82 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



diketopiperazine isolated from the hydrolysis products of silk-fibroin 

 was derived from glycyl-d-alanine. Silk-fibroin was partially hydro- 

 lysed by hydrochloric acid and then precipitated by phosphotungstic 

 acid A portion of the filtrate from this precipitate, after removal of 

 the excess of phosphotungstic acid, was treated in alkaline solution with 

 #-naphthalene-sulphonylchloride and a product was obtained which was 

 identical with $-naphthalene-sulphonyl-glycyl-d-alanine; further proof of 

 this was given by its careful hydrolysis with dilute hydrochloric acid 

 when the dipeptide chain was split, but the naphthalene-sulphonyl 

 radicle not removed ; /8-naphthalene-sulphonyl-glycine and d-alanine 

 were obtained according to the equation : 



NH . CH, . CO NH.CH{CHj).COOH + H,O = 

 NH . CH, . COOH + NH 8 . CH(CH 3 ) . COOH 



From the remainder of the filtrate glycyl-d-alanine anhydride and 

 a small quantity of glycyl-1-tyrosine anhydride were obtained by the 

 action of ammonia upon the esters, as well as another product which 

 was most probably d-alanyl-1-serine anhydride. 



The phosphotungstic acid precipitate contained several products 

 of a complex nature. A substance was isolated from them which 

 consisted of two molecules of glycine, one molecule of alanine, and one 

 molecule of tyrosine, i.e., a tetrapeptide. It had a molecular weight 

 determined by the freezing-point method of about 350, was easily 

 soluble in water, insoluble in alcohol, and was precipitated from its 

 solution in flakes by saturation with ammonium sulphate or sodium 

 chloride, as also by nitric or acetic acid. The synthetical pentapeptide, 

 1-leucyl-triglycyl-l-tyrosine, behaves in a similar way so that great 

 complexity, as formerly believed, is not an essential condition for pre- 

 cipitation by ammonium sulphate. Tyrosine was split ofF by the action 

 of trypsin, and on partial hydrolysis glycyl-d-alanine anhydride and 

 glycyl-1-tyrosine anhydride were obtained. 



In the same year (1907) Fischer and Abderhalden showed by the 

 same methods that the products of the partial hydrolysis of elastin 

 contained : 



1. d-Alanyl-1-leucine. 



2. d-Alanyl-I-leucine anhydride. This was probably formed from 

 the above dipeptide, but since it can also be formed from the isomeric 

 1-leucyl-d-alanine, this dipeptide may also be present amongst the pro- 

 ducts. 



3. Alanyl-proline anhydride, from which d-alanine and proline were 

 obtained on hydrolysis. 



