8. Hydrophobic interactions originate from
a. Coulombic forces
b. Entropic forces
c. Gravitational forces
d. None of the above

Introduction

Hydrophobic interactions originate from entropic forces and drive key biological processes like protein folding and membrane formation. These interactions explain why nonpolar molecules cluster in aqueous environments, a fundamental concept in biochemistry and cell biology for CSIR NET exams.​

Core Concept of Hydrophobic Interactions

Hydrophobic interactions arise when nonpolar molecules or regions disrupt water’s hydrogen-bonded network, increasing water molecule order around them. This creates a structured “cage” of water, reducing system entropy; clustering hydrophobes releases these water molecules, boosting entropy and stabilizing the system. Thus, the driving force is purely entropic, with minimal enthalpic contribution at physiological temperatures.​

Why Option (b) is Correct

Entropic forces dominate hydrophobic interactions as the release of ordered water molecules into bulk solvent increases overall disorder (ΔS > 0), making ΔG negative per ΔG = ΔH – TΔS. Experimental calorimetry confirms positive ΔH but large positive ΔS compensates, especially for larger hydrophobes where surface area effects amplify entropy gain. This entropic origin underpins membrane lipid bilayer self-assembly and protein tertiary structure in aqueous cells.​

Why Option (a) is Incorrect

Coulombic forces involve electrostatic attractions between charged species, like ion-ion or ion-dipole interactions, but hydrophobic interactions occur between uncharged, nonpolar groups without direct charge involvement. While water polarization near hydrophobes can induce weak long-range electrostatic effects, the primary driver remains entropic, not Coulombic.​

Why Option (c) is Incorrect

Gravitational forces act on mass at macroscopic scales and are negligible at molecular levels (10^-20 N for typical biomolecules), incapable of driving nanoscale clustering like hydrophobic effects. No biophysical model links gravity to hydrophobicity; it contradicts thermodynamic principles governing solvation.​

Why Option (d) is Incorrect

Hydrophobic interactions definitively originate from identifiable physical forces, specifically entropic contributions from water reorganization, as validated by decades of thermodynamic studies. Dismissing them as “none” ignores their central role in biomolecular stability.​

Exam Tips for CSIR NET

• Focus on thermodynamic signatures: hydrophobic effects show weak temperature dependence unlike enthalpic bonds.​

• Compare with van der Waals (dispersive, short-range) or H-bonds (enthalpic).​

• Correct choice: (b) – entropic forces power ~50-70% of protein folding free energy.​