Molecular Interactions and Properties part 6

Hydrogen Bonding

• Hydrogen bonding occurs between hydrogen and oxygen atoms of different water molecules.
• Chapter three will discuss water's properties due to hydrogen bonds.
• Individual hydrogen bonds are weak, but multiple bonds provide strength, like in DNA.
• DNA is a double-stranded molecule held together by hydrogen bonds.
• The number of hydrogen bonds between base pairs:
  • Adenine-Thymine (AT): two hydrogen bonds.
  • Guanine-Cytosine (GC): three hydrogen bonds.
• Hydrogen bonds are broken to access information in DNA strands and reformed when strands come back together.
• Ionic bonds and hydrogen bonds are individually weaker than covalent bonds.

Van der Waals Interactions

• Van der Waals interactions are based on the movement of electrons.
• Electron position is not constant; they can be on one side of a molecule at any given moment, creating temporary partial charges.
• Electrons in a nonpolar molecule are not always symmetrically distributed, leading to temporary positive and negative charges.
• Example: In a hydrogen molecule $H_2$, electrons may, at a brief instance, both be found closer to one of the Hydrogen atoms, giving that atom a partial negative charge and the other atom a partial positive charge.
• These temporary charge regions enable atoms and molecules to stick together.
• Van der Waals interactions are individually weak, but collectively strong.
• Gecko feet use van der Waals forces to adhere to surfaces, inspiring robotics research.
• Gecko-inspired adhesives:
  • Mimic gecko's foot with tiny hairs using van der Waals forces.
  • Adhesion can be turned on/off by applying shear load, increasing real area of contact.
• Applications of gecko-inspired adhesives:
  • Grabbing satellites for repair.
  • Removing space garbage.
  • Robots for space station repair and inspection.

Molecular Shape and Function

• A molecule's size, shape, and charge are key to its function. If the shape of a molecule changes, the function is often lost.
• A molecule's shape is determined by the positions of its atoms' orbitals.
• Atomic orbitals (s and p) can hybridize to form new orbitals, determining molecular shape. For example: tetrahedron.
• Water has a specific shape and bond angle based on the hybridization of its orbitals.
• Molecules with similar shapes can mimic each other's functions.
• Example: Morphine mimics natural endorphins because it has a similar shape in the receptor binding region.
  • Natural endorphins bind to receptors on cell membranes, producing a pleasant effect.
  • Morphine's similar shape allows it to bind to the same receptor thus creating similar effects.
• Changing a molecule's shape can alter or eliminate its function by affecting its ability to bind to receptors.
  Complementarity between molecules depends on their shape to recognize and bind to each other.