CHEMICAL COMPOSITION OF PLANTS 51 



In many aleurone grains, it is possible to see distinctly well- 

 defined crystals of protein substances, from which it is apparent 

 that colloidal substances may have the capacity to crystallize. 

 A more detailed description of the structure of aleurone grains 

 may be found in books on plant anatomy. In underground 

 food-storage organs, as in the tubers of the potato, reserve pro- 

 teins are often found in a crystalline form. 



Protein substances can be easily detected by the aid of color 

 reactions, of which the most important is the biuret test. In 

 this particular reaction, proteins give a blue-violet or red-violet 

 color upon the addition of a solution of sodium hydroxide and a 

 few drops of a weak solution of copper sulphate. Upon heating 

 with strong nitric acid, proteins give a bright yellow color 

 (xanthoproteic reaction) , and upon boihng with mercuric nitrate 

 in the presence of nitrous acid, a brick-red color (Millon's 

 reaction). 



It must be noted, however, that all these reactions, except the 

 biuret, are tests not for the whole protein molecule but only 

 for some of its more characteristic amino acids. These con- 

 stituent amino acids may be studied best by means of hydrolytic 

 decomposition of the protein molecule. For this purpose, the 

 protein is subjected to a prolonged boiling with mineral acids, 

 which hydrolyze it into its constituents. Almost always these 

 are a-amino acids, in which NH2 is linked to the same carbon 

 atom to which is attached the COOH group. Otherwise, their 

 atomic configuration is quite varied, being not only of the 

 aliphatic but also of the aromatic series. All of them, with the 

 exception of glycine (a-amino-acetic acid) are optically active. 



The main amino acids found in prateins are the following: 



A. Aliphatic series: 



1. Monoamino acids: 

 -Glycine, CH0NH2COOH 

 d-Alanine, CH3CHNH2COOH 

 Z-Leucine, ^HsX^jj^jj (.jj^^jj^ ^.^^^jj 



Oils/ 

 ^-Aspartic acid, COOHCH2CHNH2COOH 

 d-Glutamic acid, COOHCH2CH2CHNH2COOH. 



2. Diamino acids: 



d-Lysine, CH2NH2CH2CH2CH2CHNH2COOH 

 d-Arginine, NH2\^ j^jj^jj^^jj^^jj^(.jjj^jj^.(.Oqjj 



3. Oxyamino acids: 



Z-Serine, CH2OHCHNH2COOH 



