Questions: Monosaccharide Isomerism and Properties

Question 1

Glucose and galactose have the same molecular formula (C₆H₁₂O₆) and differ only at carbon-4. What type of isomers are they?

• A. Enantiomers — they are non-superimposable mirror images of each other
• B. Constitutional isomers — they have different functional group types
• C. Epimers — they are diastereomers differing at exactly one chiral center
• D. Anomers — they differ in configuration at the anomeric carbon

Answer: (C)

Epimers are a specific subtype of diastereomers that differ at exactly one chiral center. Glucose and galactose differ only at C4 — their carbon skeletons are otherwise identical, with the same aldehyde functional group and the same arrangement of all other hydroxyl groups. This makes them C4 epimers. They are NOT enantiomers (mirror images would require flipping all chiral centers simultaneously). They are NOT constitutional isomers (those require different connectivity — like glucose vs. fructose). Anomers are a different pairing: α-glucose and β-glucose, differing at the anomeric C1.

Question 2

Humans can digest starch but not cellulose, even though both are polymers of glucose. The fundamental reason is:

• A. Cellulose contains a different monosaccharide unit (galactose) that human amylase cannot recognize
• B. Cellulose has α-1,4 glycosidic bonds while starch has β-1,4 bonds, and human enzymes only cleave α bonds
• C. Starch has α-1,4 glycosidic bonds while cellulose has β-1,4 bonds, and human enzymes only cleave α bonds
• D. Cellulose is insoluble in water, preventing digestive enzymes from accessing it

Answer: (C)

Both starch and cellulose are made entirely of glucose — the monomer is identical. The difference is the anomeric configuration at C1: starch uses α-1,4 glycosidic linkages (from α-glucose), producing helical chains, while cellulose uses β-1,4 linkages (from β-glucose), producing rigid linear fibers that hydrogen-bond into crystalline sheets. Human amylase and intestinal glucosidases are shaped to cleave the α-configuration linkage; they cannot accommodate the β-configuration. Ruminants and termites can digest cellulose because their gut microbiota produce β-glucosidases (cellulases) that humans lack. This distinction — starch vs. cellulose from a single hydroxyl orientation — is the canonical demonstration that stereochemical detail has massive biological consequences.

Question 3

α-glucose and β-glucose are both ring forms of glucose with the same molecular formula and the same connectivity — they differ only in the orientation of the hydroxyl group at C1.

• T. True
• F. False

Answer: True

α-glucose and β-glucose are anomers — stereoisomers that differ only at the anomeric carbon (C1 in aldoses). In the α form, the C1 hydroxyl is axial (pointing down in a Haworth projection of D-glucose); in the β form, it is equatorial (pointing up). They have identical molecular formulas, identical connectivity, and differ at exactly one position. In aqueous solution they interconvert through the ring-opening/ring-closing process of mutarotation, reaching an equilibrium of approximately 36% α and 64% β. This tiny structural difference determines polymer properties: α-1,4 links give starch, β-1,4 links give cellulose.

Question 4

Glucose and fructose are enantiomers — non-superimposable mirror images of each other.

• T. True
• F. False

Answer: False

Glucose and fructose are constitutional isomers, not enantiomers. They have the same molecular formula (C₆H₁₂O₆) but different connectivity: glucose is an aldohexose with a carbonyl at C1 (aldehyde), while fructose is a ketohexose with a carbonyl at C2 (ketone). Enantiomers must have the same connectivity with all chiral centers inverted — the mirror image of D-glucose is L-glucose, not fructose. Because glucose and fructose have different functional group types, they are processed by different enzymes and have different chemical reactivities, which is the hallmark of constitutional isomers.

Question 5

What is mutarotation, and why does it occur in monosaccharide solutions?

Model answer: Mutarotation is the spontaneous interconversion between α and β anomeric forms of a cyclic monosaccharide in aqueous solution, proceeding through the open-chain form. It occurs because the ring can open and reclose, and each reclosure can produce either the α or β configuration at the anomeric carbon. The process continues until thermodynamic equilibrium is reached (for glucose: ~36% α, ~64% β). It can be observed as a change in optical rotation over time.

Mutarotation demonstrates that sugars in solution exist as a mixture of anomeric forms, not a single structure. This has biochemical relevance: enzymes that use glucose as a substrate are often anomer-specific, and the equilibrium ratio is set by the relative stabilities of the two ring conformations. The phenomenon also explains why freshly dissolved crystalline α-glucose (pure α form) shows a different optical rotation than the equilibrium mixture — a measurable change that historically was used to study sugar chemistry.