CHAPTER 2 (2)

• Dissolving salts


  Breaking ionic interactions
• High dielectric constant


  Reduces attraction between oppositely charged ions in salt crystal; almost no attraction at large distances (> 40nm)
• Solvated ions and water molecules


  Strong electrostatic interactions lower the total energy of the system
• Dissolving salt crystal


  Entropy increases
• Van der Waals interactions

  Have 2 components: attractive force (London dispersion) and repulsive force (Steric repulsion)
• Van der Waals interactions

  • Attraction dominates at longer distances (typically 0.4-0.7 nm)
  • Repulsion dominates at very short distances
  • There is a minimum energy distance (van der Waals contact distance)
• Biochemical significance of van der Waals interactions


  • Weak individually → easily broke, reversible
  • Universal → occur between any 2 atoms that are near each other
  • Determines steric complementarity
  • Stabilises biological macromolecules (stacking in DNA)
  • Facilitates binding of polarisable ligands
• Hydrophobic effect


  • Association/interaction of nonpolar molecules or components of molecules in the aqueous solution
  • One of the main factors behind: protein folding, protein-protein association, formation of lipid micelles, binding of steroid hormones to their receptors
  • Does not arise because of some attractive direct force between 2 nonpolar molecules
• Origin of the hydrophobic effect

  1. Lipid molecules disperse in the solution; nonpolar tails of lipid molecules are surrounded by ordered water molecules

  2. Entropy of the system decreases

  3. Nonpolar portions of the amphipathic molecule aggregate so that fewer water molecules are ordered and entropy increases

  4. All nonpolar groups are sequestered from water, and the released water molecules increase the entropy further

  5. Only polar "head groups" are exposed

  6. With high enough concentration of amphipathic molecules, complete aggregation into micelles is possible
• Hydrophobic effect favours ligand binding


  • Binding sites in enzymes and receptors are often hydrophobic
  • Such sites can bind hydrophobic substrates and ligands, such as steroid hormones, which displace water and increase entropy of the system
  • Many drugs are designed to take advantage of the hydrophobic effect
• Colligative properties


  • Boiling point, melting point, and osmolarity
  • Do not depend on the nature of the solute, just the concentration
• Non-colligative properties


  • Viscosity, surface tension, taste, and colour
  • Depend on the chemical nature of the solute
• Cytoplasm of cells are highly concentrated solutions and have high osmotic pressure due to dissolved solutes
• Osmotic pressure


  • Water moves from areas of high-water concentration (low solute concentration) to areas of low water concentration (high solute concentration)
  • Osmotic pressure (π) is the force necessary to resist the movement
  • Osmotic pressure is influences by the concentration of each solute in solution
  • Dissociated components of a solute individually influence the osmotic pressure
• Effect of osmotic pressure on cells