Partition of Proteins Between Water and Non-Polar Phases

Abstract

The partition of small molecules between water and a non-polar phase follows simple and well defined rules. The most important macroscopic characteristic that determines partition between these phases is their difference in dielectric constant. The high dielectric constant of water favors the separation of charges with formation of independent ions of opposite charge but in media of much lower dielectric constant, separation of charges cannot take place and in consequence the ions present in the water cannot be transferred to the second phase. The high dielectric constant of water results from the permanent dipole moment of the water molecules and the high dipole density. These properties determine the very different molecular interactions prevalent in water and in a typical non-polar medium. On the water side we have the strong attractive forces between permanent dipoles and in a lipid phase the much weaker dispersion forces that result from the attractive effects of mutually induced dipoles. Partition is determined by the work necessary to transfer a given molecule from one of the phases to the other. We can consider it as the result of two successive operations: transfer from one phase to the vacuum, followed by transfer from the vacuum to the second phase. In the first operation it will be necessary to break the bonds between the solute and the molecules of the solvent phase.