338 



IMMUNOLOGY 



NiTRoso Compounds. — For diazotization it is also necessary that 

 tlie group be an amino (-NHo) group and not an imino (=NH) 

 group. In the case of trj'ptophane the =NH group in the ring 

 is not diazotizable, -NH^ being required for diazotization. How- 

 ever, nitroso compounds can be formed lierc by reaction with 

 nitrous acid in the following manner : 



/\ 



N 



H H O 



I I II 

 -C— C— C— OH 



I I 

 H NH.. 



; H + HO 



-N=:0 



N=0 



Azo Dyes. — Fur diazotization it is customary to use sodium 

 nitrite and hydrochloric acid, the reaction between sodium nitrite 

 and hydrocliloric acid that yields nitrous acid being as follows: 



NaNO., + HCl = HNO, +NaCl 



The preparation of azo dyes is accomplished by coupling dia- 

 zonium chlorides in solution with phenols or aromatic amino com- 

 pounds, 



\ 



^— N==N— jCi'TH'"!— <^ N— OH = 



<(^ ^— N=N— <^ ^-OH + HCl 



^— NH, 



-N=N— i CI + H 





<^ ^— N=N— / ^— NH„ + HCl 



In the protein a similar coupling can be accomplished whenever 

 the aromatic nucleus has an -OH or an -NHo in it, as has been 

 illustrated heretofore. 



Cysteine and Cystine. — In the case of cysteine, diazotization 

 would not be possible, nor would it be in the case of cystine, since 

 neither of these two compounds contains the aromatic nucleus. 



