8 Water Interactions

Water and Ions

The attraction between oppositely charged ions in an ionic compound is strong. However, because of the polarity of water, when many ionic compounds are in aqueous solutions they become dissociated into ions. For instance, when an ionic compound such as table salt (NaCl) is dissolved in water, it separates into Na+ and Cl ions. The water molecules surround the ions to form polar interactions such that the positive ends of the water molecules are arranged around negative ions and the negative ends of the water molecules surround positive ions. Thus the ions become encapsulated by water spheres, which are called spheres of hydration. The biological world is very ionic, and spheres of hydration are important in a cell because they maintain the separation of the many ions of the cell from each other. The sphere of hydration must be broken in order for binding to take place with a specific binding partner.

Hydrophilic Interaction

The nature of polar molecules is that they contain highly electronegative atoms. Consequently, many are capable of hydrogen bonding with aqueous or polar solvents. Because polar molecules are generally water soluble, they are referred to as being hydrophilic, or water-loving. The one-carbon alcohol, methanol, is an example of a polar molecule.

Hydrophobic Interaction

The final type of interaction occurs between neutral hydrophobic, or water-fearing, molecules. These nonpolar molecules do not interact with water and are characterized by atoms with the same or nearly the same electronegativities. In aqueous solutions, the hydrophobic molecules are driven together to the exclusion of water. For example, shaking a bottle of oil and vinegar (acetic acid in water), such as in a salad dressing, results in the oil being dispersed as tiny droplets in the vinegar. As the mixture settles, the oil collects in larger and larger drops until it only exists as a layer, or phase, above the vinegar.

A similar effect occurs in biological systems. As a protein folds to its final three-dimensional structure, the hydrophobic parts of the protein are forced together and away from the aqueous environment of the cell. Similarly, biological membranes are stabilized by the exclusion of water between layers of lipids as we will see later.

Amphipathic molecules are molecules that have a distinct nonpolar, or hydrophobic, region, and a distinct polar region. These molecules do not form true solutions in water. Rather, the nonpolar parts are forced together into a nonpolar aggregate, leaving the polar part of the molecule to interact with the aqueous phase. Detergents and long-chain carboxylic acids are examples of amphipathic molecules.


Energy Associated with Bonds

Each of the bond types represents a measurable amount of energy. To break a bond, the equivalent amount of energy must be expended. In metabolism, bonds are broken in molecules, such as glucose, to “release” the energy. The cell utilizes this energy to drive other energy-consuming reactions. The covalent bond has the most energy associated with it, on average approximately 100 kilocalories/mole (kcal/mol). The noncovalent bonds, ionic and hydrogen, and hydrophobic interactions, have approximately five kcal/mol associated with each of them.

It should be noted here that throughout the presentation of this course approximations will be used for certain values so that estimations can be made as we move to more complex systems. It is to be acknowledged that very precise values for each of the measurements are not available.

Thus the noncovalent bonds that have been introduced have approximately 20 times less energy associated with them and, thus, are more easily broken individually. However, hydrogen bonds generally form extensive networks, and the total energy associated with the network is the sum of the individual interactions (van der Waals force). As anyone who has done a “belly flop” into a swimming pool knows, breaking a large surface area of water is extremely difficult (and painful).

Table 1: Energy associated with the different bonds
Bond Energy, kcal/mol
Covalent 100
Ionic 5
Hydrogen 5
Hydrophobic interactions 5
van der Waals 5 (depends on surface area)

When NaCl dissolves in water, each ion becomes surrounded by at least 20 water molecules. As NaCl there is 5 kcal/mol of energy associated with the ionic attraction of the cation and anion, but when a Na ion is surrounded by 20 water molecules, there is 100 kcal/mol of energy associated with just the Na ion. Thus, NaCl in an aqueous solution is energetically more favored than NaCl as the ionically bonded molecule due to the resulting hydrated state. You will explore what happens to molecules that only partially dissociate in water, or weak electrolytes, in the next module.

Example: Salt Crystals and Nonpolar Solvent

How would you explain what happens to a salt crystal that remains in a nonpolar solvent or a protein that finds itself buried in a sea of lipids?

The salt crystal won’t dissolve because the nonpolar solvent cannot interact favorably with the individual ions. A water soluble protein, if found in a sea of lipids, is likely to unfold, because the nonpolar residues that are in the core of the protein are no longer driven to the core by the hydrophobic effect; they are “solvated” by the nonpolar part of the membrane.


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