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 exhibit amphoteric behavior because they contain both acidic (carboxyl group, \(-COOH\)) and basic (amino group, \(-NH_2\)) functional groups in the same molecule. This dual nature allows them to act as both acids and bases depending on the pH of their environment.

Key reasons for amphoteric behavior:

1. Proton Donor (Acidic Property):
   The carboxyl group (\(-COOH\)) can donate a proton (\(H^+\)) in basic conditions, forming a carboxylate ion (\(-COO^-\)).
2. Proton Acceptor (Basic Property):
   The amino group (\(-NH_2\)) can accept a proton (\(H^+\)) in acidic conditions, forming an ammonium ion (\(-NH_3^+\)).
3. Zwitterion Formation:
   In neutral pH (near their isoelectric point), amino acids exist as zwitterions - molecules with both a positive (\(-NH_3^+\)) and negative (\(-COO^-\)) charge, making them electrically neutral overall.

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

Factors influencing amphoteric nature:

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

Biological significance:
This amphoteric property is crucial for:

• Maintaining pH buffering capacity in biological systems
• Protein structure and folding
• Enzyme function
• Transport of molecules across membranes