JEE MainChemistryBiomolecules
The carbohydrate moiety present in RNA is a cyclic monosaccharide. Which of the following structural descriptions corresponds to the Haworth projection of this naturally occurring sugar?
Options
- AA 5-membered cyclic hemiacetal where the C1 -OH is cis to the C4 -CH₂OH group, and C2 lacks a hydroxyl group.
- BA 5-membered cyclic hemiacetal where the C1 -OH is trans to the C4 -CH₂OH group, and C2 has a hydroxyl group.
- CA 6-membered cyclic hemiacetal where the C1 -OH is cis to the C5 -CH₂OH group, and C2 has a hydroxyl group.
- DA 5-membered cyclic hemiacetal where the C1 -OH is cis to the C4 -CH₂OH group, and C2 has a hydroxyl group.
Correct answer
D. A 5-membered cyclic hemiacetal where the C1 -OH is cis to the C4 -CH₂OH group, and C2 has a hydroxyl group.
Step-by-step solution
The sugar present in RNA is D-ribose, which exists in nucleic acids as a 5-membered furanose ring ( -D-ribofuranose). In its Haworth projection, being a -anomer means the anomeric hydroxyl group at C1 is on the same side (cis) as the terminal -CH₂OH group at C4 (both point UP in the standard orientation). Unlike the sugar in DNA (which is 2-deoxyribose and lacks an -OH at C2), the RNA sugar retains its hydroxyl group at the C2 position. Therefore, it is a 5-membered cyclic hemiacetal with the C1 -OH cis to the C4 -