308 DYEING 



HON=N— <^ ^<^ \— N=NOH 



- Tetra-azotized benzidine 



ation of what is essentially an insoluble, non-sulphonated azo dye. 

 Since the phenyl ring of the tyrosine is part of the dye, no question 



NH 



HC.CH2— / NoH 



:=0 \n=N— / \— / V-N=NOH 



The tyrosine of a protein chain linked to tetra-azotized benzidine 



?= 



of attachment through an auxochrome presents itself. The 

 colour is brown. A bright red can be produced by coupling p- 

 naphthol at the other end of the benzidine through another azo- 

 link.^*^ Histidine and tryptophane behave similarly to tyrosine. 

 The three amino-acids can be distinguished by the use of 'block- 

 ing' agents; that is, substances that prevent one or other of the 

 amino-acids from forming a link with tetra-azotized benzidine. ^^^ 

 An important histochemical test for aldehydes provides another 



OH OH 



H— C— R H— C— R 



s=o s=o 



NH, NH NH 



NH2 



Schiff's reagent '^' ^^' +N^ 



2 



Schiff's reagent, in the presence of 

 hydrochloric acid, after reaction 

 with an aldehyde. ^^^'^^^ (R varies 

 according to the aldehyde used.) 



example of the localized synthesis of a dye in certain particular 

 tissue-constituents. Sections are treated with Schiff's reagent, 

 which is made by the action of sulphurous acid on basic fuchsine. 



