8 BIOLOGICAL CHEMISTRY 



Molisch Reaction. By adding a few drops of an alcoholic 

 solution of a-naphthol to a solution of carbohydrate, and by 

 floating the mixture on strong sulphuric acid a purple red 

 ring is produced at the junction of the two liquids.* This 

 test, unlike some of the previous tests, which depend upon 

 some special molecular arrangement, is given by all carbo- 

 hydrates. 



POL YS ACCH ARIDE S 



The union of two or more hexose groups usually leads to the 

 masking of the aldehyde and ketone groups. Under these 

 circumstances many of the carbohydrate tests fail until the 

 compound has been hydrolysed by acid. The substances of 

 high molecular weight, containing a number of monosaccharicle 

 groups can be distinguished by certain colours given by them 

 with iodine. 



Starch, the most frequently occurring carbohydrate in 

 plants, gives a blue colour ; glycogen, a similar substance from 

 animals, gives a mahogany brown colour ; and erythro-dextrin 

 gives a reddish brown colour. Anothtr dextrin (achroo-dextrin) 

 gives no colour with iodine. Cellulose, which is an insoluble 

 substance, sometimes gives a faint blue colour with iodine, 

 but this is greatly intensified by the addition of concentrated 

 sulphuric acid. These substances can be distinguished by 

 their relative solubilities. Starch and glycogen are colloids 

 (see p. 50), whilst the dextrins are more like true solutions. 



Returning to the structural formulae it can be shown that 

 when one carbon atom has four different groups united to it 

 a three-space model can be arranged in two different ways. 

 The projection of these models on the plane of the paper 

 gives rise to the two following diagrams : 



2 C 4 4 C 2 



I I 



3 3 



In these illustrations it is seen that the one compound is 

 related to the other, as the image of the one is to its reflection, 

 or as the right to the left hand of a pair of gloves. This 

 relation corresponds with the power of rotating the plane of 

 polarised light, one to the right (clockwise) and the other to 

 the left (anti-clockwise), and is spoken of as asymmetry. 



* H. Molisch, Sitzungsberichte d. k. k. Akad. in Wien, 1886, Part II, 

 vol. 93, p. 912. 



