70 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



N 



x\ 



C CHO + CH 2 .NHCOC 6 H g C CH=C C . C 6 H 5 



C 6 H 4 <()CH ioOH =C 6 H 4 ()CH O J O 



NH NH 



which, when boiled with dilute soda, gives indolyl-a-benzoylaminoacrylic 

 acid, 



N 



C CH=C C.C 6 H 5 C CH=C.NH.COC 6 H 5 



o_0 +NaOH = C 6 H 



NH NH 



This compound on reduction with sodium amalgam and hydrolysis 

 with water gives tryptophane : 



C CH=C . NHCOC 6 H 5 C CH 2 . CH(NH 2 ) 



C 6 H 4 /^CH ioOH +H 2 +H 2 = C 6 H 4 /^CH COOH + C 6 H 5 COOH 

 NH NH 



E. THE OPTICALLY ACTIVE AMINO ACIDS. 



With the exception of glycine, all the ammo acids contain an asym- 

 metric carbon atom, and they are therefore capable of existence in two 

 optically active forms. In one of these forms they are present in the 

 protein molecule, and the synthesis of a naturally occurring amino acid 

 is only completed when the synthesised compound has been separated 

 into its optically active components. 



Three methods are known, all due to Pasteur, by which an inactive 

 mixture can be separated into its optically active isomers : 



1. The fractional crystallisation and mechanical separation of the 

 two isomers. 



2. The action of micro-organisms moulds, yeasts which destroy 

 the one isomer more rapidly than the other. This is known as the bio- 

 logical method. 



3. The fractional crystallisation of the salts of these compounds with 

 optically active bases or acids. 



By all these methods the optically active forms of the amino acids 

 have been prepared. 



By the first method Piutti, in 1887, obtained dextro- and laevo-aspartic 

 acid. He fractionally crystallised natural asparagine and separated it 

 into dextro- and laevo-asparagine, from which compounds he prepared 

 both the dextro- and the laevo-aspartic acids. As, however, Piutti used 

 asparagine from vetch seedlings, this separation cannot strictly be said 



