CARBOHYDRATES 



41 



D-glucose. This particular disaccharidc is maltose. The "disaccharide 

 hnkage" connecting the two simple sugar units in this case is an a-type 

 and goes from carbon 1 of one unit to 4 of the other. This is often 

 abbreviated to an "a-l,4-Unkage." 



Any alteration in the nature of the disaccharidc linkage, or in the 

 component sugars, results in a different disaccharidc. For example, two 

 glucose units combined by a /?-l,4 linkage form the disaccharidc, cello- 

 biose, which is a substance distinctly different from maltose. The chem- 

 ical make-up of the more common disaccharides is shown in Table 3-3. 



Common name and 

 component sugars 



Sucrose : 



D-glucose 



D-fructose 

 Lactose: 



D-glucose 



D-galactose 

 Melibiose : 



D-glucose 



D-galactose 

 Maltose: 



D-glucose 



D-glucose 

 i.so-Maltose : 



D-glucose 



D-glucose 

 Cellobiose: 



D-glucose 



D-glucose 

 Gentiobiose: 



D-glucose 



D-glucose 

 Trehalose : 



D-glucose 



D-glucose 



* For simplicity, 

 from these names, 

 structural formulas 



Table 3-3 



Chemical constitution of disaccharides 



Disacchaiide 

 linkage 



a,P-l,2 



P-1,4 



a-1,6 



a-1,4 



a-1,6 



P-1,4 



P-1,6 



a, a-1, 1 



Chem.ical 

 name* 



1-a-D-glucosyl-P-D- 

 fructose 



4-D-glucosyl-(3-D- 

 galactose 



6-D-glucosyl-a-D- 

 galactose 



4-D-glucosyl-a-D- 

 glucose 



6-D-glucosyl-a-D- 

 glucose 



4-D-glucosyl-P-D- 

 glucose 



6-D-glucosyl-P-D- 

 glucose 



1-a-D-glucosyl-a-D- 

 glucose 



the designation of furanose and pyranose rings has been omitted 

 All are pyranose except for the fructose unit in sucrose (see 

 for the individual disaccharides). 



The disaccharides may or may not have reducing properties, depending 

 on whether the disaccharide linkage involves the aldehyde (or ketone) 

 group of only one of the component simple sugars, or of both. In the 

 latter case since no free, or potentially free, aldehyde or ketone group 

 remains in the disaccharide, it therefore gives no osazone and does not 

 respond to the Fehling's or other similar tests. The structures of sucrose 

 and trehalose are of this nonreducing type. 



