Interactions in Large Biomolecules (College Board AP® Chemistry): Revision Note

Interactions in Large Biomolecules

• Noncovalent interactions (hydrogen bonds, dipole-dipole and London dispersion forces) are also found in large biomolecules such as proteins, carbohydrates and lipids

• These interactions are possible either between biomolecules or within different regions of the same molecule

• For example, double-helix DNA molecules are held together by hydrogen bonds

  • Hydrogen bonds form between the nitrogenous bases on the two strands: three between guanine and cytosine, two between adenine and thymine

Hydrogen bonds in DNA

Double helix DNA strands show the hydrogen bonds between the nitrogenous base components of each strand

• Interactions between biomolecules and water explain their solubility:

  • Lipids have long nonpolar hydrocarbon tails, which are hydrophobic (“water fearing”): the attractions between water and these tails are much weaker than the hydrogen bonds between water molecules, so lipids do not dissolve in water

  • Carbohydrates have many O—H groups, which are hydrophilic (“water loving”): they form hydrogen bonds with water molecules, so many carbohydrates dissolve in water

Lipids and water

Nonpolar lipid molecules form a separate layer because their attractions to water are weaker than water's hydrogen bonds

Sugars and water

Water forms hydrogen bonds with the O—H groups and oxygen atoms of sucrose, so sucrose dissolves in water