THE CHEMICAL CONSTITUTION OF THE UNITS 131 



Fischer and Zemplen have also found that proline is formed by 

 boiling benzoyl-S-amino-a-bromovalerianic acid (p. 119) with concen- 

 trated hydrochloric acid : 



C 6 H 3 CO NH . CH 2 . CH 2 . CH 2 . CHBr . COOH + H 2 O = 



HBr + C 6 H 6 COOH + CH 2 CH . COOH 



\X 

 NH 



There was no difficulty in identifying the natural substance with the 

 synthetical one, and its presence in egg-albumin, gelatin and other pro- 

 teins was soon afterwards established. 



The question at once arose whether this a-pyrrolidine carboxylic 

 acid, or a-proline as Fischer termed it in 1904, was a primary product, 

 or a secondary product formed by the action of mineral acids upon 

 other products^ but its formation by hydrolysis by alkali and by the 

 action of pepsin followed by trypsin pointed to its being a primary 

 product and therefore one of the units of the protein molecule. Still 

 further proof of the primary nature of proline was given by Fischer 

 and London in 1911. By a prolonged digestion of gliadin they ob- 

 tained almost as much proline as was obtained from this protein by 

 hydrolysis. 



Sorensen, in 1905, suggested that it might arise from a-amino-8- 

 oxyvalerianic acid which he synthesised, but the fact that this amino 

 acid has not yet been obtained by hydrolysis of protein and the above 

 facts seem to exclude this possibility. Fischer and Boehner also ob- 

 tained a greater amount of proline by hydrolyses with baryta than is 

 usually obtained by acid hydrolysis. Its isolation, under these condi- 

 tions, excludes Sorensen's amino-oxyvalerianic acid as a primary 

 product. 



Pyrrolidone carboxylic acid is readily made from glutamic acid 

 (p. 1 1 6). Proline can be obtained, by reduction, from this compound 

 as Fischer and Boehner showed in 1911 : 



CH 2 CH 2 CH 2 CH 2 



CO CH . COOH > CH 2 CH . COOH 



v \/ 



NH NH 



This conversion of glutamic acid into proline is of interest as it is 

 possible that proline may be formed in nature in this manner. 



