THE CHEMISTRY OF THE PROTEINS in 



octaglycylglycine. This decapeptide, on combination with 

 bromisocapronyl diglycylglycine, and subsequent replacement 

 of Br by NH 2 , yields the tetradecapeptide 



NH,CH(C 4 H 9 )CO . [NHCH,CO] 3 . NHCH(C 4 H 9 )CO . [NHCH 2 CO] 8 . 



NHCH,COOH, 



and a repetition of the process of combining with bromisocap- 

 ronyldiglycylglycine gives the octadecapeptide 



NH 2 CH(C 4 H 9 )CO . [NHCH,CO] :) . NHCH(C 4 H 9 )CO . [NHCH.,CO] 3 . 

 NHCH(C,H 9 )CO . [NHCH 2 CO] 8 . NHCH.COOH. 



Many technical difficulties had to be surmounted in its 

 preparation which were successfully overcome ; e.g. prevention 

 of frothing by shaking up with glass beads, etc. 



The molecular weight of this compound is 12 13. It far 

 exceeds that of the fats, of which tristearin has the molecular 

 weight 891, and it belongs to the most complicated combinations 

 ever obtained synthetically without our losing the knowledge of 

 its constitution. If, in place of the glycine residues, it contained 

 tyrosine, phenylalanine, or leucine residues, its molecular 

 weight would be increased two- to three-fold, i.e. to values 

 taken as the normal for many natural proteins. The values of 

 12,000 — 15,000 for other proteins, according to Fischer, are 

 uncertain, as we have no guarantee of the purity of these 

 natural substances. 



Up to the present time nearly one hundred different poly- 

 peptides have been prepared. Their structure is derived directly 

 from their synthesis, and they are generally represented as in 

 the formulae given in the above equations. Other structures are 

 also possible ; indications of these have been observed in their 

 study, and they are at present only of theoretical interest. 



The configuration of the polypeptides is of extreme im- 

 portance, for, with the exception of glycine, all the amino acids 

 contain an asymmetric carbon atom. According to van't Hoff's 

 formula, 2 n , the number of isomers possible when two amino 

 acids are combined together is four, namely, the dd, 11, dl, Id 

 forms ; and two racemic forms — dd, 11 and dl, Id — can be obtained 

 from these. With more amino acids coupled together the 

 complications are still greater, and their separation well-nigh 

 impossible. Fortunately, these conditions do not seem to be 

 present in the protein molecule, since the amino acids as they 

 result on hydrolysis are optically active andl hence exist in 



