Q.Lysine, $\text{H}_2\text{N}(\text{CH}_2)_4\text{CH}(\text{NH}_2)\text{COOH}$, is _______. (Note: one or more of the following options may be correct.)

The key idea is to classify lysine based on its structure and properties.

1. $\alpha$-Amino acid? The amino group ($-\text{NH}_2$) and the carboxyl group ($-\text{COOH}$) are attached to the same carbon (the $\alpha$-carbon). The side chain is $(\text{CH}_2)_4\text{NH}_2$. This makes it an $\alpha$-amino acid, not a $\beta$-amino acid. So (A) is correct, (D) is wrong.
2. Basic amino acid? The side chain contains an additional $-\text{NH}_2$ group, making the molecule basic (it has two amino groups). So (B) is correct. …

Lysine is one of the 20 standard amino acids, and its structure tells you almost everything you need to know. The formula given is $\text{H}_2\text{N}(\text{CH}_2)_4\text{CH}(\text{NH}_2)\text{COOH}$. Let’s break down what each part means and why the options fall the way they do.

The key idea: an $\alpha$-amino acid has the amino group ($-\text{NH}_2$) attached to the carbon atom next to the carboxyl group ($-\text{COOH}$). That carbon is called the $\alpha$-carbon. In lysine, the central carbon in $\text{CH}(\text{NH}_2)\text{COOH}$ is that $\alpha$-carbon — it carries both the carboxyl and one amino group. So it’s an $\alpha$-amino acid, not a $\beta$-amino acid (where the amino group would be on the second carbon away).

Now, why is it basic? Because there’s a second amino group at the end of the side chain: $(\text{CH}_2)_4\text{NH}_2$. That extra $-\text{NH}_2$ can accept a proton, making the overall molecule basic. In fact, at physiological pH, lysine carries a net positive charge — it’s one of the three basic amino acids (along with arginine and histidine).

What about synthesis? The human body cannot make lysine. It is an essential amino acid, meaning it must come from the diet. So option (C) is false.

Let’s go step by step.

1. Identify the $\alpha$-carbon. The general structure of an $\alpha$-amino acid is $\text{R}-\text{CH}(\text{NH}_2)\text{COOH}$. In lysine, the $\text{CH}(\text{NH}_2)\text{COOH}$ part matches exactly — the amino group is on the carbon adjacent to the carboxyl. So it is an $\alpha$-amino acid. …

Concept: Classification of Amino Acids

Amino acids are classified based on:

• Position of amino group relative to carboxyl group ($\alpha$, $\beta$, $\gamma$, etc.)
• Nature of side chain (acidic, basic, neutral)
• Biological essentiality (essential vs non-essential)

Method: Structural Analysis & Classification

Step 1: Identify the amino group position

The given structure is:

• The carbon directly attached to the $-\text{COOH}$ group is the $\alpha$-carbon.
• On this $\alpha$-carbon, there is one $-\text{NH}_2$ group.
• Therefore, it is an $\alpha$-amino acid.

✓ Option (A) is correct.

✗ Option (D) is wrong (that would require the $-\text{NH}_2$ on the $\beta$-carbon).

Step 2: Check the side chain nature

• The side chain is $-\text{(CH}_2)_4 - \text{NH}_2$, which contains an additional amino group. …

Here are the common mistakes students make on this question, along with how to avoid each.

Mistake 1: Confusing $\alpha$-Amino vs $\beta$-Amino Acids

The Mistake:

Students see the long carbon chain $(\text{CH}_2)_4$ and assume the amino group is far from the carboxyl group, incorrectly selecting β-amino acid (Option D) or rejecting α-amino acid (Option A).

Why it happens:

They focus on the total chain length instead of the position of the first amino group relative to the carboxyl carbon.

How to Avoid:

• Always locate the carboxyl carbon first (the carbon of $-\text{COOH}$).
• The α-carbon is the carbon immediately next to the carboxyl carbon.
• In lysine, the structure is: $\text{H}_2\text{N} - \text{CH}_2 - \text{CH}_2 - \text{CH}_2 - \text{CH}_2 - \text{CH}(\text{NH}_2) - \text{COOH}$ The carbon attached to $\text{NH}_2$ and $\text{COOH}$ is the α-carbon.
• Rule: If the amino group is on the α-carbon, it is an α-amino acid. The second amino group on the side chain does not change this classification.

Correct choice: (A) is correct. (D) is wrong.

Mistake 2: Misclassifying Lysine as Neutral or Acidic

The Mistake:

Students see two amino groups and one carboxyl group but forget the basicity of the side chain, marking lysine as neutral or acidic.

Why it happens:

They only count the groups without considering the pKa values or the net charge at physiological pH.

How to Avoid:

• Amino acids are classified by the nature of their R-group (side chain).
• Lysine’s side chain is $(\text{CH}_2)_4\text{NH}_2$. This is a basic group (it can accept a proton).
• At physiological pH (~7.4), the side chain amino group is protonated ($-\text{NH}_3^+$), giving lysine a net positive charge.
• Memory aid: Basic amino acids have extra nitrogen atoms in their side chains (Lysine, Arginine, Histidine).

Correct choice: (B) is correct.

Mistake 3: Assuming All Amino Acids Are Essential

The Mistake: …