Disaccharides are sugar pairs with unique structures and properties. Maltose, cellobiose, lactose, and sucrose each have distinct glycosidic bonds and compositions, affecting their reducing abilities and roles in nature.

Understanding these differences is key to grasping carbohydrate chemistry. From maltose in grains to lactose in milk, disaccharides showcase how small structural changes can lead to diverse functions in biological systems.

Disaccharide Structures and Properties

Maltose vs cellobiose structure

• Maltose and cellobiose are both disaccharides composed of two glucose units
  • Maltose is formed from two $\alpha$-D-glucose units connected by an $\alpha(1\rightarrow4)$ glycosidic bond (found in malt and germinating cereal grains)
  • Cellobiose is formed from two $\beta$-D-glucose units connected by a $\beta(1\rightarrow4)$ glycosidic bond (structural unit of cellulose)
• The glycosidic linkages between the glucose units differ in their stereochemistry and anomeric configuration
  • Maltose has an $\alpha(1\rightarrow4)$ glycosidic bond with both anomeric carbons in the $\alpha$ configuration
  • Cellobiose has a $\beta(1\rightarrow4)$ glycosidic bond with both anomeric carbons in the $\beta$ configuration
• Reducing properties are determined by the presence of a free hemiacetal group at the C1 position of the non-reducing end glucose unit
  • Maltose is a reducing sugar due to the free hemiacetal group on the non-reducing end glucose (can be oxidized by Fehling's or Benedict's reagent)
  • Cellobiose is also a reducing sugar for the same reason as maltose (can be detected by reducing sugar tests)
• The anomeric carbon of the reducing end glucose unit in both maltose and cellobiose can undergo mutarotation in solution

Composition of lactose

• Lactose is a disaccharide composed of galactose and glucose units connected by a $\beta(1\rightarrow4)$ glycosidic bond
  • The galactose unit is $\beta$-D-galactose (a C-4 epimer of glucose)
  • The glucose unit is $\beta$-D-glucose
• Comparison to other common disaccharides highlights similarities and differences in structure and composition
  • Like maltose and cellobiose, lactose has a $(1\rightarrow4)$ glycosidic bond connecting its monosaccharide units (common structural feature)
  • Lactose differs from maltose and cellobiose in its monosaccharide composition, as it contains galactose instead of two glucose units (unique sugar combination)
  • Lactose is a reducing sugar, similar to maltose and cellobiose, due to the free hemiacetal group at the C1 position of the glucose unit (can reduce copper(II) ions in Fehling's solution)

Properties of sucrose

• Sucrose is a non-reducing disaccharide composed of glucose and fructose units connected by an $\alpha(1\rightarrow2)\beta$ glycosidic bond
  • The glucose unit is $\alpha$-D-glucose (pyranose form)
  • The fructose unit is $\beta$-D-fructose (furanose form)
• The glycosidic bond in sucrose is unique among common disaccharides
  • Formed between the C1 of glucose and the C2 of fructose (only disaccharide with a $(1\rightarrow2)$ linkage)
  • Connects the anomeric carbons of both monosaccharides, resulting in a non-reducing disaccharide (no free hemiacetal group)
• Hydrolysis of sucrose yields equimolar amounts of glucose and fructose (monosaccharides)
  • The hydrolysis products are referred to as invert sugar due to the change in optical rotation (from dextrorotatory to levorotatory)
  • Invert sugar is sweeter than sucrose and is used in confectionery products (candy, syrups)
• Sucrose is a non-reducing sugar because it lacks a free hemiacetal group
  • The glycosidic bond formation between C1 of glucose and C2 of fructose eliminates the reducing ends of both monosaccharides (no open-chain form with free aldehyde or ketone group)
  • Sucrose does not react with Benedict's or Fehling's reagent (negative reducing sugar test)