For a hapten like PCP (phencyclidine), which interaction dominates tight antibody binding?

Introduction / Context:
Hapten–antibody interactions reveal how small, nonpolar molecules are recognized. PCP (phencyclidine) is largely hydrophobic, making it a classic example to illustrate dominant binding forces.

Given Data / Assumptions:

• PCP is predominantly nonpolar and aromatic/alicyclic.
• Antibody combining sites can create hydrophobic pockets.
• Multiple noncovalent forces contribute to affinity.

Concept / Approach:
For nonpolar haptens, hydrophobic interactions drive binding by displacing ordered water and fitting the hapten into a complementary nonpolar pocket, yielding favorable entropy and enthalpy. Electrostatic contacts are limited when the ligand lacks strong charges. Van der Waals forces contribute but usually in combination with hydrophobicity rather than alone.

Step-by-Step Solution:
Characterize PCP as hydrophobic.Map antibody pocket: nonpolar residues line the paratope.Conclude hydrophobic forces dominate tight binding.

Verification / Alternative check:
Solvent isotope effects and pocket mutagenesis often show decreased affinity when hydrophobic contacts are disrupted, supporting the dominant role of hydrophobic interactions.

Why Other Options Are Wrong:

• Electrostatic forces: Less relevant for largely uncharged ligands.
• Van der Waals only: Present but not typically the principal driver alone.
• Covalent bonding: Antigen–antibody binding is noncovalent under physiological conditions.

Common Pitfalls:
Assuming a single force explains all binding; in reality, multiple weak forces sum, with hydrophobic effects often leading for nonpolar haptens.

Final Answer:
Hydrophobic forces.