104 



Biological Stains 



photometric characteristics of the thiazins (Knapp. Emery and 

 Stotz, 1950). From this information the absorption curves given 

 in Figs. 11 and 12 have been derived for seven of the dyes in the 

 group. Reference to these curves will be given under the indi- 

 vidual dyes as discussed below. 



J5 



THIONIN 



Synonym: LautKs violet. 



C. I. NO. 910 



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 

 VIII). One type is known as the orthoquinoid, the other as the 

 paraquinoid. 



It is supposed (see p. 25) that when the quinoid ring is formed 

 the two hydrogen atoms replaced by atoms or groups v/ith 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: 



HoN 



.NH2 



OH 



"N^ 



paraquinoid fornmla 



OH 



I 

 H2N_/\_ S =/\_^NH2 



orthoquinoid formula' 



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

 monium base of the type discussed on p. 27, which is capable of 

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

 orthoquinoid formula the salt formation takes place through 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 pre- 

 ferred. Possibly both forms actually exist simultaneously. For 



