THE CHEMICAL CONSTITUTION OF THE UNITS 91 



Isoleucine. 



This amino acid was first obtained by F. Ehrlich in 1903 from the 

 nitrogenous constituents of beet-sugar molasses, and was subsequently 

 isolated by him from the decomposition products of fibrin and other 

 proteins. Like leucine, to which it is very similar in properties, it thus 

 appears to be a widely distributed constituent of the protein molecule. 



Of the various isomeric amino-caproic acids only leucine and iso- 

 leucine occur in the protein molecule ; both of them, combined with 

 tyrosine arid valine in the form of polypeptides from which they are 

 easily split off by enzymes, seem to form a very important part of most 

 proteins. 



Ehrlich showed that leucine, when heated to 200 C., was converted 

 into d-amylamine with loss of carbon dioxide : 



CH 3 \ CH 3 \ 



>CH . CH(NH 2 )COOH = CO 2 + >CH CH 2 . NH 3 



C 2 H/ C 2 H/ 



and that, when fermented by yeast in the presence of cane sugar, it 

 yielded d-amylalcohol : 



CH 3 \ CH 3X 



/CH . CH(NH 2 )COOH + H 2 O = >CH . CH 2 OH 



C 2 H/ C 2 H/ +C0 2 + NH, 



This was determined by oxidising to methylethylacetic acid : 



CH 3 \ CH 3 \ 



^>CH . CH 2 OH + 20 = ">CH . COOH + H 2 O, 



C 2 H 5 C 2 H 5 



from which the constitution of isoleucine appeared to be a-amino-/:?- 

 methyl-/3-ethyl-propionic acid. 



Ehrlich proved this by synthesising it from d-amylalcohol ; this was 

 first oxidised to valeraldehyde, which, on treatment with hydrogen 

 cyanide and ammonia, gave valeroaminonitrile, and then, on hydrolysis, 

 d-isoleucine mixed with allo-isoleucine ; isoleucine has two asymmetric 

 carbon atoms, and allo-isoleucine is formed by the rearrangement of the 

 groups attached to one of them. By heating natural isoleucine with 

 baryta water under pressure, it was found to also undergo a rearrange- 

 ment, and the product seemed identical with that obtained by syn- 

 thesis : 



CH 3 \ CH 3 \ CH 3 \ /NH 2 CH 3 \ /NH 2 



>CH.CH 2 OH-> >CH.CHO-> >CH . CH/ ^ \CH . CH/ 



C 2 H/ C 2 H/ C 2 H/ X CN C 2 H/ \COOH 



d-amylalcohol d-valeraldehyde d-valeroaminonitrile d-isoleucine 



Allo-isoleucine 



