Liz SCIENCE: PROGRESS 
only one form. Optically active polypeptides, in addition to 
glycyl-l-tyrosine and leucyl-l-tyrosine, have quite recently been 
prepared by Fischer. These are lleucylglycine, 1-leucyl-d- 
alanine, l-leucyl-l-leucine, and they are composed of the 
stereoisomers obtained from the proteins. Glycyl-d-alanine, 
d-alanyl-d-alanine, and d-alanyl-l-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 levo form, termed 
Walden’s rearrangement, has been observed by Fischer, thus 
d-leucine is converted into isobromcaproic acid chloride; this 
when combined with ]-leucine and treated with ammonia gives 
1-leucy]-l-leucine, instead of the expected d-leucy]-l-leucine. 
The material employed in the preparation of nearly all the 
polypeptides consisted of the racemic amino acids, from which 
two racemic compounds would be expected to result. These 
first appear in the halogen intermediate compounds, 
Br .C HR, CO: NH .CHR-COOr 
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-leucy]-l-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. 
