Question

Arrange the following amino acids on the basis of increasing side chain size
(A) Alanine
(B) Glycine
(C) Isoleucine
(D) Leucine
Choose the correct answer from the options given below :

• A. (A), (B), (C), (D)
• B. (B), (D), (A), (C)
• C. (B), (A), (D), (C)
• D. (A), (D), (C), (B)

Hint:

Remember that beta-branched amino acids like Isoleucine and Valine have bulkier side chains close to the main chain than their isomers.
This steric hindrance restricts their flexibility in protein secondary structures.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Amino acids have a common backbone but differ in their side chains (R-groups).
The size, bulk, and steric hindrance of an amino acid depend on the molecular structure, branching, and number of carbon atoms in its side chain.

Step 3: Detailed Explanation:
Let us analyze the side-chain structures of the given amino acids:
1. (B) Glycine:
The side chain of Glycine is a single hydrogen atom ($-\text{H}$).
This is the smallest and simplest side chain among all amino acids, providing minimal steric hindrance.
2. (A) Alanine:
The side chain of Alanine is a single methyl group ($-\text{CH}_3$).
This is slightly larger than Glycine's hydrogen atom.
3. (D) Leucine:
The side chain of Leucine is an isobutyl group ($-\text{CH}_2-\text{CH}(\text{CH}_3)_2$).
It contains four carbon atoms with branching at the gamma-carbon.
4. (C) Isoleucine:
Isoleucine is a structural isomer of Leucine with a sec-butyl side chain ($-\text{CH}(\text{CH}_3)-\text{CH}_2-\text{CH}_3$).
It contains four carbon atoms with branching at the beta-carbon.
Because the methyl branch is closer to the peptide backbone (at the beta-carbon), the side chain of Isoleucine is bulkier and exerts greater steric hindrance than Leucine.
Therefore, the increasing order of side chain size is: (B) Glycine $\rightarrow$ (A) Alanine $\rightarrow$ (D) Leucine $\rightarrow$ (C) Isoleucine.

Step 4: Final Answer:
The correct increasing sequence is (B), (A), (D), (C).
This matches Option (C).