Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
2.1 Weak Interactions in Aqueous Systems
This chapter discusses how weak interactions—especially hydrogen bonds—in aqueous environments determine the solubility of biomolecules, the structure of water, and the stability of biological macromolecules.
Hydrogen Bonding and Water’s Properties
• Water’s high melting and boiling points are due to hydrogen bonds, which provide internal cohesion among water molecules.
• The polar nature of water allows each molecule to form up to four hydrogen bonds, leading to a unique structure in both liquid water and ice.
• Hydrogen bonds are individually weak and transient but collectively contribute to water’s unusual physical properties.
• Polar biomolecules dissolve readily in water by forming favorable interactions, whereas nonpolar molecules are poorly soluble and tend to cluster together.
Hydrogen Bonds in Biological Molecules
• Hydrogen bonds can form between water and polar solutes like alcohols, aldehydes, ketones, and compounds containing N-H bonds, making these solutes water-soluble.
• Hydrogen bonds are strongest when donor, hydrogen, and acceptor atoms are aligned in a straight line, which confers directionality and specificity to biological structures.
• Intramolecular hydrogen bonding is critical for the three-dimensional structure of proteins and nucleic acids.
Electrostatic Interactions and Solubility
• Water is a polar solvent that dissolves ionic and polar substances by hydrating and stabilizing ions, reducing electrostatic interactions between them.
• The high dielectric constant of water allows it to effectively screen ionic charges, weakening attractions between dissolved ions and increasing solubility.
• Dissolving salts like NaCl in water increases the system’s entropy, making the process thermodynamically favorable.
Nonpolar Molecules and the Hydrophobic Effect
• Nonpolar gases (O₂, N₂, CO₂) and hydrophobic compounds are poorly soluble in water due to their inability to form hydrogen bonds.
• Introducing hydrophobic solutes into water decreases entropy, as water molecules become more ordered around them.
• Amphipathic molecules, with both polar and nonpolar regions, form micelles or bilayers in water; this clustering minimizes the hydrophobic surface area exposed to water and increases entropy—a phenomenon known as the hydrophobic effect.
• The hydrophobic effect drives the formation of biological membranes and the folding of proteins, stabilizing their structures.
van der Waals Interactions
• When two uncharged atoms are very close, transient dipoles induce weak attractive forces called van der Waals interactions.
• Although individually weak, the cumulative effect of many van der Waals interactions contributes significantly to the stability of macromolecules.
• Each atom has a characteristic van der Waals radius, influencing how closely atoms can approach in space-filling molecular structures.
Importance of Weak Interactions in Biology
• Hydrogen bonds, ionic interactions, hydrophobic effects, and van der Waals forces are all much weaker than covalent bonds but, in large numbers, have a decisive influence on the folding, structure, and function of proteins, nucleic acids, and other macromolecules.
• The native structure of a macromolecule maximizes these weak interactions, making it energetically stable.
• Weak interactions are responsible for molecular recognition, enzyme-substrate binding, and the stability of large complexes.
• Bound water molecules can be integral to the structure and function of macromolecules.
Osmosis and Colligative Properties
• Solutes affect the physical properties of water—vapor pressure, boiling and melting points, and osmotic pressure—based solely on the number of particles present (colligative properties).
• Osmosis is the movement of water across a semipermeable membrane from regions of higher to lower water concentration, generating osmotic pressure.
• Cells prevent osmotic lysis by structural adaptations (cell walls), regulatory mechanisms, or maintaining isotonic environments.
• Macromolecules have less impact on osmolarity than an equal mass of their monomeric components, so cells store fuel as polymers like glycogen to minimize osmotic pressure.
• Plants use osmotic pressure (turgor) for structural rigidity.
In a Nutshell
Weak, noncovalent interactions—hydrogen bonds, ionic interactions, hydrophobic effects, and van der Waals forces—are fundamental to the behavior of water and the structure, solubility, and function of biological molecules. These interactions drive molecular recognition, stabilize macromolecular conformations, and underlie key biological processes like membrane formation, protein folding, and osmotic regulation in cells.