Question

Amino acids show amphoteric behaviour, why?

Hint:

Amino acids can exist in different ionic forms (zwitterions) depending on the pH, where the amino group is protonated (\( NH_3^+ \)) and the carboxyl group is deprotonated (\( COO^- \)).

Answer 1

Amino acids are amphoteric due to the presence of both acidic carboxyl (\(-COOH\)) and basic amino (\(-NH_2\)) groups within the same molecule. This duality allows them to act as either acids or bases based on the surrounding pH.

Key factors for amphoteric behavior:

1. Acidic nature (proton donation):
   The carboxyl group (\(-COOH\)) can release a proton (\(H^+\)) under alkaline conditions, forming a carboxylate ion (\(-COO^-\)).
2. Basic nature (proton acceptance):
   The amino group (\(-NH_2\)) can accept a proton (\(H^+\)) under acidic conditions, forming an ammonium ion (\(-NH_3^+\)).
3. Zwitterion formation:
   At neutral pH (close to their isoelectric point), amino acids exist as zwitterions, molecules bearing both positive (\(-NH_3^+\)) and negative (\(-COO^-\)) charges, resulting in an overall neutral charge.

Glycine example:
\[ H_2N-CH_2-COOH \rightleftharpoons ^+H_3N-CH_2-COO^- \]

Influencing factors:

• Solution pH
• pKa values of the amino and carboxyl groups
• Presence of additional acidic or basic groups in side chains (for non-standard amino acids)

Biological importance:
This amphoteric property is vital for:

• Maintaining pH buffering in biological systems
• Protein structure and folding
• Enzyme activity
• Transport across membranes