GATE 2025 CY – Question 17

Organic Chemistry · Biomolecules: Structure, properties and reactions of mono- and di-saccharides, physicochemical properties of amino acids, chemical synthesis of peptides, chemical structure determination of peptides and proteins, structural features of proteins, nucleic acids, lipids, steroids, terpenoids, carotenoids, and alkaloids. · 1 mark · Multiple choice

A disaccharide X does NOT show mutarotation in aqueous solution. Acidic hydrolysis of X affords two different monosaccharides. The disaccharide X is

Four disaccharide structures with glycosidic linkages.

Option A in the figure
Option B in the figure
Option C in the figure
Option D in the figure

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Correct answer: (D) Option D in the figure

Explanation

**Mutarotation** is the slow change in optical rotation of a sugar solution, caused by the ring opening and closing of a **free hemiacetal (anomeric) group**. A sugar that has no free hemiacetal group cannot open to the aldehyde or ketone form, so it does not mutarotate.

A disaccharide with a **glycosidic bond between the two anomeric carbons** has no free hemiacetal group. It is a **non-reducing sugar** and does not show mutarotation.
Hydrolysis of that disaccharide gives two **different** monosaccharides.

**Sucrose** fits both clues: the glycosidic bond joins the anomeric carbon of glucose (C-1) to the anomeric carbon of fructose (C-2), so there is no free hemiacetal, and acid hydrolysis gives **glucose and fructose**. Lactose, maltose and cellobiose have a free anomeric centre, so they mutarotate. The answer is the sucrose structure (option D).