Lecture 17: Non-Covalent Bonds

Noncovalent Bonds Overview

• Noncovalent bonds play a crucial role in the structure and function of biological macromolecules.

Types of Interactions

Covalent Bonds

• Strong (150-1000 kJ/mol)

• Directional; convey shape

Noncovalent Interactions

• Weaker (~2-40 kJ/mol) compared to covalent bonds

Main Types of Noncovalent Interactions:

• Ionic interactions (~20 kJ/mol)

• Dipole interactions (~5-20 kJ/mol)

• Hydrogen bonds (~10-30 kJ/mol)

• Dispersion forces (~2 kJ/mol)

• Steric repulsion (very strong, short-range)

• Hydrophobic interactions (<40 kJ/mol)

Importance of Noncovalent Interactions

• Individually weak but collectively strong

• Provide flexibility in biological structures, similar to Velcro

• Essential for transient molecular interactions

Atom Interaction Across Distances

• Repulsion: Electron clouds overlap at close distances

• Attraction: Occurs at optimal distance for minimized energy

Stability of Proteins

• Stability comparisons:

  • A. Several covalent bonds

  • B. A single covalent bond

  • C. A few hydrogen bonds

  • D. A dipole-dipole interaction

  • E. Less than above

• Comparison of native and denatured protein stability

Biologically Important Ions

• Sodium (Na+), Magnesium (Mg2+), Chloride (Cl-)

• Bind to proteins' charged/polar regions

• Help in stabilizing protein structures

Ionic Bonds and Salt Bridges

• Common in biological systems; formed between charged amino acids.

• Salt bridges enhance protein stability, especially in ion-rich environments.

Polar Covalent Bonds

• Key Characteristics:

  • Oxygen is more electronegative than hydrogen, causing uneven electron sharing.

  • Polar bonds: important for molecule interactions.

Hydrogen Bonds in Biological Processes

• Form between hydrogen bound to electronegative atoms (like O and N) and lone pairs of other electronegative atoms.

• Contributes to molecular shape and stability in proteins and nucleic acids.

Properties of Water

• Makes up approximately 66% of the human body.

• Unusual properties due to strong hydrogen bonding:

  • High heat capacity and boiling point.

  • Strong cohesion and adhesion; critical for capillary action.

• Acts as a solvent for many biochemical reactions.

Hydrophobic Interactions

• Occur as hydrophobic molecules clump together to minimize exposure to water.

• Vital for maintaining the structure of protein molecules and membranes.

Take Home Messages

• Bonds give shape and strength necessary for the function of molecules:

  • Examples include:

    • Gecko feet

    • DNA double helix

    • Protein structure

    • Protein-protein interactions

• Many weak bonds add up to strong but flexible interactions.