42 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



the carboxyl group is converted into the acid amide group ; here one of 

 the conditions necessary, as shown by Schiff, is added. 



The polypeptides yield the same derivatives as the amino acids ; 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 naphthalene-sulpho 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 important as those of the amino acids for 

 purposes of characterisation. 



On the other hand, the esters of the polypeptides are of the greatest 

 importance and they are prepared by the action of alcoholic hydrochloric 

 acid. Hydrolysis of the polypeptide does not occur if prolonged heat- 

 ing 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 been generally 

 effected. 



The polypeptides are attacked by nitrous acid with evolution of 

 nitrogen, but the amino and imino groups show no great difference in 

 their behaviour as might have been expected, and consequently a 

 differentiation of the amino and imino groups in the protein molecule 

 with nitrous acid seems impossible. They are not acted upon by 

 potassium permanganate 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 100 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. 



