246 DYEING 



the fact that the chromotropic objects seen in microscopical 

 preparations are basiphil. 



The acidic groups present in chromotropes are sulphuric, 

 phosphoric, and carboxyl. As Lison ^^^ showed, most of the 

 familiar, strikingly chromotropic objects owe their character to 

 the presence of sulphuric esters of polysaccharides of high 

 molecular weight. It will be recollected that a sulphuric ester 

 differs from a sulphonic acid by the possession of an extra oxygen 

 atom linking the organic radicle to the sulphur. Several such com- 



RO^ ^O R^ ^O 



Sulphuric ester Sulphonic acid 



pounds are mucosubstances. Chrondroitic acid, for instance, makes 

 the matrix of cartilage chromotropic ; mucoitic acid does the same 

 for certain mucous secretions ; heparin for the granules of Mast- 

 zellen. Chromotropic sulphuric esters need not, however, be muco- 

 substances. Agar, for instance, is the calcium salt of a sulphuric 

 ester of a pentose polysaccharide that lacks any amino-group. 



The phosphate group does not generally confer such a strongly 

 chromotropic character as the sulphuric. Adenosine triphosphate is 

 not chromotropic, RNA only very slightly so, DNA rather weakly 

 (see p. 258). Some of the metaphosphates, however, are rather 

 strongly chromotropic. These substances tend to polymerize. The 

 general formula for potassium metaphosphates is (KPOg),^. When 

 n is very large, the substance is strongly chromotropic; when 

 moderate, weakly ; sodium trimetaphosphate is not chromotropic at 

 all. A potassium metaphosphate of rather high molecular weight 

 can be extracted from the mould, Aspergillus niger, and this is 

 strongly metachromatic. It has been shown ^^^ that the meta- 

 chromatic particles present in yeast {Sac char omyces cerevisiae) and 

 certain bacteria contain a metaphosphate associated with protein. 

 There seems to be no doubt that this substance corresponds to the 

 Volutin' of earlier authors. 



The carboxyl group has less chromotropic effect than the phos- 

 phate. It occurs in uronic acids as a component of many muco- 

 substances. Some of these are sulphuric esters, and the sulphuric 

 component then overshadows the carboxylic in chromotropic 

 effect. When, as in hyaluronic acid, there is no acidic group other 

 than the carboxylic, the substance is only feebly chromotropic. 



Other acidic radicles than sulphuric, phosphoric, and carboxylic 



