Interactions of polarizable media in water and the hydrophobic interaction

Water plays an essential role in determining the interactions between biological molecules in solution. A major component of these interactions is the hydrophobic force, the water-mediated interaction deemed to provide an impetus for the organization of living matter. Hydrophobic forces can be traced back to the dynamic and structural changes undergone by water at biological surfaces. Since biological surfaces are quite heterogeneous with regard to their interactions with water, rapid variations of the water properties at length scales on the order of molecular dimensions occur at these surfaces. This behavior is expected to be significant for proteins, where experiments show that the protein polarizability, as measured by the dielectric permittivity, can range from 4 for hydrophobic regions to 10 for regions of catalytic importance. These results emphasize the "patchy" character of these biomolecules regarding polarizability, an effect that may be relevant for understanding the conformational changes of proteins and other biomolecules in solution. In this chapter, we provide a critical discussion of the current state of hydrophobic interactions, with particular emphasis on biomolecules. Most of the theoretical approximations to date do not address specifically polarization effects. Polarization is, nonetheless, expected to play a major role in determining the interactions of biological water with uncharged surfaces. We discuss recent work that suggests that polarization can impart dramatic changes in the water behavior at hydrophobic surfaces and also can modify the strength of the hydrophobic interaction.