Introduction

Amino acids can exist in different ionic forms depending on the pH of the solution.
Their net charge is governed by the pKa values of ionizable groups:

• α-carboxyl group (–COOH)
• α-amino group (–NH₃⁺)
• Side chain (R group), if ionizable

In this article, we solve a conceptual MCQ and explain why only one option is correct.
This topic is important for CSIR-NET, GATE, IIT-JAM, and MSc entrance exams.

Question Overview

An amino acid has one proton-donating group in the side chain (R).
The pKa values are:

• pK_COOH = 2.19
• pK_NH2 = 9.67
• pK_R = 4.25

Which of the following statements is CORRECT?

Understanding Ionization of Each Group

1. α-Carboxyl Group (pK = 2.19)

At pH values greater than 2.19, the carboxyl group is deprotonated.

Charge: –1

2. Side Chain R Group (pK = 4.25)

The side chain is acidic and loses a proton when pH exceeds 4.25.

Charge: –1 (when deprotonated)

3. α-Amino Group (pK = 9.67)

Below pH 9.67, the amino group remains protonated.

Charge: +1

Correct Answer

Option (A): Majority of the molecules will have a net charge of –1 at pH 7.0

Why Option (A) is Correct

At pH = 7.0:

• Carboxyl group → –1
• R group → –1
• Amino group → +1

Net Charge Calculation:

(–1) + (–1) + (+1) = –1

Hence, the majority of molecules possess a net charge of –1.

Explanation of All Options

Option (A)

Majority of the molecules will have a net charge of –1 at pH 7.0.

Status: ✅ Correct

Option (B)

Majority of the molecules will have a net charge of 0 at pH 4.25.

At pH equal to pK_R, the R group is only 50% deprotonated,
resulting in a mixture of species rather than a dominant neutral form.

Status: ❌ Incorrect

Option (C)

All molecules will have a deprotonated R group at pH 3.22.

Since pH is less than pK_R, the R group remains mostly protonated.

Status: ❌ Incorrect

Option (D)

During titration with a strong base, deprotonation starts with the R group.

Deprotonation always begins with the group having the lowest pKa value,
which is the carboxyl group (pK = 2.19).

Status: ❌ Incorrect

Key Concept to Remember

Lower pKa means stronger acid and earlier deprotonation.
Net charge depends on the ionization state of all functional groups.