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