Amino acids are classified based on the nature of their side chains (R groups), because the side chain decides how each amino acid behaves in water and inside proteins.
Here’s a explanation:
1. Non-polar (Hydrophobic) Amino Acids
- Their side chains do not mix with water.
- They are usually found inside proteins (away from water).
- Examples: Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Tryptophan, Proline.
2. Polar (Uncharged) Amino Acids
- Their side chains can form hydrogen bonds with water, but they are not charged.
- They help make proteins soluble in water.
- Examples: Serine, Threonine, Asparagine, Glutamine, Tyrosine, Cysteine.
3. Acidic Amino Acids
- Their side chains have a negative charge at normal body pH.
- They can donate protons (H⁺).
- Examples: Aspartic acid, Glutamic acid.
4. Basic Amino Acids
- Their side chains have a positive charge at normal pH.
- They can accept protons (H⁺).
- Examples: Lysine, Arginine, Histidine.
In short:
- Non-polar: water-hating
- Polar: water-loving (no charge)
- Acidic: negatively charged
- Basic: positively charged
These side chain differences decide each amino acid’s role and position in a protein.