40 



CARBOHYDRATES 



both being in the oxide ring form. The process may be illustrated by 

 the combination of glucose with methyl alcohol: 



CHjOH 



CH2OH 



+ CH3OH 



OH 



HO 



H OH 



a-D-Glucose 



+ H2O 



OCH3 



H OH 



Methyl a-D-Glucoside 



As a class such substances are termed glycosides, but individual members 

 are named from the component parts, as indicated in the above example. 

 Both a- and ^-glycosides may be formed from the corresponding a- and 

 y8-sugars. 



The glycosides do not show the simple sugars' characteristic proper- 

 ties of reducing power or osazone formation because the aldehyde or 

 ketone group is covered up. Glycosides are rather stable to alkalies 

 but are hydrolyzed by acids to form the original components. 



If the second molecule with which a monosaccharide combines happens 

 to be that of another monosaccharide, the product is a disaccharide. 

 A disaccharide may therefore be defined as a glycoside formed from two 

 simple sugar molecules by removing one molecule of water.^ The second 

 simple sugar may be either the same kind or a different kind from the 

 first. For example, two molecules of glucose may combine as follows: 



CH2OH 



CH2OH 



HO 



H OH 



a-D-Glucose 



H OH 



a-D-Glucose 



CHjOH CH2OH 



a XT Tx J^-— 0. 



+ n,o 



H OH H OH 



4-D-Glucosyl-o;-D-glucose 



The product in this case is an a-D-glucoside with the second glucose unit 

 attached through its carbon 4; it is therefore called 4-D-glucosyl^a- 



^ This statement is intended to be a definition only. Disaccharides probably are 

 not actually produced in living cells by removing water from simple sugars (see 

 p. 399). 



