Question

Identify neutral amino acid from following list represented by three letter symbols.

• A. Arg
• B. Asp
• C. Leu
• D. His

Hint:

Memorize the extremes to save time!
Only 2 are Acidic: Aspartic acid (Asp) & Glutamic acid (Glu).
Only 3 are Basic: Histidine (His), Arginine (Arg), & Lysine (Lys).
Almost everything else on a standard list is Neutral!

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
We must classify standard $\alpha$-amino acids (given by their 3-letter abbreviations) into acidic, basic, or neutral categories based on the nature of their side chain (R-group).

Step 2: Detailed Explanation:
An amino acid's acidity or basicity is determined by the functional groups on its side chain:
Acidic amino acids: Side chain contains an extra carboxyl ($-\text{COOH}$) group.
Basic amino acids: Side chain contains an extra basic nitrogen (amine, guanidino, or imidazole) group.
Neutral amino acids: Side chain contains neither (it is alkyl, aromatic, or a neutral polar group like hydroxyl or amide).
Let's evaluate the options:
(a) Arg (Arginine): Contains a highly basic guanidino group in its side chain. It is a basic amino acid.
(b) Asp (Aspartic acid): The name itself gives it away. It contains an extra carboxyl group. It is an acidic amino acid.
(d) His (Histidine): Contains a basic imidazole ring in its side chain. It is a basic amino acid.
(c) Leu (Leucine): Its side chain is purely a non-polar, uncharged isobutyl alkyl group ($-\text{CH}_2\text{-CH}(\text{CH}_3)_2$). Because it lacks any extra acidic or basic functional groups, it is structurally balanced and classified as a neutral amino acid.

Step 3: Final Answer:
The neutral amino acid is Leu, matching option (c).