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

Noncovalent Interactions in Large Molecules

• Many substances with very large molar masses make up the structure of living organisms and tissues

• Some examples include starch and cellulose, which abound in plants, as well as proteins, which are found in both plants and animals

  • These molecules are known as polymers and are made of smaller molecules called monomers

Structures of starch and cellulose

Diagram showing the structures of the starch polymer made from the α-glucose monomers while the cellulose polymer is made from β-glucose monomers

• Some important noncovalent interactions which occur in large molecules include:

  • Hydrogen bonding

    • Involves interaction between the polar hydrogen atom from an O-H or N-H bond and the lone electron pairs of very electronegative atoms such as nitrogen, oxygen and fluorine

  • Electrostatic/Ionic interactions

    • These are attractions between oppositely charged groups, such as –NH₃⁺ on one part of a protein and –COO^- on another molecule or region

  • Hydrophobic interactions

    • These are attractions between nonpolar groups (such as hydrocarbon chains and rings), which cluster together in water because they cannot hydrogen bond with it

    • This is why the nonpolar regions of a protein fold inward, away from the surrounding water

  • London dispersion forces

    • Present between all molecules and regions, polar or nonpolar

  • Dipole-dipole interactions

    • Present between polar regions, in addition to London dispersion forces

  • These interactions can occur between different molecules or between different regions of the same large molecule

  • Image 2 shows these interactions between a protein and ferulic acid molecules

Noncovalent interactions between a protein and ferulic acid

Possible noncovalent interactions between a protein and ferulic acid