THIONIN C. I. NO. 920 



Synonym: Lauth's violet. 

 (A basic dye; absorption maximum about 602.)* 



Thionin, having two amino groups, is a strongly basic dye. The 

 exact structural formulae of this dye and its derivatives, as well as 

 many others in which two benzene rings are similarly joined, are in 

 some dispute. At least two types of formulae are possible for the 

 thiazins and oxazins, as well as for the xanthene dyes (Chapter 

 VII). One t>pe is known as the orthoquinoid, the other as the 

 paraquinoid. 



It will be recalled (see p. 13) that when the quinoid ring is formed 

 the two hydrogen atoms replaced by atoms or groups with double 

 valency bonds may be either in the para or in the ortho position to 

 each other. It will also be recalled from elementary chemistry 

 that sulfur and oxygen may be either bivalent or tetravalent. These 

 facts make it possible for a thiazin or an oxazin to have either one 

 or the other of the different structures represented by the following 

 two formulae for the theoretical thionin base: 



OH 



paraquinoid formula 

 OH 



H.X 



NH. 



"N= 



orthoquinoid formula 

 In the case of the paraquinoid formula the compound is an am- 

 monium base of the type discussed on p. 15, which is capable of 

 salt formation thru its pentavalent nitrogen. In the case of the 

 orthoquinoid formula the salt formation takes place thru the 

 tetravalent sulfur, the base being of the type known as a sulfonium 

 base. There are arguments in favor of either formula, and from 

 the standpoint of the biologist it does not matter which is preferred. 

 Possibly both forms actually exist simultaneously. For the sake of 

 uniformity the paraquinoid form will be sho^-n in the following 

 pages wherever possible; but with the understanding that the 

 orthoquinoid form is equally permissable. 



The dye, thionin, is a salt, generally a chloride, of the above 

 mentioned base; and on the assumption of paraquinoid structure, 

 it has the following formula : 



H.X /\ S _/\=XH, 



I 

 CI 



"See Fig. 5. p. 30. 



