Question

Which of the following amino acid is basic in nature?

• A. Alanine
• B. Lysine
• C. Threonine
• D. Methionine

Hint:

Remember the three basic amino acids: Histidine, Arginine, and Lysine (mnemonic: HAL). Arginine is the most basic, due to its guanidinium group.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Amino acids are classified based on the chemical properties of their side chain (R-group). Basic amino acids have side chains containing nitrogenous groups that can accept a proton (H⁺) at physiological pH, thereby acting as bases and carrying a positive charge.
Step 2: Detailed Explanation:
Let's analyze the side chains of the given amino acids:

(A) Alanine: Has a simple methyl group (-CH₃) as its side chain. It is nonpolar and neutral.

(B) Lysine: Its side chain contains a terminal amino group (-NH₂) at the end of a four-carbon chain (- (CH₂)₄-NH₂). This amino group readily accepts a proton to become -NH₃⁺, making lysine a basic amino acid.

(C) Threonine: Has a hydroxyl group (-OH) in its side chain. It is polar but uncharged (neutral).

(D) Methionine: Contains a sulfur atom (thioether) in its side chain. It is nonpolar and neutral.

Step 3: Final Answer:
Among the given options, only lysine has a side chain with an amino group that imparts basic properties to the molecule.