Water, Weak Bonds, and the Generation of Order Out of Chaos

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  • 2.1 Thermal Motions Power Biological Interactions

  • 2.2 Biochemical Interactions Take Place in an Aqueous Solution

  • 2.3 Weak Interactions Are Important Biochemical Properties

  • 2.4 Hydrophobic Molecules Cluster Together

  • 2.5 pH Is an Important Parameter of Biochemical Systems

Our senses—vision, taste, smell, hearing, and touch—allow us to experience the world. We delight in the softness of a kitten’s fur and the loudness of its purr through touch and hearing. remarkably, these sensuous pleasures depend on weak, reversible chemical bonds.

Cells, as shown in Chapter 1, present a remarkable display of functional order. Millions of individual molecules are the cell’s building blocks, consisting of the four key biomolecules of life—proteins, nucleic acids, lipids, and carbohydrates. These molecules are, for the most part, stable because they are constructed with strong covalent bonds—bonds in which the electrons are shared by the participating atoms. However, the remarkable structure and function of the cell itself are stabilized by weak interactions that have only a fraction of the strength of covalent bonds.

Two questions immediately come to mind: How is such stabilization possible? And why is it advantageous? The answer to the first question is that there is stability in numbers. Many weak bonds can result in large stable structures. The answer to the second question is that weak bonds allow transient interactions. A substrate can bind to an enzyme, and the product can leave the enzyme. A hormone can bind to its receptor and then dissociate from the receptor after the signal has been received. Weak bonds allow for dynamic interactions and permit energy and information to move about the cell and organism. Transient chemical interactions form the basis of biochemistry and life itself.

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Water is the solvent of life and greatly affects weak bonds, making some weaker and powering the formation of others. For instance, hydrophobic molecules, such as fats, cannot interact with water at all. Yet, this chemical antipathy is put to use. The formation of membranes and the intricate three-dimensional structure of biomolecules, most notably proteins, are powered by an energetic solution to the chemical opposition between water and hydrophobic molecules.

DID YOU KNOW?

One angstrom (Å) = 0.1 nanometer (nm) = 1 × 10−10 meter (m). It is named after Swedish physicist Anders Jonas Ångström (1814–1874) who expressed wavelengths as multiples of 1 × 10−10 meter. That length was subsequently named an angstrom.

Our experience of life happens at a distance of 4 angstroms (4 Å, or 0.4 nm), the typical length of noncovalent bonds. The pressure of a held hand, the feeling of a kiss, the reading of the words on this page—all of these sensations are the result of large, covalently bonded molecules interacting noncovalently with a vast array of other large molecules or with sodium ions, photons, or an assortment of other signal molecules, all at a distance of approximately 4 Å.

In this chapter, we will focus on transient interactions between molecules—weak, reversible but essential interactions. We will see how molecules must meet before they can interact and will then examine the chemical foundations for the various weak interactions. Finally, we will examine especially prominent weak, reversible interactions—the ionization of water and weak acids.