Question

Given below are two statements:

Statement I:

Amino acids have a property of ionizable nature of -NH2 and -COOH groups, hence have different structures at different pH.

Statement II:

Amino acids can exist as Zwitterionic form at acidic and basic pH.

• A. Statement I is true and Statement II is true.
• B. Statement I is true and Statement II is false.
• C. Statement I is false and Statement II is true.
• D. Statement I is false and Statement II is false.

Answer 1

Answer: (B)

Statement I is true and Statement II is false.

Answer 2

Amino acids possess both an amino group (-NH2) and a carboxyl group (-COOH) attached to a central alpha-carbon. Both these groups are ionizable:

• The carboxyl group can lose a proton (-COOH $\to$ -COO⁻).
• The amino group can gain a proton (-NH2 $\to$ -NH3⁺).

The ionization state of these groups is highly pH-dependent.

Statement I Analysis: Due to the ionizable nature of the -NH2 and -COOH groups, amino acids exhibit different structural forms at different pH values. At acidic pH, they exist in cationic form ($+NH_3-CHR-COOH$). At the isoelectric point (pI), they exist as zwitterions ($+NH_3-CHR-COO^-$). At basic pH, they exist in anionic form ($NH_2-CHR-COO^-$). Thus, Statement I is true.

Statement II Analysis: The zwitterionic form is the predominant species only at the isoelectric point (pI), where the net charge on the amino acid is zero. At acidic pH, the amino acid carries a net positive charge (cationic form), and at basic pH, it carries a net negative charge (anionic form). Therefore, amino acids do not predominantly exist as zwitterions at acidic and basic pH. Thus, Statement II is false.