12 CHEMICAL STATICS 



In other words, the chloracetyl group, introduced to protect 

 the NH2 group of the amino-acid is, after it has performed its 

 protective function, itself transformed into an amino-acid group, 

 through the replacement of the halogen atom by NH2. Obvi- 

 ously, other halogen-containing acid groups may be used in place 

 of chloracetyl, and in this way a great variety of amino-acid groups 

 can be introduced into the NH2 group. Thus Fischer employs: 



Chloracetyl-chloride for the introduction of glycyl. 



a-Bromopropionjd-chloride for the introduction of alanyl. 

 1-a-Bromopropionyl-chloride for the introduction of d-alanyl. 



a-Bromobutyryl-chioride for the introduction of a-aminobutyryl, 



a-Bromisocapronyl-chloride for the introduction of leucyl. 



a-Bromophenylacetyl-chloride for the introduction of phenylglycyl. 



a-Bromohydrocinnamyl-chloride for the introduction of phenylalanyl. 



a-Phenylbromopropionyl-chloride for the introduction of phenylalanyl. 

 a-5-Dibromovaleryl-chloride for the introduction of prolyl. 

 Fumaryl-chloride for the introduction of asparagyl. 



By this method the chain of amino-acids is lengthened at the 

 amino-group end. Theoretically, it appeared possible to also 

 lengthen the chain at the carboxyl end of the molecule, by acting 

 upon the esters of the amino-acids with the acid chlorides of 

 other amino-acids. Until 1904, however, the acid chlorides of 

 amino-acids were unknown and all attempts to prepare them had 

 failed, owing to the same reason which limits the use of the first 

 method of synthesizing poly-amino-acids, described above, namely 

 the reactivity of the NH2 group. It will be recollected that 

 Fischer found that the NH2 group could be protected by the 

 introduction of radicals, and, utilizing this fact, in 1904 Fischer 

 succeeded in devising a method of preparing the acid chlorides 

 of the amino-acids (17). The acid chlorides thus prepared react 

 with the esters of other amino- or poly-amino-acids to form poly- 

 amino-acid chains of greater length. Thus: 



C4H9.CHBr.CO.NH.CH2COCl + 2 NH2.CH2.COOC2H5 



(Bromiaocapronylglycyl chloride) (Glycin ester) 



= HCINH2.CH2.COOC2H5 + C4H9.CHBr.CO.NH.CH2CO.- 



NH.CH2COOC2H5. 



(Glycin ester hydrochloride) (Bromisocapronylglycyl-glycin ester) 



Subsequent saponification of the bromisocapronyl-glycyl-glycin 

 ester and treatment with ammonia yields the poly-amino-acid (tri- 

 peptid) leucyl-glycyl-glycin : 



C4H9.CH(NH2).CO.NH.CH2.CO.NH.CH2.COOH. 



