CHAPTER VII. 

 THE PROTEINS AND THE AMINO-ACIDS. 



GENERAL CHARACTERISTICS OF THE PROTEINS. 



THE greater and most characteristic part of the organic matter in 

 protoplasm consists of colloidal substances containing nitrogen which 

 are designated Proteins. As examples of the proteins we may recall 

 white of egg, which is practically a solution of protein in dilute sodium 

 chloride solution, or casein, which is flocculated out of milk by the 

 addition of acids, and gelatin, which is derived from the connective 

 tissues by extraction with hot water. 



The proteins all contain carbon, hydrogen, nitrogen and oxygen, 

 while the great majority of them also contain sulphur and a very 

 great many of them contain phosphorus. Other constituents, for 

 example, iron, copper and iodine are found in certain exceptional 

 proteins or in. compounds of the proteins with non-protein radicals 

 containing these elements. The average composition of the more 

 typical proteins is represented in the following table: 



Element. Per cent. 



C . . . . . .... . . . . 50. 6 to 54.5 



H 6. 5 to 7.3 



N 15.0to 17.6 



S 0.3 to 2.2 



P 0.4to 0.9 



O 21. 5 to 23. 5 



When perfectly free from water, the proteins form loose white 

 powders, but when imperfectly dry, and especially if exposed to heat, 

 they tend to form horny semi-transparent flakes or plates, so that in 

 most of the older literature, before the modern methods of dehydration 

 at low temperature by absolute alcohol and ether were employed, 

 the proteins are usually described as horny substances when in the dry 

 condition. 



While drying, and in the presence of traces of moisture the proteins 

 show a marked tendency to discoloration, with the production of 

 heavily pigmented insoluble substances which are probably related 

 to the " humin-substances" which are produced in the presence of 

 carbohydrates by boiling the tryptophane radical of proteins with 

 acids. Many proteins have curious and characteristic faint odors, 

 but they are generally tasteless and amorphous. 



Notwithstanding their colloidal character and very slight diffusi- 

 bility in solutions, many proteins may, nevertheless, under suitable 



