THE CLASSIFICATION OF DYES l8l 



discussed elsewhere (p. 243). It is their metachromatic property 

 that makes these dyes so important in the staining of blood- 

 corpuscles (p. 268). 



Methylene blue is a valuable vital dye (p. 287). 



Azines 



The structural formulae for all the diverse dyes that have been 

 mentioned so far can be written down so as to include a para- 

 quinonoid ring, and this has been done. In some dyes, however, 

 there may in fact be resonance between the paraquinonoid and 

 orthoquinonoid configurations, and for the azine dyes it becomes 



1+ 



Skeleton-formula for azines 



necessary to write orthoquinonoid formulae. In this group an 

 imino-nitrogen once more forms one of the links between two 

 rings, but, in contrast to the oxazines and thiazines, the second 

 link is also formed by a nitrogen atom. The skeleton-formula shown 

 here is satisfactory for most of the azine dyes. Most of the ones 



H,cl ' ^1- 



Safranine 



used in microtechnique are rather complex. The safranines have 

 an aryl ring attached to the charged nitrogen atom. Safranine O, a 

 useful red dye for chromatin, consists of the dye shown here mixed 

 with another dye diflfering only in the absence of the methyl 

 group from the attached aryl ring. 



Two other basic azines, neutral red and Janus green B, are of 

 outstanding importance as vital dyes. The former, a dull red, is an 

 exceptionally innocuous dye that colours certain kinds of globules 

 in the cytoplasm. It turns yellow on the alkaline side of neutrality, 

 and one can thus learn something of the acidity or alkalinity of 

 cytoplasmic inclusions by using it. Janus green B, a very complex 

 dye that contains also an azo chromophore, has been much used 

 in the study of mitochondria (p. 292). 



