DETERMINATION OF STRUCTURE OF PROTEINS 87 



Complete Hydrolysis after Treatment with Alkali. 



Kossel and Weiss have observed that the optical rotation of the pro- 

 tamine, clupeine, when dissolved in semi-normal alkali, gradually fell 

 a rotation of - 2'6 became - I '4 in sixty-four hours and - o - 5 in 

 seven days at the ordinary temperature, a rotation of - 275 became 

 -0*51 in fifty hours at a temperature of 38 C., and the rotation of 

 a gelatin solution under the same conditions fell from - 5*84 to - 0-57 

 in fourteen days at the ordinary temperature. This change in rotation 

 was not due to the hydrolysis of the compound and the subsequent 

 racemisation of the amino acids ; there was only slight hydrolysis and 

 arginine, the chief constituent of clupeine, does not undergo such a 

 marked change in rotation when treated with alkali. The only con- 

 clusion which could be drawn was that racemisation of the arginine 

 occurs very readily whilst it is in combination in the protein. Subse- 

 quent hydrolysis of the protein yielded dl-arginine and dl-ornithine. 

 The same result was obtained with gelatin. 



An explanation of this phenomenon has been given by Dakin : 

 Uramido acids are converted by mineral acids into hydantoins : 



/NH.CO.NH. /NH CO 



R.CH( - R.CH< 



\COOH \ CO NH 



The optical rotation of the uramido acids in alkali is constant; the 

 rotation of the optically active hydantoin diminishes and becomes zero ; 

 and the hydantoin obtained on acidifying is optically inactive. 



The racemisation process is connected with the group 



CH a CO 

 which can undergo tautomeric change : 



NH . CO . NH NH CO NH 



R C CO *~ R C==C.OH 



I, 



This supposition was proved by an examination of the optically 

 active methyl-ethyl hydantoin, 



NH CO NH 

 CH.xl 



>C CO 



C a H.X 



prepared from isovaline by the action of potassium cyanate. It suffered 

 no racemisation on treatment with alkali. 



Amino acids in combination possess a structure similar to the 

 hydantoins and can undergo this keto-enol tautomerism as follows : 



