30 Biological Stains 



pound assume a quinoid structure, as for example the following 

 grouping : 



H 



H3N_/\_N_/\^NH 

 II LI 



/ \ 

 \ X 



3. The indamin group, — N = , as observed in the indamins, 

 thiazins, and so forth. Methylene blue is the best known repre- 

 sentative of this group. In these dyes, two benzene rings are at- 

 tached to the nitrogen atom, one of these being in the quinoid form 

 and hence adding a second chromophore. The typical indamin 

 formula is: 



HN^/~\^N_/"~\_NH 



In the thiazins, such as methylene blue, the two benzene rings are 

 further joined together by a sulfur atom, forming three closed 

 rings of atoms. The simplest thiazin base would be: 



. S _/\_NH 



ACID CHROMOPHORES 



1. The nitro group, ■ — NO2, as in picric acid 



2. The quinoid benzene ring. 



fl 



which occurs in a long series of dyes, such as the indamins above 

 mentioned, the xanthenes and the di- and tri-phenyl methanes, 

 which include many well known stains, such as rosolic acid, 

 fuchsin, methyl green and the methyl violets. A typical triphenyl 

 methane formula is that of pararosanilin base: 



H2N 



C_/~"\_NH 

 H2N 



