BackAqueous Chemistry: Water – Structure, Interactions, and Properties
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Aqueous Chemistry: Water
Introduction
Water is a fundamental molecule in biochemistry, serving as the medium for most biological processes. Its unique structure and properties are essential for the structure and function of biomolecules. This chapter explores the types of noncovalent interactions, the nature of hydrogen bonding, and the physical properties of water that make it indispensable for life.
Noncovalent Interactions in Biomolecules
Definition and Importance
Noncovalent interactions are weak, reversible forces that play a critical role in the structure and function of biomolecules, such as proteins, nucleic acids, and membranes.
These interactions allow biomolecules to recognize and bind to each other, as seen in hormone-receptor binding.
Types of Noncovalent Interactions
Charge–charge (ionic) interactions: Electrostatic attractions between oppositely charged groups.
Charge–dipole interactions: Attraction between a charged group and a polar molecule.
Dipole–dipole interactions: Attraction between two polar molecules.
Charge–induced dipole interactions: A charged group induces a dipole in a nonpolar molecule.
Dipole–induced dipole interactions: A polar molecule induces a dipole in a nonpolar molecule.
Dispersion (van der Waals) forces: Weak attractions due to transient dipoles in all molecules.
Hydrogen bonds: A special type of dipole–dipole interaction involving hydrogen bonded to electronegative atoms (O, N, F).
Bond Energies
Noncovalent bonds are much weaker than covalent bonds, allowing them to be broken and reformed easily, which is essential for dynamic biological processes.
Type of Interaction | Approximate Energy (kJ/mol) |
|---|---|
Charge–Charge | 13–17 |
Hydrogen Bond | 2–21 |
Van der Waals | 0.4–0.8 |
Additional info: Covalent bonds are typically >300 kJ/mol, much stronger than noncovalent interactions.
Hydrogen Bonding
Nature of Hydrogen Bonds
A hydrogen bond forms when a hydrogen atom covalently bonded to an electronegative atom (donor) interacts with another electronegative atom (acceptor).
Hydrogen bonds are directional and contribute to the stability and specificity of biomolecular structures.
Major Types of Hydrogen Bonds in Biomolecules
Donor → Acceptor | Distance (Å) | Comment |
|---|---|---|
O–H...O | 2.8–3.0 | H bond formed in water |
N–H...O | 2.8–3.0 | Bonding of water to other molecules |
N–H...N | 2.9–3.1 | Important in protein and nucleic acid structures |
O–H...N | 2.9–3.1 | Weaker than above |
Other (e.g., S–H...O) | 3.5–3.7 | Much weaker |
Hydrogen Bonding in Biological Molecules
Hydrogen bonds stabilize the secondary and tertiary structures of proteins and nucleic acids.
In proteins, backbone N–H and C=O groups form hydrogen bonds in α-helices and β-sheets.
In DNA, hydrogen bonds between base pairs hold the double helix together.
Hydrogen Bond Donors and Acceptors
Donor: Atom to which hydrogen is covalently bonded (e.g., N–H, O–H).
Acceptor: Electronegative atom with a lone pair (e.g., O, N).
Summary Table: Types of Noncovalent Interactions
Type of Interaction | Model | Example |
|---|---|---|
Charge–charge | Opposite charges attract | Na+ and Cl– |
Charge–dipole | Charge interacts with dipole | Na+ and H2O |
Dipole–dipole | Two dipoles align | H2O and H2O |
Charge–induced dipole | Charge induces dipole | Na+ and benzene |
Dipole–induced dipole | Dipole induces dipole | H2O and benzene |
Dispersion (van der Waals) | Transient dipoles | Two benzene rings |
Hydrogen bond | Donor and acceptor | O–H...O in water |
Key Concepts and Applications
Noncovalent interactions are essential for molecular recognition, enzyme-substrate binding, and the formation of complex biological structures.
Hydrogen bonds are particularly important in stabilizing the three-dimensional structures of proteins and nucleic acids.
Understanding the types and strengths of noncovalent interactions helps explain the behavior of biomolecules in aqueous environments.
Example: Human Growth Hormone and Receptor
The binding of human growth hormone to its receptor is mediated by a network of noncovalent interactions, including hydrogen bonds and ionic interactions, which ensure specificity and reversibility.
Additional info: The dynamic nature of noncovalent interactions allows for the regulation of biological processes, as these bonds can be easily formed and broken under physiological conditions.
