THE SYNTHESIS OF PROTEINS 139 



This experiment of Schiitzenberger's left us, however, very much 

 where we were, so far as real knowledge of the structure of the protein 

 molecules is concerned. The knowledge of the fact that a mixture of 

 amino-acids and urea yields, under certain treatment, a body or bodies 

 more or less closely resembling the proteins, furnished us with little 

 or no information regarding the structure of the protein molecule 

 which we did not already possess in the fact that the disintegration- 

 products of the proteins are predominantly amino-acids. Prior to 

 Schiitzenberger, Grimaux, in 1881, had shown that condensatioa- 

 products of aminobenzoic acid and aspartic acid resemble the proteins 

 in many of their properties; but these experiments also threw no light 

 upon the structure of the protein molecule beyond emphasizing the 

 already sufficiently evident probability that the amino-acid grouping 

 plays an important part in the building up of the protein molecule. 



The clue which led, through a series of remarkable researches, to 

 our present comparatively extensive knowledge of the groupings 

 within the protein molecule, was obtained in 1883 by Curtius when 

 he discovered that ethyl glycocollate (the ethyl ester of glycine) in 

 watery solution tends to form Glycine Anhydride: 



(In the absence of water) 



H 2 N.CH 2 .COOH + C 2 H 6 OH = H 2 N.CH 2 COOC 2 H 6 + H 2 O 



(Glycine) + (Ethyl alcohol) = (Ethyl glycocollate). 



(In the presence of water). 



H 2 N.CH 2 COOC 2 H 6 + Hj-N.CH^COOCzHs = 



(Ethyl glycocollate) + (Ethyl glycocollate) . 



CH 2 N HL 



O = C C = O + 2C 2 H 6 OH 



^NH CH/ 



(Glycine anhydride) + (Ethyl alcohol) . 



Obviously, if the closed ring representing the glycine anhydride 

 molecule could be opened up without destroying the stability of the 

 molecule, a new amino-acid would be formed, one degree more complex 

 than the original amino-acid (glycine) . This possibility was realized 

 by Emil Fischer, who found that if the glycine anhydride which is 

 thus prepared be boiled for a short time with concentrated hydro- 

 chloric acid, the following change occurs: 



/CH 2 .NHx /CH ? .NH 2 .HC1 



O = C C - O + HC1 + H 2 O = O = 



\ / 



(Glycine anhydride.) (Glycyl-glycine chloride.) 



