MODIFIED AND CONJUGATED ANTIGENS 339 



However, acetylalion or niethylation of cysteine might be possible 

 through replacement of the hydrogen on the sulphur as follows: 



H O 



H— C— S— rir+ ■ CTi— C— CH., 



I " " 



H— C— NH„ 



I 



c=o 



I 



OH 



Since there is no replaceable hydrogen on either of the sulphurs 

 of cystine, it is not possible to acetylatc or methylale as shown by 

 the structural formula : 



H H 



I I 



H— C— S fi— 0— H 



I I 



H~C— NH., H— C— NH., 



I ' I ' 



C=0 0=0 



I I 



OH OH 



Cystine 



The chance for aeetylation or methylation of the amino grou]i 

 of either of these two compounds or of any amino acid when 

 combined in a protein is extremely remote, as explained under 

 protein structure. 



Stereoisomers or Spatial Relationships. — Landsteiner calls at- 

 tention to another interesting factor, spatial relationship, that 

 may influence specificity, especially in the natural antigens con- 

 taining carbohydrates. This type of specificity is due to the con- 

 figuration around an asymmetric carbon atom. He finds when he 

 couples stereoisomers to a protein such as horse serum, by diazo- 

 tization, that each optical isomer-azo-protein complex constitutes 

 a distinctly specific antigen for which corresponding antibodies 

 can be produced. This is another example of hapten specificity. 



When phenyl glycine is treated with p-aminobenzoyl chloride 

 there results phenyl-p-aminobenzoylamino acetic acid, of which 

 Landsteiner prepared the two optical isomers as shown: 



