Disaccharide Structures

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 Name	Molecular Formula	Reducing Sugar	Hydrolysis Products
Melibiose	C₁₂H₂₂O₁₁	Yes	D-Glucose & D-Galactose
Gentiobiose	C₁₂H₂₂O₁₁	Yes	D-Glucose only
Trehalose	C₁₂H₂₂O₁₁	No	D-Glucose only
Primeverose	C₁₁H₂₀O₁₀	Yes	D-Glucose & D-Xylose
Sucrose	C₁₂H₂₂O₁₁	No	D-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 Name	Cleavage Enzyme	O-CH₃ Groups formed by Permethylation	Hydrolysis Products following Permethylation
Melibiose	Maltase α-glycosidase	8	2,3,4,6-tetra-O-methyl-D-galactose & 2,3,4-tri-O-methyl-D-glucose
Gentiobiose	Emulsin β-glycosidase	8	2,3,4,6-tetra-O-methyl-D-glucose & 2,3,4-tri-O-methyl-D-glucose
Trehalose	Maltase α-glycosidase	8	2,3,4,6-tetra-O-methyl-D-glucose only
Primeverose	Emulsin β-glycosidase	7	2,3,4-tri-O-methyl-D-xylose & 2,3,4-tri-O-methyl-D-glucose
Sucrose	Invertase β-fructosidase	8	1,3,4,6-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?