Structural Differences Between Peptides and Proteins

The peptide-protein boundary is fundamentally about structural stability rather than simply chain length.

The Hydrophobic Effect

• Nonpolar amino acids bury themselves away from water
• This releases ordered water molecules, increasing entropy
• The net effect drives formation of a compact **hydrophobic core**

Why Peptides Don't Fold Stably Short peptides have high **surface-to-volume ratios**: - Cannot bury sufficient hydrophobic residues - No critical mass for hydrophobic collapse - Remain as flexible ensembles in solution

Proteins, being longer, can create dense nonpolar interiors that lock them into specific shapes.

Secondary Structure Elements

Both peptides and proteins can form secondary structures, but with different dynamics:

α-Helices - 3.6 residues per turn - Stabilized by i→i+4 hydrogen bonds - In peptides: often transient or **induced upon binding** - In proteins: stable elements of tertiary structure

β-Sheets - Extended chains connected by hydrogen bonds - Parallel or antiparallel arrangements - Require multiple strand interactions for stability

3₁₀-Helices - Tighter helix (3 residues per turn) - More common in short peptides than α-helices - Often found at helix termini

Peptide Secondary Structures

Dynamic Ensembles Peptides in solution fluctuate between conformations: - **Polyproline II (PPII)** — Extended, left-handed helix; common in "random coil" - **β-turns** — Tight reversals important for recognition - **Amphipathic helices** — Form upon membrane contact (e.g., AMPs)

Induced Folding Many bioactive peptides are **intrinsically disordered** until they: - Bind their target receptor - Contact lipid membranes - Interact with metal ions

This "induced fit" provides high specificity with low affinity.

Engineering Peptide Structure

Therapeutic peptide development often focuses on stabilizing transient structures: