2. Natural proteins are composed primarily of 20 α-amino acids. Which one of the following statements
in true for any of these amino acids in a solution of pH 1.5?
(a) Only the amino group is ionized.
(b) Only the carboxylic acid group is ionized.
(c) Both amino and carboxylic acid group are ionized.
(d) Both amino and carboxylic acid groups are neutral.

Amino acids, the building blocks of proteins, exhibit distinct ionization states based on solution pH due to their pKa values. At pH 1.5, only the amino group ionizes for standard α-amino acids, making option (a) correct.​​

Ionization Basics

Standard α-amino acids feature a carboxylic acid group (pKa ≈ 2.0-2.4) and an amino group (pKa ≈ 9.0-10.0). In highly acidic conditions like pH 1.5, the low pH protonates both groups: -COOH remains neutral (protonated), while -NH3+ forms as the ionized amino group.​​

Correct Answer: Option (a)

Only the amino group is ionized (-NH3+). The carboxylic group stays protonated (-COOH) because pH 1.5 is below its pKa, preventing deprotonation to -COO-.​​

Option Analysis

• (a) Only the amino group is ionized: Accurate, as pH < pKa_carboxyl keeps -COOH neutral, but amino group protonates fully.​​

• (b) Only the carboxylic acid group is ionized: Incorrect; ionization means -COO-, which requires pH > pKa (~2.2).​

• (c) Both amino and carboxylic acid groups are ionized: Wrong; carboxylic stays -COOH at pH 1.5.​

• (d) Both amino and carboxylic acid groups are neutral: False; amino protonates to -NH3+.​