Short Answer

Amino acids behave as zwitterions because they contain both an acidic carboxyl group (–COOH) and a basic amino group (–NH₂).

• In water, the carboxyl group loses a proton forming –COO⁻, while the amino group gains a proton forming –NH₃⁺.
• This makes amino acids electrically neutral overall but internally charged, allowing them to act as both acids and bases depending on the surrounding pH.

Detailed Explanation :

Zwitterionic Nature of Amino Acids

Amino acids are the building blocks of proteins and have a general structure:

• R group represents the side chain.
• Amino acids possess both a carboxyl group (acidic) and an amino group (basic).

In aqueous solutions, they exist as zwitterions, meaning the molecule contains both positive and negative charges but has no net charge.

1. Formation of Zwitterions

• When an amino acid is dissolved in water:

• Mechanism:
  1. The carboxyl group (–COOH) loses a proton → –COO⁻ (negative charge).
  2. The amino group (–NH₂) gains a proton → –NH₃⁺ (positive charge).
• As a result, the amino acid carries both charges internally and remains electrically neutral overall.
• This form is most prevalent at the isoelectric point (pI), where the net charge of the amino acid is zero.

2. pH-Dependent Behavior

1. Acidic pH (pH < pI)
   • Excess H⁺ in solution protonates the carboxyl group, so the amino acid exists mainly in cationic form (H₃N⁺–CH(R)–COOH).
2. Neutral pH (pH ≈ pI)
   • Amino acid exists predominantly as a zwitterion, carrying both –NH₃⁺ and –COO⁻ groups.
3. Basic pH (pH > pI)
   • OH⁻ deprotonates the amino group, forming anionic form (H₂N–CH(R)–COO⁻).

• This amphoteric property allows amino acids to act as buffers, resisting changes in pH.

3. Examples of Zwitterionic Amino Acids

• Glycine (simplest amino acid): H₃N⁺–CH₂–COO⁻
• Alanine: H₃N⁺–CH(CH₃)–COO⁻
• Serine: H₃N⁺–CH(CH₂OH)–COO⁻

All amino acids form zwitterions in aqueous solution under physiological conditions.

4. Importance of Zwitterionic Behavior

1. Biological Significance
   • Zwitterionic form allows amino acids to remain soluble in water.
   • Facilitates electrostatic interactions in protein structures.
   • Essential for enzyme activity, transport, and metabolic processes.
2. Buffering Capacity
   • Amino acids can resist pH changes, important in blood and cellular fluids.
3. Protein Structure
   • Internal charges help in forming hydrogen bonds and ionic interactions, stabilizing secondary and tertiary protein structures.

5. Summary

• Amino acids behave as zwitterions due to proton transfer between carboxyl and amino groups.
• Charge distribution: –NH₃⁺ (positive) and –COO⁻ (negative).
• Predominant form at isoelectric point.
• This behavior is crucial for solubility, buffering, protein structure, and biological activity.

Conclusion

Amino acids exhibit zwitterionic behavior, meaning they carry both positive and negative charges internally while remaining neutral overall. This occurs because the amino group accepts a proton and the carboxyl group donates a proton in aqueous solution. The zwitterionic nature of amino acids is vital for protein formation, enzymatic reactions, buffering capacity, and biological solubility, making it a fundamental concept in chemistry and biochemistry.