Overview
This lecture introduces hydrogen bonds, explaining their formation and significance in water, DNA, and proteins, and compares them with covalent and ionic bonds.
Review of Covalent Bonds
- Polar covalent bonds involve unequal sharing of electrons due to differences in electronegativity.
- Non-polar covalent bonds involve equal sharing of electrons between atoms with similar or identical electronegativity.
- Highly electronegative elements like nitrogen, oxygen, and fluorine pull electrons more strongly in bonds.
Hydrogen Bonding in Water
- A water molecule (H₂O) has polar covalent bonds due to oxygen's high electronegativity compared to hydrogen.
- Oxygen becomes partially negative (δ-) and hydrogen becomes partially positive (δ+) in water molecules.
- The partial charges on different water molecules attract each other, forming hydrogen bonds between molecules.
- Hydrogen bonds in water are intermolecular (between molecules), not intramolecular (within a molecule).
Hydrogen Bonds in DNA and Proteins
- Hydrogen bonds occur between bases in DNA, holding the two strands together.
- In proteins, hydrogen bonds help maintain secondary structure (e.g., alpha helices and beta sheets).
- Hydrogen bonds in biological molecules commonly involve hydrogen bonded to oxygen or nitrogen due to their high electronegativity.
Requirements for Hydrogen Bonding
- Hydrogen must be covalently bonded to an electronegative atom (usually oxygen, nitrogen, or sometimes fluorine).
- The electronegative atom attracts shared electrons, leading to partial charges and weak attractions to other molecules or molecular regions.
Key Terms & Definitions
- Hydrogen Bond — a weak attraction between a partially positive hydrogen atom and a partially negative atom (like oxygen or nitrogen) on another molecule.
- Polar Covalent Bond — a bond where electrons are shared unequally due to differences in electronegativity.
- Non-polar Covalent Bond — a bond where electrons are shared equally between atoms.
- Electronegativity — the ability of an atom to attract shared electrons in a bond.
- Intermolecular — occurring between separate molecules.
- Partial Charge (δ+, δ-) — a small electrical charge due to uneven electron distribution in a molecule.
Action Items / Next Steps
- Review notes on polar vs. non-polar covalent bonds.
- Prepare for next lecture on the properties of water related to hydrogen bonding.