32 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



In the preparation of this octadecapeptide complete combination of 

 the polypeptide with the acid chloride was very essential, since if the 

 greater part of the compound be not used up, but remained unchanged, 

 it was precipitated with the bromo compound on acidifying ; this was 

 only attained by using a very large excess of the acid chloride. At the 

 same time there was the technical difficulty of frothing ; this was over- 

 come by shaking with glass beads in large flasks. Liquid ammonia was 

 also necessary for the conversion of the halogen compound into its 

 amino derivative. Analysis of the polypeptides hardly sufficed for the 

 determination of their synthesis, since the variations in the figures are 

 so small, but a determination of the bromine in the corresponding 

 halogen derivative indicated that the synthesis was being effected in the 

 stages represented. 



This octadecapeptide has the highest molecular weight of any com- 

 pound as yet prepared by synthesis and of which we know the constitu- 

 tion. Its molecular weight is 1213, a figure which far exceeds that of 

 the fats, tristearin having a molecular weight of only 891. If the 

 compound contained other amino acid residues, such as leucine, tyrosine, 

 phenylalanine in the place of the glycine residues, the molecular weight 

 would be increased two to three times. 



Such a figure of 3,000-5,000 has been found for the molecular 

 weight of many proteins, and it would appear that they are composed 

 of some twenty amino acids. The higher values of 12,000-15,000 

 which have been found for the molecular weight of other proteins, are, 

 according to Fischer, very doubtful, since we have no indication of their 

 purity, in spite of the crystallisability of many of them ; the admixture 

 of a small quantity of another protein might easily raise the value to 

 this extent. 



Fischer subsequently found that the acid chloride of other acyl 

 derivatives of the amino acids could be prepared by the same process. 

 Thus, by treating finely powdered hippuric acid with phosphorus penta- 

 chloride in the presence of acetyl chloride, he obtained hippuryl chloride, 

 a compound which numerous investigators had tried to synthesise, 

 but unsuccessfully. By combining hippuryl chloride with glycine ester, 

 benzoyl-glycyl-glycine was obtained, and this compound, when con- 

 verted into its acid chloride and combined with glycine, yielded 

 benzoyl-diglycyl-glycine. In this way, by means of the acid chlorides, 

 Fischer has prepared the same compounds which Curtius has prepared 

 by means of hippurazide. 



By applying this method of preparing the acid chlorides to the 

 amino acids themselves, Fischer ultimately succeeded in obtaining the 



