Understanding peptide chemistry is fundamental to appreciating their biological functions and therapeutic potential.
The Peptide Bond
The peptide bond forms through dehydration synthesis between the carboxyl group of one amino acid and the amino group of another, releasing water.
Key Properties: - **Partial double-bond character** — Due to resonance, the C-N bond is intermediate between single and double - **Planar geometry** — Atoms in the peptide bond lie in a single plane - **Trans configuration** — Most peptide bonds adopt the thermodynamically favored trans form - **Restricted rotation** — The φ (phi) and ψ (psi) angles define backbone conformation
Peptide Directionality
- **N-terminus** — Free amino group (written on the left)
- **C-terminus** — Free carboxyl group (written on the right)
By convention, peptide sequences are written from N to C terminus.
Building Blocks: The 20 Amino Acids
Amino acids are classified by their side chains (R groups):
| Category | Examples | Properties |
|---|---|---|
| Hydrophobic | Leu, Ile, Val, Phe | Drive protein folding |
| Polar | Ser, Thr, Asn, Gln | Hydrogen bonding |
| Charged (+) | Lys, Arg, His | Ionic interactions |
| Charged (-) | Asp, Glu | Salt bridges |
| Special | Cys, Gly, Pro | Disulfides, flexibility, helix-breaking |
What Distinguishes Peptides from Proteins?
- **Peptides**: Typically < 50 amino acids; often lack stable 3D structure
- **Proteins**: > 50 amino acids; fold into stable tertiary structures
Functionally, proteins fold into stable shapes essential for enzymatic roles, while peptides remain flexible until they interact with their targets.