54 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Its /3-naphthalene sulphonyl-derivative, 



/CH 2 OH /CH 2 OH 



C 10 H 7 SO 2 C1 + H a N . CH/ = HC1 + C 10 H 7 SO 2 . NH . CH< 



\COOH \COOH 



which is prepared by shaking in alkaline solution with /3-naphthalene 

 sulphonyl-chloride, serves for the isolation of the remainder from the 

 filtrate. This compound is very suitable for its characterisation. 

 (M.P. = 2i4corr.) 



(g) The Distillation Residue. 



If the higher boiling esters are distilled a dark reddish-brown and 

 thick syrupy residue remains in the distilling 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 repeated. 



(h) 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 (p. 38). 

 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 di-amino acids are precipitated with phosphotungstic acid 

 (see p. 60). 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 precipitated in the usual way, the solution is eva- 

 porated in vacuo to a small volume and 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] D - - 81*04 in aqueous solution 

 and by conversion into its /9- naphthalene sulphonyl-derivative. 



