THE PROTEINS 91 



of protein, impure leucine crystallises out in the form of imperfect 

 crystals, the so-called ' leucine cones.' 



Lately another isomer of leucine has been discovered, namely, a-amino- 

 methyl ethyl propionic acid. This is called isoleucine. 



(2) Mono-amino Derivatives of Dibasic Acids 

 Of these two are known, namely, aspartic and glutamic acids. 

 ASPARTIC ACID is a-amino-succinic acid : 



COOH 



CH.NH 2 



CH 2 



COOH 



and glutamic acid is the next homologue, namely, a-amino-glutaric 

 acid : 



COOH 



CH.NH 2 



CH 2 



CH 2 



COOH 



Owing to the possession of two carboxyl groups these amino- acids have 

 a much more pronounced acid character than is the case with the 

 other members of the group which we have been studying. 



Aspartic acid was first found in the shoots of asparagus in the form of the 

 amide, asparagine : 



COOH 



I 

 CHNH 2 



CH 2 



CONH 2 



This substance is very widely distributed throughout the vegetable kingdom 

 and is present in seedlings in very large quantities, as much as 25 per cent, 

 of the dried weight. In plants it apparently serves either as a reserve material 

 or as the form in which the greater part of the nitrogen is conveyed from the 

 reserve organs to be built up into the protoplasm of the growing parts of the 

 plant. 



