THE AMINO ACIDS 



139 



(2) By the addition of hydrogen cyanide and ammonia to aldehydes 

 and the subsequent hydrolysis of the aminocyanohydrin : 



CH, CH 3 



+ NH 3 = |/NH 2 

 CHO HC\ 



X OH 

 CH 3 



CH 



NH, 



X OH 

 CH 3 



HC NH 2 



CN 



HCN = 



NH, 



CN 



CH 



= HC 



NH + NH 



COOH 



Alanine. 



These reactions cannot always be used in the synthesis of 

 amino acids since the halogen derivative or aldehyde is sometimes 

 difficult to obtain. These amino acids have therefore been prepared by 

 indirect methods. A full account of the syntheses is given in the 

 author's "Chemical Constitution of the Proteins," Part II., 3rd edition. 

 The amino acid is not always prepared most easily by synthesis, but 

 by the hydrolysis of proteins, e.g. tyrosine, cystine, etc. 



Properties. 



The amino acids in a pure state are white crystalline substances 

 having characteristic forms. Glycine and leucine crystals are shown in 

 Figs. 28 and 29. 



FIG. 28. Glycine. FIG. 29. Leucine. 



(From Funke's " Atlas of Physiological Chemistry ".) 



They are usually easily soluble in water (cystine and tyrosine are 

 exceptions) but insoluble in alcohol (proline and hydroxyproline are 

 exceptions) and ether. They have generally high melting-points and 

 decompose when they melt, losing carbon dioxide. 



