METACHROMASY 247 



do not confer the chromotropic property. Thus nitrocellulose is 

 acidic and therefore basiphil, but not a chromotrope. (For the 

 reaction to dyes of nitrocellulose in the form of collodion, see 

 p. 193.) 



COOH 



Skeleton formula of glucuronic, galacturonic, and mannuronic acids 



There is a strong correlation between degree of polymerization 

 and chromotropic effect. Thus glucuronic acids by themselves and 

 hyaluronic acid are scarcely chromotropic, while sodium alginates 

 (high polymers of mannuronic acid) and concentrated gels and 

 solid films of hyaluronic acid show a definite colour-shift.^^* 

 There appear to be two separate factors affecting the degree of 

 chromotropy achieved: the nature of the acid groups, and their 

 degree of separation in space. With any particular acid group, the 

 greatest colour-shift will be shown by a highly polymeric gel or film 

 crowded with sites of negative charge.*^* This crowding will natur- 

 ally result also in strong basiphilia towards orthochromatic dyes. 



It was shown by Lison ^^^ that various non-chromotropic high 

 polymers occurring in the tissues of organisms, such as glycogen, 

 starch, cellulose, gum arabic, and chitin, can be rendered strongly 

 chromotropic by their artificial conversion into sulphuric esters. 

 Sylven *^* has extended this work by observing the gradual in- 

 crease in degree of chromotropy as more and more carboxymethyl 

 groups are introduced into cellulose. 



Similar effects can be observed in microscopical preparations. 

 If a section be treated briefly with concentrated sulphuric acid, 

 any neutral polysaccharides will be transformed to sulphuric 

 esters and will therefore become basiphil and metachromatic."* 

 Glycogen and neutral mucopolysaccharides (such as the 'mucoid' 

 of the cells lining the mammalian stomach) give this reaction. The 

 same result may be achieved in a different way, by simply placing 

 a section in a solution of chromium trioxide.*^^' -^^^ Lison ^^^ con- 



siders that a ^C. group of the saccharide component is 



/ \h .0 



oxidized through aldehyde to — C. , presumably with 



breakage of the ring. 



