SECTION III. 



GLUCOSIDES. 



The glucosides are compounds of some complexity which on 

 decomposition yield glucose together with one or more other 

 substances, usually of an aromatic nature ; they are, therefore, 

 often described as ether-like compounds of carbohydrates 

 with aromatic compounds. The carbohydrate in the majority 

 of cases is glucose, but occasionally it may be an isomeric 

 hexose, such as galactose or even a pentose such as rham- 

 nose. Thus, for example, digitonin, one of the glucosides of 

 Digitalis purpurea, yields on hydrolysis both glucose and 

 galactose, while hesperidin and quercitrin, the glucosides con- 

 tained respectively in the unripe orange and in the bark of 

 Quercus tinctoria, give rhamnose. 



The reaction by means of which glucosides are split up 

 into their constituent parts is in almost all cases one of hydro- 

 lysis,* and can, therefore, usually be brought about by boiling 

 with dilute mineral acids or, in some cases, alkalis. In 

 nature, however, the decomposition is effected by means ot 

 suitable ferments which often exist in the same part, although 

 generally in different cells to the glucoside. 



Each glucoside may have an enzyme appropriate to itself, 

 but any one particular ferment may have the power of splitting 

 several glucosides. 



Thus, for example, the glucoside amygdalin is hydrolysed 

 by its appropriate enzyme emulsin to glucose, benzaldehyde 

 and hydrocyanic acid, according to the equation : — 



CaoHo^NO,, + 2H.p = 2C«Hi20u + C^HjCHO + HCN 



*The decomposition of potassium myronate into potassium hydrogen 

 sulphate, glucose and allyl mustard oil can hardly be described as hydrolysis 

 (see p. i88). 



169 



