Determining the Structure of Disaccharides

Some properties and characteristics of five disaccharides are listed in the following table. The entry in the "Reducing Sugar" column refers to whether the compound is oxidized by Tollens', Benedict's or Fehling's reagent. Sugars that are oxidized are called reducing sugars, since the cationic silver or copper oxidant is reduced in the reaction. The hydrolysis referred to in the last column is accomplished by aqueous acid or by enzymes (see below).

Compound NameMolecular FormulaReducing SugarHydrolysis Products
MelibioseC12H22O11YesD-Glucose &
D-Galactose
GentiobioseC12H22O11YesD-Glucose
only
TrehaloseC12H22O11NoD-Glucose
only
PrimeveroseC11H20O10YesD-Glucose &
D-Xylose
SucroseC12H22O11NoD-Glucose &
D-Fructose

To establish the structure of these compounds by chemical means, two kinds of reactions are conducted. First, enzymatic cleavage is used to determine the configuration of the glycosidic linkage (α or β). Second, the disaccharide is permethylated (exhaustively methylated) by reaction with excess methyl iodide & silver oxide. This converts every free hydroxyl group in the molecule into a methyl ether. The permethylation is then followed by acid-catalyzed hydrolysis to mixtures of methylated monosaccharides having known structures. From the results of these experiments we can deduce the structure/configuration of the initial disaccharide.

Compound NameCleavage EnzymeO-CH3 Groups formed
by Permethylation
Hydrolysis Products
following Permethylation
MelibioseMaltase
α-glycosidase
82,3,4,6-tetra-O-methyl-D-galactose
& 2,3,4-tri-O-methyl-D-glucose
GentiobioseEmulsin
β-glycosidase
82,3,4,6-tetra-O-methyl-D-glucose
& 2,3,4-tri-O-methyl-D-glucose
TrehaloseMaltase
α-glycosidase
82,3,4,6-tetra-O-methyl-D-glucose
only
PrimeveroseEmulsin
β-glycosidase
72,3,4-tri-O-methyl-D-xylose
& 2,3,4-tri-O-methyl-D-glucose
SucroseInvertase
β-fructosidase
81,3,4,6-tetra-O-methyl-D-fructose
& 2,3,4,6-tetra-O-methyl-D-glucose

The following questions may be answered using the first letter of each compound's name as its designator: Melibiose, Gentiobiose, Trehalose, Primeverose & Sucrose. If none of the five disaccharides satisfy the specified condition, enter X for an answer.

1. Which compounds can be reduced by NaBH4?
2. In your answer to (1), which of the reduced units are glucose?
3. Which compounds have a 1,6'-glycoside link?
4. Which compounds have a 1,4'-glycoside link?
5. Which compounds have a 1,2'-glycoside link?
6. Which compounds have a 1,1'-glycoside link?






This script written by William Reusch, Dept. of Chemistry, Michigan State University. Please send comments and corrections to whreusch@pilot.msu.edu.