CONSTITUTION 



>4i 



terminal carbon atom accounts for the ability of glucose to 

 react with methyl alcohol to form two isomeric a- and j8- 

 methyl glucosides * according to the equation— 



r 



-o- 



CH2OH CH (CHOH)3 CHOH + CH3OH 



I 



-0 



= CH2OH CH(CHOH)3 CHOCH3 + HjO 



The a-glucoside, which is dextro-rotatory, is hydrolysed 

 by maltase, but not by emulsin, while the j3-glucoside, on 

 the contrary, is unaffected by maltase, but is hydrolysed by 

 emulsin, t there result on hydrolysis the two isomeric a- 

 and jS-glucoses, whose constitutions are represented by the 

 following formulae :— 



H . C . OH 



I 

 H . C . OH 



I 

 HO . C . H O 



HC .OH 



I 

 HC 



HO . C . H 

 Hi 



OH 



HOC— H O 

 H.i 



OH 



HC 



CH2OH 



a-Glucose a^ = + 110° 



CH2OH 

 j3-Glucose CTjj = + 19° 



The fact that either of these sugars tends to change at 

 once into the ordinary form of glucose, the so-called equilibrium 

 mixture having a^ = 52-9°, may be employed as a means 

 for determining the nature of a given glucoside since the 

 rotation of a freshly hydrolysed a-glucoside solution will tend 

 to decrease, while that of a j3-glucoside will increase. 



As the result of the study of the action of maltase and 

 emulsin upon other glucosides, Fischer divided these sub- 

 stances into two classes known as a-glucosides and ^-glucosides, 

 according as they are hydrolysed by maltase or emulsin re- 

 spectively. Other examples of a-glucosidases besides maltase 



* A number of analogous compounds have since been prepared by 

 Fischer and his co-workers from mannose, galactose, and fructose, the result- 

 ing compounds being termed mannosides, galactosides, and fructosides 

 respectively. 



f See also section on Enzymes. 



16 



