Question

Do epinephrine and norepinephrine exist in the body as a racemic mixture?

Answer 1

No, natural epinephrine is (`R`)-(-)-epinephrine, and natural norepinephrine is (`R`)-(-)-norepinephrine.

Explanation:

The drugs, however, are often supplied as racemic mixtures.

According to theory, the chiral molecule interacts with three sites on the receptor:

• an anionic site for the ammonium ion
• a hydrogen-bonding site for the β-hydroxyl group
• a flat site for π-π interaction with the aromatic ring

(`S`)-Norepinephrine and (`R`)-norepinephrine have the same function in the body, but the less active isomer can establish only a two-point contact.

This loss of the H-bonding interaction equals about 12 kJ/mol, and this corresponds to a 100-fold decrease in activity.

The only difference between epinephrine and norepinephrine is that the latter lacks an N-methyl group.

This does not affect its three-point connection to the receptor.