O

hydrogen bond between a barged amino group and a charged carboxyl group

FIGURE 3-8 Example of hydrogen bonds in biological molecules.

FIGURE 3-7 Antibody-antigen interaction. The structure shows the complex between Fab D 1 i and lyicsyine (Ftschmann T.O., Bentiey C.A., Bhat T.W., Bculot C., Msriuzza R.A., Phillips SE„ Tello D, and Poljak Rl 1991.1 Biol. Cfiem 266: 12915.)

The electrostatic forces acting hetween the oppositely charged groups are called ionic bonds, Their average bond energy in an aqueous solu-tjon is about 5 kcal/mol.

in many cases, either an inorganic cation like Na1, K\ or Mg or an inorganic anion like CI or S042 neutralizes the charge of ionized organic molecules. When this happens in aqueous solution, the neutralizing cations and anions do not carry fixed positions because inorganic ions are usually surrounded by shells of water molecules and so do not directly bind to oppositely charged groups. Thus, in water solutions, electrostatic bonds to surrounding inorganic cations or anions are usually not of primary importance in determining the molecular shapes of organic molecules.

On the other hand, highly directional bonds result if the oppositely charged groups can form hydrogen bonds to each other. For example, COO" and NH3+ groups are often held together by hydrogen bonds. Since these bonds are stronger than those that involve groups with less than a unit of charge, they are correspondingly shorter. A strong hydrogen bond can also form between a group with a unit charge and a group having less than a unit charge. For example, a hydrogen atom belonging to an amino group (NHJ bonds strongly to an oxygen atom of a carboxyl group (COO ),

Weak Interactions Demand Complementary Molecular Surfaces

Weak binding forces are effective only when the interacting surfaces are close. This proximity is possible only when the molecular surfaces have complementary structures, so that a protruding group (or positive charge) on one surface is matched by a cavity (or negative charge) on another. That is, the interacting molecules must have a lock-and-

Weak Bonds in Biological Systems 49

key relationship. In cells, this requirement often means that some molecules hardly ever bond to other molecules of the same kind, because such molecules do not have the properties of symmetry necessary for self-interaction. For example, some polar molecules contain donor hydrogen atoms and no suitable acceptor atoms, whereas other molecules can accept hydrogen bonds but have no hydrogen atoms to donate. On the other hand, there are many molecules with the necessary symmetry to permit strong self-interaction in cells. Water is the most important example of this.

Water Molecules Form Hydrogen Bonds

Under physiological conditions, water molecules rarely ionize to form IT* and OH ions. Instead, they exist as polar H-O-H molecules with both the hydrogen and oxygen atoms forming strong hydrogen bonds. In each water molecule, the oxygen atom can bind to two external hydrogen atoms, whereas each hydrogen atom can bind to one adjacent oxygen atom. These bonds are directed tetrahedrally (Figure 3-10), so in its solid and liquid forms, each water molecule tends to have four nearest neighbors, one in each of the four directions of a tetrahedron. In ice, the bonds to these neighbors are very rigid and the arrangement of molecules fixed. Above the melting temperature (0 L'C), the energy of thermal motion is sufficient to break the hydrogen bonds and to allow the water molecules to change their nearest neighbors continually. Even in the liquid form, however, at any given instant most water molecules are bound by four strong hydrogen bonds.

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