54 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



in the cold, but on boiling they are oxidised, as was shown by Pollak in 

 the case of glycyl-glycine. 



The synthetical polypeptides are completely hydrolysed by boiling 

 with concentrated hydrochloric acid for five hours ; 10 per cent, hydro- 

 chloric acid at 1 00 C. hydrolyses them very slowly, and normal alkali 

 has only a very slight action. Their hydrolysis by enzymes, especially 

 by trypsin, is of such importance that a special section is required for 

 the description of these results (p. 63). 



///. Derivatives. 



The polypeptides yield the same derivatives as the amino acids. 

 Of the greatest importance are the esters of the polypeptides ; they 

 are prepared by the action of alcoholic hydrochloric acid. Hydrolysis 

 of the polypeptide does not occur if prolonged heating be avoided, nor 

 does hydrolysis occur when the esters are saponified by dilute cold 

 caustic alkali. The esters have served in particular for the further 

 synthesis of polypeptides and for the isolation of dipeptides from 

 mixtures ; on treatment with alcoholic ammonia, the dipeptide esters 

 are converted into their diketopiperazines. They are not soluble in 

 petroleum ether and they are soluble with difficulty in ether, and they 

 thus differ from amino acid esters. Chloroform dissolves them, and in 

 this solvent their combination with acid chlorides has generally been 

 effected. 



The carboxyl group can be converted into the acid chloride and a 

 halogen acyl group can be attached to the amino group. Further, the 

 benzoyl, the carbethoxyl and the napththalene-sulphonyl compounds 

 can be easily obtained ; the derivatives with the last mentioned are 

 soluble with difficulty and may be made use of for characterising the 

 polypeptides. The combination with phenylisocyanate also takes place 

 readily, but these compounds are not so useful as those of the amino 

 acids for purposes of characterisation. 



