Configurations

Molecular Identity and Structure

• Molecular identity is determined not only by atomic composition but also by 3D structure.

• Example: Malic acid and fumaric acid

  • Same atomic composition.

  • Different configurations due to a double bond preventing rotation about the carbon-carbon axis.

Configurations and Stereoisomers

• Molecules can exist in different configurations, leading to the concept of stereoisomers.

• Malic Acid:

  • Bulky groups on the same side of the double bond.

  • Corresponds to cis configuration.

• Fumaric Acid:

  • Bulky groups on opposite sides of the double bond.

  • Corresponds to trans configuration.

• A double bond is not always required for configurational isomers.

Chiral Centers

• Chiral Center Definition:

  • A carbon atom bonded to four distinct chemical groups.

• Example:

  • Chiral carbon linked to 1 hydrogen and 3 distinct groups (r1, r2, r3).

  • If linked to two identical groups, it is achiral.

Enantiomers

• Different configurations of chiral molecules result in two enantiomers.

• They are mirror images of each other.

• Illustration:

  • Can't reach one configuration from another without breaking covalent bonds.

  • Comparison with human hands and feet (mirror images but non-superimposable).

Biochemical Consequences of Configurations

• Molecules with different configurations interact differently with receptors.

• Example: Different smells from enantiomers (R and S carbon):

  • R carbon → fresh spermine smell.

  • S carbon → pungent caraway smell.

Conformations vs Configurations

• Conformations: Different spatial arrangements due to free rotation around single bonds.

  • No breaking of covalent bonds required.

• Stability of conformations:

  • Ethane example:

    • Two conformations: eclipsed vs. staggered.

    • Staggered conformation is more stable due to lower potential energy (steric hindrance).

  • Flexibility of biomolecules allows reaching stable conformations.