METACHROMASY 251 



When a strongly chromotropic substance is added to the solu- 

 tion, a new trough (y trough) appears, still further in the direction 

 of shorter wave-lengths. The addition of quite a small amount of 

 an intensely chromotropic substance, such as the heparin of 

 Mastzellen, suffices to hollow out a considerable y trough at the 

 expense of a and ^ (fig. 27, b). With toluidine blue the wave- 

 length of the y trough is about 550 m/x (yellowish green). The 

 colour of the solution is purple, since light is freely transmitted 

 from both ends of the spectrum. 



Varying degrees of colour-shift will be produced by variations 

 in the tendency of chromotropes to flatten out the a transmission 

 trough and deepen the p and y. Most chromotropes produce their 

 main visible effect by lowering the y trough, though they lower the 

 P at the same time. RNA, however, a feebly chromotropic sub- 

 stance, makes a low p trough at certain concentrations, without 

 affecting the y region. ^*^ 



Some metachromatic dyes show quite a low trough in the 

 transmission curve in the region of the ultra-violet, but this is not 

 affected by the presence of chromotropes. ^^^ 



We naturally ask ourselves whether there are any features of chem- 

 ical composition that separate metachromatic from orthochromatic 

 dyes. Certain general remarks may be made under this head. 



Dyes in which all the -NH2 auxochrome groups have the 

 hydrogen atoms replaced by -CH3 or -C2H5 are not meta- 

 chromatic. This applies, for instance, to crystal violet and methyl- 

 ene blue. These dyes are often regarded as somewhat meta- 

 chromatic, but there is reason to believe that they would be quite 

 orthochromatic if perfectly pure.^°^ The former is generally 

 contaminated with the highly metachromatic methyl violet, and 

 methylene blue always gives rise by allochromasy to the azures, so 

 that perfectly pure specimens have not been obtained. 



Not all dyes that have an unsubstituted -NHg group are meta- 

 chromatic. It is a remarkable fact that the dyes that are meta- 

 chromatic are, in general, those that are capable of being trans- 



+ 

 formed to imino-bases. The change from an =NH2 group to an 



imino (^NH) group does not involve a loss of quinonoid structure 

 in the ion. The imino-bases are therefore coloured, unlike the 

 leucobases. The chemical structure of the imino-base of pararos- 

 aniline, for instance, may be compared with that of the leuco- 

 base (p. 163). The transformation from dye to imino-base involves 



