CHEMICAL COMPOSITION OF PROTEIN MOLECULE 33 



The Distillation Residue. 



A dark reddish-brown and thick syrupy residue remains in the distill- 

 ing flask and sometimes crystals are observed clinging to the walls of 

 the vessel. The mass consists chiefly of the anhydrides of the amino 

 acids. Leucinimide may be extracted by boiling it with ethyl acetate. 

 The residue is generally so small in amount that it is not further 

 examined, but if this be necessary the mass is boiled with baryta for 

 sixteen hours, the baryta removed and the process of separation re- 

 peated. 



The Isolation of Oxyproline. 



Only in a few cases has this compound been isolated from the 

 products of hydrolysis of proteins, since its separation is extremely 

 laborious. Its ester is not extracted by ether, and it consequently 

 remains behind in the mass of carbonate ; if its isolation be required 

 a second esterification is generally not performed. 



The mass of carbonate is treated as previously described (pp. 20, 21). 

 The aqueous solution is evaporated in vacuo to remove hydrochloric 

 acid as completely as possible. The organic matter is dissolved in 

 water so that its content is about I per cent. ; sulphuric acid is added 

 and the diamino acids are precipitated with phosphotungstic , acid 

 (see p. 1 1). The excess of reagents are removed from the filtrate 

 with baryta, the solution is concentrated in vacuo to a small volume 

 and hydrochloric acid is removed with silver sulphate. The silver and 

 sulphuric acid are then precipitated in the usual way and the solution 

 is evaporated in vacuo to a small volume and then allowed to stand in 

 a desiccator over sulphuric acid. Oxyproline slowly crystallises out. 



Oxyproline is more easy to obtain when the esters have been 

 separated with sodium ethylate. It is then present in the distillation 

 residue ; this is treated in a similar way. It is identified by its melt- 

 ing-point of 270, rotation of [a] 2 D = - 81*04 in aqueous solution 

 and by conversion into its /3-naphthalene sulphonyl-derivative. 



PT. 



