Hydration of Polar Molecules and Ions

Water molecules are able to hydrogen bond not only to each other but also to polar groups of dissolved compounds. Thus, every group that is capable of forming a hydrogen bond to another organic group is also able to form hydrogen bonds of a somewhat similar strength with water. For this reason, hydrogen bonding is usually not a significant force in holding small molecules together in aqueous solutions. Polar molecules that stick together through hydrogen bonding when dissolved in a nonpolar solvent often do not associate in water. How then can biochemists assert that hydrogen bonding is so important in biochemistry? Part of the answer is that proteins and nucleic acids can be either properly folded with hydrogen bonds
formed internally or denatured with hydrogen bonds from those same groups to water. The Gibbs energy change between these two states is small.
Every ion in an aqueous solution is surrounded by a shell of oriented water molecules held by the attraction of the water dipoles to the charged ion. The hydration of ions has a strong influence on all aspects of electrostatic interactions and plays a dominant role in determining such matters as the strength of acids and bases, the Gibbs energy of hydrolysis of ATP, and the strength of bonding of metal ions to negatively charged groups. For example, the previously considered interaction between carboxylate and calcium ions would be much weaker if both ions retained their hydration shells.
Consider the following example. ΔG° for dissociation of acetic acid in water is +27.2 kJ/mol. The enthalpy change ΔH° for this process is almost zero (–0.1 kJ/mol) and ΔS° is consequently –91.6 J K–1. This large entropy decrease reflects the increased amount of water that is immobilized in the hydration spheres of the H+ and acetate– ions formed in the dissociation reaction. In contrast, dissociation of NH4 + to NH3 and H+ converts one positive ion to another. ΔH° is large (+52.5 kJ/mol) but the entropy change ΔS° is small.
Although effects of hydration are important in almost all biochemical equilibria, they are difficult to assess quantitatively. It is hard to know how many molecules of water are freed or immobilized in a given reaction. Charged groups in proteins are often hydrated. However, if they are buried in the interior of the protein, they may be solvated by polarizable protein side chain groups such as –OH or by backbone or side chain amide groups.