CARBOHYDRATES 65 



in the furanose form and the glucose portion in the pyranose form. 

 The structural formula is shown on page 64 where the numbers desig- 

 nate the positions linked \i;i the ox)gen connecting the two mono- 

 saccharide fragments. 



Sucrose cannot show mutarotation since there is no semiacetal struc- 

 ture. Hence sucrose exists in but one form and is readily crystallized. 

 An aqueous solution at 20°C. rotates the plane of polarization clock- 

 wise. When the sucrose has been hydrolyzed, the direction of optical 

 rotation is reversed. This change arises when the strong counterclock- 

 wise (levo) rotation for D-fructose is added algebraically to the rela- 

 tively smaller dextro rotation of an equivalent solution of D-glucose. 

 Hydrolysis of sucrose is effectively catalyzed by acids and by enzymes 

 called invertases, named in this case from the inversion in optical rota- 

 tion. The hydrolysis mixture of D-ghicose and o-fructose is called in- 

 vert sugar for the same reason. 



Maltose 



This disaccharide occurs in small amounts in cell sap and to a 

 greater degree in germinating and malted grains. In these cases the 

 sugar is formed by partial hydrolysis of starch occurring in the pres- 

 ence of another group of enzymes called amylases. Maltose is a re- 

 ducing sugar displaying reducing power equal to approximately half 

 its weight of glucose. Presumably, then, maltose has one acetal and 

 one semiacetal or free carbonyl group. Hydrolysis of maltose yields 

 only glucose, the molecular ratio being one maltose to two glucose. 

 Chemical studies of the linkage involved in coupling the two glucoses 

 reveal the following structure for a-maltose: 



H OH H OH 



1 -a-D-glucosido-4-a-D-ghicose 



With a free semiacetal structure, maltose shows the mutarotation char- 

 acteristic of most reducing sugars and exists in solution in two equilib- 

 rium forms with perhaps a trace of intermediate having only the left- 

 hand ring closed, the other opening and closing reversibly to yield 

 the a and /? forms. 



