Amino Acids: Structure and Classification
General amino acid structure:
-CHR-COOH at physiological pH → zwitterion: ^{+}-CHR-C
SMILES for L-alanine (simplest chiral amino acid): N[C@@H](C)C(=O)O
SMILES for glycine (only achiral): NCC(=O)O
Zwitterion formation at isoelectric point (pI):
At pH < pI: cationic form (^{+}-CHR-COOH) At pH = pI: zwitterion (^{+}-CHR-C) — no net charge At pH > pI: anionic form (-CHR-C)
Classification of amino acids by R group:
| Category | Examples | R group character |
|---|---|---|
| Non-polar aliphatic | Glycine, Alanine, Valine, Leucine | Alkyl chains, hydrophobic |
| Aromatic | Phenylalanine, Tryptophan, Tyrosine | Aromatic rings |
| Polar uncharged | Serine, Threonine, Cysteine | -OH, -SH groups |
| Positively charged (basic) | Lysine, Arginine, Histidine | -, guanidinium, imidazole |
| Negatively charged (acidic) | Aspartate, Glutamate | -COOH in R group |
Essential amino acids (cannot be synthesised by humans):
PVT TIM HALL: Phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Arginine (semi), Leucine, Lysine
Special amino acids for NEET:
- Glycine: only achiral amino acid (no chiral centre)
- Cysteine: contains -SH (thiol) → forms disulfide bonds in proteins
- Proline: cyclic structure, disrupts alpha-helices, imino acid (tertiary N)
- Tryptophan: precursor of niacin (B3); amino acid with highest molecular weight among common ones