Opening image: the KcsA K⁺ channel. Four identical protein subunits surround two potassium ions in the "selectivity pore."

Introduction

In previous tutorials we considered only interactions between uncharged atoms and nonpolar molecules. These van der Waals interactions are short-range and diminish with distance r as 1/r⁶. Thus, van der Waals attractions are felt only when two atoms are so close to each that they nearly contact. Charge interactions are very different; they are long-range and diminish with distance as 1/r (Coulomb's law). Proteins and nucleic acids carry numerous ionizable groups. Hence charge-charge interactions, both intra- and intermolecular, are very important to stability and function of these macromolecules. Next are the weaker -- but directional -- ion-dipole interactions. If the ions and dipoles are mobile, then the dipole stabilizes the ion by aligning itself with the strong electrostatic field of the ion. A good example is the hydration of ions in aqueous solutions, a topic we'll consider in some detail. Finally, we'll explore the fascinating topic of K⁺ diffusion through an ion channel and learn how inner shell exchange reactions are decisive to selectivity of biological ion channels.

Charge-charge Interactions in Ionic Compounds

Gases, most common liquids, and many solids consist of molecules in which the atoms are held together by covalent bonds. However, not all substances are made up of molecules. For example, sodium chloride consists of sodium ions and chloride ions so arranged that each sodium ion is surrounded by six equidistant chloride ions and each chloride ion is surrounded by six equidistant sodium ions. Ionic bonds hold the salt crystals together. Hence, no distinct group is identifiable as a molecule of sodium chloride.

NaCl crystal. The centers of electron densities lie on regularly spaced lattice points with a mean Na–Cl center-to-center distance of 2.81 Å. The ions vibrate about these mean positions. The ionic radii are 0.95 Å for Na⁺ and 1.81 Å for Cl^-.

The charge-charge interactions in the crystal are so strong that NaCl melts at 850°C. However, when a crystal of NaCl is placed in water, water molecules are so strongly attracted to the charge centers that the salt dissociates into free ionic particles in solution. The underlying interactions are called hydration.