328 IMMUNOLOGY 



amino acids in a single protein." Wells regards this as strong 

 evidence that immunological and biological specificity depend 

 upon the proteins. Subsequent work has shown, however, that a 

 feiv apparently nonprotein polysaccharides or carbohydrate-lipoid 

 complexes can stimulate specific antibody production. 



Importance of Aromatic Amino Acids in Protein Antigens. — 

 Starin (1918) working in Wells's laboratory found that gelatin, 

 which is a derived protein lacking tyrosine, is nonantigenic. From 

 an extensive chemical examination of protein antigens Obermayer 

 and Pick (1906) conclude that aromatic amino acids are essential 

 to all antigens and conversely that compounds made up of ali- 

 phatic amino and diamino acids are not antigenic. 



Acid and Basic Groups of Amino Acids. — It will be remembered 

 that the aliphatic amino acids have an open chainlike structure 

 while the aromatic amino acids have somewhere in their struc- 

 ture a closed chain. The benzene ring is quite common. Each 

 possesses one or more amino (NHo) groups and also a carboxyl 

 (COOH) group. The former endows it with basic properties 

 Avhile the latter, through its ionizable hydrogen, gives it acid 

 properties. Normally these tend to balance each other. If the 

 basic properties are removed, through linkage of some ethyl or 

 methyl group to the nitrogen of the amino group by replacement 

 of a hydrogen, the acid property is increased through this diminu- 

 tion of the basic property. In a similar way the acid property 

 may be reduced and the basic property increased. These basic 

 and acid groups are shown in the following formula for alanine : 



( Amino pr ] 

 I basic grouj) ^ 



H NH, O 



I I II [Acid property due 1o ion- "j 



H-— C— C— C— OH-I izable hydrogen of the }- 

 I I [ carboxyl ' (COOH) group J 



H H 

 Alanine 



Aliphatic and Aromatic Amino Acids. — The student should re- 

 member that the aliphatic or open chain amino acids can be 

 acetylated or methylated but cannot be diazotized. This will be 

 referred to again in the next chapter. The aromatic amino acids, 

 on the other hand, can be methylated, acetylated, azotized or 



