Question

Which of the following amino acids has four isomers?

• A. Glycine
• B. Isoleucine
• C. Leucine
• D. Methionine

Hint:

Amino Acids with 2 Chiral Centers (4 Stereoisomers): ISOLEUCINE and THREONINE. Both possess chiral $\alpha$ and $\beta$ carbons.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Amino acids with two chiral (asymmetric) carbon centers form $2^n = 2^2 = 4$ stereoisomers (two pairs of enantiomers: D/L-isomers and D/L-alloisomers).
Key Formula or Approach:
\[ \text{Number of Stereoisomers} = 2^n = 2^2 = 4 \quad (n = 2 \text{ chiral centers}) \]

Step 2: Detailed Explanation:
Analyzing chirality in standard amino acids:
1. Glycine: Achiral ($0$ chiral centers $\rightarrow 0$ optical isomers).
2. Leucine Methionine: $1$ chiral $\alpha$-carbon center $\rightarrow 2^1 = 2$ stereoisomers (D- and L-forms).
3. Isoleucine (and Threonine): Possesses two asymmetric chiral carbons (the $\alpha$-carbon $\text{C}_\alpha$ and the $\beta$-carbon $\text{C}_\beta$). Therefore, it exists as $2^2 = 4 stereoisomers$: L-isoleucine, D-isoleucine, L-alloisoleucine, and D-alloisoleucine.

Step 3: Final Answer:
Hence, Isoleucine has four isomers, matching option (B).