Study Notes on Salts, Noncovalent Interactions, and the Hydrophobic Effect

Dissolving Salts

• Dissolving NaCl increases entropy by transitioning from a crystal (ordered) to a liquid (disordered) state.

Solubility in Water

• Methane (CH$4$) has low solubility in water compared to glycerol, ethanol, fructose, and MgCl$2$.

Noncovalent Interactions

• Types:

  • Ionic (Coulombic) interactions: electrostatic interactions between charged species.

  • Hydrogen bonds: interactions between polar molecules.

  • van der Waals interactions: weak interactions, consist of attractive (dispersion) and repulsive (steric) components.

  • Hydrophobic Effect: ordering of water molecules around nonpolar substances.

• Noncovalent interactions do not involve sharing of electrons.

Strength of Noncovalent Interactions

• Relative Strength (kJ/mol) and Distance (nm):

  • Van der Waals: 0.4-4.0 kJ/mol, 0.4-0.7 nm

  • Hydrogen Bonds: 12-30 kJ/mol, 0.3 nm

  • Ionic Interactions: ~20 kJ/mol, <0.25 nm

  • Hydrophobic Interactions: varies

Biochemical Significance of van der Waals Interactions

• Weak and reversible yet universal.

• Important for steric complementarity and structural stability of macromolecules (e.g., DNA).

The Hydrophobic Effect

• Involves interactions of nonpolar molecules in aqueous solutions.

• Key in processes like protein folding and lipid micelle formation.

• Does not result from direct forces between nonpolar molecules.

Entropy Considerations

• Water surrounding nonpolar solutes exhibits lower entropy, causing low solubility of hydrophobic substances.

• Amphipathic lipids in water lower the overall entropy when dispersed.

• Aggregation of nonpolar portions leads to higher entropy as fewer water molecules are ordered.

Ligand Binding and the Hydrophobic Effect

• Enzymes often have hydrophobic binding sites for substrates and ligands.

• Removal of ordered water increases entropy, facilitating binding.

• Substrate binding involves shape complementarity after water displacement.