ii2 SCIENCE PROGRESS 



only one form. Optically active polypeptides, in addition to 

 glycyl-1-tyrosine and leucyl-1-tyrosine, have quite recently been 

 prepared by Fischer. These are 1-leucylglycine, 1-leucyl-d- 

 alanine, 1-leucyl-l-leucine, and they are composed of the 

 stereoisomers obtained from the proteins. Glycyl-d-alanine, 

 d-alanyl-d-alanine, and d-alanyl-1-leucine have since been syn- 

 thesised. Their synthesis is far more tedious, since it 

 necessitates the preparation of the optically active forms. In 

 their preparation the acid chloride method has been used, and 

 a curious change from the dextro to the laevo form, termed 

 Walden's rearrangement, has been observed by Fischer, thus 

 d-leucine is converted into isobromcaproic acid chloride; this 

 when combined with 1-leucine and treated with ammonia gives 

 1-leucyl-l-leucine, instead of the expected d-leucyl-1-leucine. 



The material employed in the preparation of nearly all the 

 polypeptides consisted of the racemic amino acids, from which 

 two racemic compounds w r ould be expected to result. These 

 first appear in the halogen intermediate compounds, 



Br . C . HR . CO . NH . CHR . COOH l 



and a separation in a few cases has been effected at this stage. 

 In other cases both the dipeptides have been isolated after their 

 formation. In the majority of cases, however, only one product 

 has been obtained ; this may either consist of mixed crystals of 

 the two forms, or only the one form may have resulted. Instances 

 of this kind have been noticed in other classes of compounds, 

 and it can only be assumed that in the synthesis one arrange- 

 ment is more favourable than the other, and therefore appears 

 in much larger amount under the influence of asymmetry, or 

 the velocity of the reaction between one pair is greater than 

 between the other pair. The influence of asymmetry is well 

 known to play a great part in the rate of reactions, as exemplified 

 in the action of enzymes. Both forms may also be present, but 

 the one in greater quantity than the other. 



The nomenclature of the polypeptides in the case of those 

 consisting of optically active amino acids is quite simple, e.g. 

 d-leucyl-1-alanine, but where the racemic forms are concerned it 

 is more difficult. The examples, dl-leucylglycine and glycyl-dl- 

 leucine, designate the two isomeric inactive dipeptides, containing 

 only one asymmetric carbon atom. The two racemic dipeptides 



1 The asymmetric carbon atoms are in thicker type. 



