LECTURE VIII. 



ALBUMINS OR PROTEINS. 

 II. 



THE COMPONENTS OF PROTEIN. 



ALTHOUGH the number and distribution of the proteins in Nature are 

 very great, still they show great similarity in the way they are constructed. 

 Various methods have been tried for effecting the cleavage of proteins. 

 Up to the present time, hydrolysis, whether brought about by the action 

 of acids, of alkalies, or of ferments, has alone been productive of results. 

 Experiments performed in the attempt to obtain known products by 

 oxidation have thus far been unfruitful. 1 It is, moreover, hardly to be 

 expected that in the case of such a complicated substance as albumin we 

 shall obtain much idea of its chemical constitution by means of oxidation 

 or reduction processes. We shall limit ourselves here, therefore, to a 

 consideration of those investigations which have been of service in the 

 further development of the entire chemistry of the proteins. We shall 

 first of all take up those substances which are formed by the hydrolysis 

 brought about by the action of acids or alkalies. If a protein is boiled 

 for some time with fuming hydrochloric acid, or with twenty-five per 

 cent sulphuric acid, its character is completely changed. It is broken 

 down into numerous, simpler cleavage-products. These are of various 

 kinds. They possess, however, certain common characteristics. As far 

 as we know, they nearly all crystallize well, and contain nitrogen, hydro- 

 gen, and oxygen. These cleavage-products are known in general as amino 

 acids. They can be easily identified and prepared in a pure condition. 

 Besides these amino acids we find varying amounts of ammonia, and fre- 

 quently humin substances. We shall presently see that we are acquainted 

 with a large number of the cleavage-products of the proteins, although an 

 appreciable part of the molecule is still unexplained. We shall also 

 soon learn that all proteins, as far as our present knowledge goes, contain 

 these same amino acids. Occasionally one or another amino acid is 

 missing, although, as a whole, there is a very general agreement in the 

 qualitative composition. The following amino acids have been isolated 

 up to the present time: 



1 Cf. Otto von Fiirth: Hofmeister's Beitrage, 6, 296 (1905). The older literature is 

 carefully compiled in this article. 



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