CONSTITUTION 171 



This latter formula for glucose, showing the asymmetry 

 of the terminal carbon atom, marked with an asterisk, explains 

 the possibility of the existence of two optically isomeric forms 

 of glucose,* and also accounts for the ability of glucose to re- 

 act with methyl alcohol to form two isomeric a- and /3-methyl 

 ghicosides f according to the equation : — • 



in 



CH^OH CHOH CH CHOH CHOH CHOH + CH3OH 



lu ' 



= CH„OH CHOH CH CHOH CHOH CH OCH., + H^O 



A number of analogous compounds have since been pre- 

 pared by Fischer and his co-workers from mannose, galactose, 

 and fructose, the resulting compounds being termed manno- 

 sides, galactosides, and fructosides respectively.^ 



As the result of studying the action of the two enzymes, 

 maltase and emulsin, upon other glucosides, Fischer divides 

 these substances into two classes known as a-glucosides and 

 yS-glucosides, of which maltase can only split the a-glucosides 

 and emulsin the yS-glucosides. Amygdalin is, therefore, an a- 

 yS-glucoside, e.g. of its two glucose molecules, one is the a- and 

 the other is the y8-modification. 



Since most natural glucosides yield on hydrolysis a sub- 

 stance containing a hydroxyl group, it seems reasonable to 

 assume that the original glucoside was formed by a reaction 

 similar to the one given for the artificial glucosides mentioned 

 above, that is by the elimination of water between a hydroxyl 

 group of the sugar and one from the other compound. On 

 this assumption the constitution of some of the better known 

 natural glucosides could be represented as follo\/s : — 



*Tanret : " Compt. rend.," 1895, 120, 1060; cf. also Fischer: " Ber. deut. 

 chem. Gesells.," 1893, 26, 2400; Lowry : "J. Chem, Soc , Lond.," 1S99, 7Si 

 213 ; and Perkin : " J. Chem. Soc, Lond.," 1902, 81, 177. 



t The specific nature of enzymic hydrolysis is exhibited in the case of these 

 two artificially synthesized o- and j8-inethyl glucosides which Fischer prepared 

 by the action of hydrochloric acid on a solution of glucose in methyl alcohol. 

 The a-glucoside, which is dextro-rotatory, is hydrolysed by maltase, but not by 

 emulsin, while the j8-glucoside, on the contrary, is unaffected by maltase, but is 

 hydrolysed by emulsin (see also section on Enzymes, p. 353). 



:J: Fischer and Fischer: "Ber. deut. chem. Gesells.," 1910, 43, 2521. 



