Water and Polarity
Water is the principal component of most cells. The geometry of the water molecule and its properties as a solvent play major roles in determining the properties of living systems.
The tendency of an atom to attract electrons to itself in a chemical bond (i.e., to become negative) is called electronegativity. Atoms of the same ele-ment, of course, share electrons equally in a bond-that is, they have equal electronegativity-but different elements do not necessarily have the same electronegativity. Oxygen and nitrogen are both highly electronegative, much more so than carbon and hydrogen (Table 2.1).
When two atoms with the same electronegativity form a bond, the electrons are shared equally between the two atoms. However, if atoms with differing electronegativity form a bond, the electrons are not shared equally and more of the negative charge is found closer to one of the atoms. In the O2H bonds in water, oxygen is more electronegative than hydrogen, so there is a higher probability that the bonding electrons are closer to the oxygen. The difference in electronegativity between oxygen and hydrogen gives rise to a partial positive and negative charge, usually pictured asd1andd2, respectively (Figure 2.1). Bonds such as this are called polar bonds. In situations in which the electronegativity difference is quite small, such as in the C-H bond in methane (CH4), the sharing of electrons in the bond is very nearly equal, and the bond is essentially nonpolar.
The bonds in a molecule may be polar, but the molecule itself can still be nonpolar because of its geometry. Carbon dioxide is an example. The two C=O bonds are polar, but because the CO2 molecule is linear, the attraction of the oxygen for the electrons in one bond is cancelled out by the equal and opposite attraction for the electrons by the oxygen on the other side of the molecule.
Water is a bent molecule with a bond angle of 104.3° (Figure 2.1), and the uneven sharing of electrons in the two bonds is not cancelled out as it is in CO2. The result is that the bonding electrons are more likely to be found at the oxygen end of the molecule than at the hydrogen end. Bonds with positive and negative ends are called dipoles.
The polar nature of water largely determines its solvent properties. Ionic compounds with full charges, such as potassium chloride (KCl, K+ and Cl- in solution), and polar compounds with partial charges (i.e., dipoles), such as ethyl alcohol (C2H5OH) or acetone [(CH3)2C==O], tend to dissolve in water (Figures 2.2 and 2.3). The underlying physical principle is electrostatic attraction between unlike charges.
The negative end of a water dipole attracts a positive ion or the positive end of another dipole. The positive end of a water molecule attracts a negative ion or the negative end of another dipole. The aggregate of unlike charges, held in proximity to one another because of electrostatic attraction, has a lower energy than would be possible if this interaction did not take place. The lowering of energy makes the system more stable and more likely to exist. These ion–dipole and dipole–dipole interactions are similar to the interactions between water molecules themselves in terms of the quantities of energy involved. Examples of polar compounds that dissolve easily in water are small organic molecules containing one or more electronegative atoms (e.g., oxygen or nitrogen), including alcohols, amines, and carboxylic acids. The attraction between the dipoles of these molecules and the water dipoles makes them tend to dissolve. Ionic and polar substances are referred to as hydrophilic (“water-loving,” from the Greek) because of this tendency.
Hydrocarbons (compounds that contain only carbon and hydrogen) are nonpolar. The favorable ion–dipole and dipole–dipole interactions respon-sible for the solubility of ionic and polar compounds do not occur for nonpolar compounds, so these compounds tend not to dissolve in water. The interac-tions between nonpolar molecules and water molecules are weaker than dipolar interactions. The permanent dipole of the water molecule can induce a temporary dipole in the nonpolar molecule by distorting the spatial arrangements of the electrons in its bonds. Electrostatic attraction is possible between the induced dipole of the nonpolar molecule and the permanent dipole of the water mol-ecule (a dipole–induced dipole interaction), but it is not as strong as that between permanent dipoles. Hence, its consequent lowering of energy is less than that produced by the attraction of the water molecules for one another. The associa-tion of nonpolar molecules with water is far less likely to occur than the associa-tion of water molecules with themselves.
A full discussion of why nonpolar substances are insoluble in water requires the thermodynamic arguments. However, the points made here about intermolecular interactions will be useful background information for that discussion. For the moment, it is enough to know that it is less favorable thermodynamically for water molecules to be asso-ciated with nonpolar molecules than with other water molecules. As a result, nonpolar molecules do not dissolve in water and are referred to as hydrophobic (“water-hating,” from the Greek). Hydrocarbons in particular tend to sequester themselves from an aqueous environment. A nonpolar solid leaves undissolved material in water. A nonpolar liquid forms a two-layer system with water; an example is an oil slick. The interactions between nonpolar molecules are called hydrophobic interactions or, in some cases, hydrophobic bonds.
Table 2.2 gives examples of hydrophobic and hydrophilic substances.
A single molecule may have both polar (hydrophilic) and nonpolar (hydro-phobic) portions. Substances of this type are called amphipathic. A long-chain fatty acid having a polar carboxylic acid group and a long nonpolar hydrocarbon portion is a prime example of an amphipathic substance. The carboxylic acid group, the “head” group, contains two oxygen atoms in addition to carbon and hydrogen; it is very polar and can form a carboxylate anion at neutral pH. The rest of the molecule, the “tail,” contains only carbon and hydrogen and is thus nonpolar (Figure 2.4). A compound such as this in the presence of water tends to form structures called micelles, in which the polar head groups are in contact with the aqueous environment and the nonpolar tails are sequestered from the water (Figure 2.5). A similar process is responsible for the separation of oil and water, such as you would see in Italian salad dressing. When shaken, initially the substances mix. Immediately thereafter you can see small spheres or oil droplets. As these float on water, they move to the top and coalesce into the oil layer.
Interactions between nonpolar molecules themselves are very weak and depend on the attraction between short-lived temporary dipoles and the dipoles they induce. A large sample of nonpolar molecules will always include some molecules with these temporary dipoles, which are caused by a momen-tary clumping of bonding electrons at one end of the molecule. A temporary dipole can induce another dipole in a neighboring molecule in the same way that a permanent dipole does. The interaction energy is low because the association is so short-lived.
It is called a van der Waals interaction (also referred to as a van der Waals bond). The arrangement of molecules in cells strongly depends on the molecules’ polarity, as we saw with micelles.
Water is a polar molecule, with a partial negative charge on the oxygen and partial positive charges on the hydrogens.
Forces of attraction exist between the unlike charges.
Polar substances tend to dissolve in water, but nonpolar substances do not.
The properties of water have a direct effect on the behavior of biomolecules.