per cent of arginin, while sturin con- 

 tains 67 per cent of its nitrogen as arginin, 10 per cent in 

 the form of histidin and from 6 to 7 per cent in the form of 

 lysin (41). 



Kossel (35) has expressed the view that all proteins are built 

 up around a protamin nucleus and that this accounts for the 

 fact that the majority of the proteins yield, on hydrolysis, a 

 certain proportion of diamino-acids. 



