MODIFIED AND CONJUGATED ANTIGENS 337 



It is important that the diazotization of aromatic amino groups, 

 i.e., amino groups attached directly to a carbon atom in a benzene 



nucleus or ring as in aniline <y y — NH., be carried out in 

 the cold, for otherwise hydrolysis ensues with the formation of 

 liydrogen chloride (HCl), free nitrogen (N.) and the correspond- 

 ing phenol as shown lielow: 



/ ^— N rHr+ OT=N— FohTh'; CI = / \— N=N-C1 + 2Hp 



In a cold solution, this is the only reaction, but if the solution is 

 warm a second reaction takes place which is the hydrolysis reac- 

 tion yielding the phenol : 



^ N— N=N— CI + H,0 =<^ ^— OH + K + HCl 



thus giving in the end products analogous to the reaction in the 

 aliphatic series. 



Protein Structure. — It might be thought that the amino groups 

 on the side chain of the amino acids constituting the protein 

 would be attacked by the diazotization thus giving hydroxy acids 

 rather than amino acids, but it will be remembered that the pro- 

 tein molecule is conceivably built up by condensation of an amino 

 group of one amino acid molecule with the carboxyl (-COOH) 

 group of another amino acid molecule as shown : 



H OH 



I .,-.-... II I 



CH3— C— N— i H + HO i— C— C— CH3 



I "'"" ' I 



0=0 NH,, 



I 

 OH 



Alanyl-alanine, a dipeptid. 



until finally a compound of high molecular weight and having 

 emulsoid colloid properties is formed. In this type of combina- 

 tion all but a few of the side chain amino groups of the amino 

 acids constituting the protein are i)roteeted against the action of 

 the nitrous acid. 



