Hands in fruit and medicine

Section 4 of 4

Living things usually make just one hand of a chiral molecule. The hand matters most when the molecule meets something else that is handed, such as a receptor or another protein in your body.

Limonene in orange peel

Limonene, C₁₀H₁₆, has the same ring, methyl group and branch as carvone, with no C=O group, and it is chiral too. Squeeze an orange peel and the oil that sprays out is about 90% limonene. More than 99.9% of that limonene is one hand, (R)-⁠limonene. Lemon peel oil holds the same hand.

Ibuprofen

Medicines meet handed proteins all over the body, so the two hands of a medicine can act differently. Ibuprofen, a common painkiller, has one stereocentre: the carbon carrying -⁠H, -⁠CH₃, -⁠COOH and a six-carbon benzene ring. Ibuprofen tablets hold both hands in equal amounts.

The (S) hand is the one that eases pain and swelling. The (R) hand does very little on its own. Enzymes, proteins that speed up the chemistry in your body, turn much of it into the (S) hand.

(S)-ibuprofen A skeletal formula: a benzene ring with a branched four-carbon chain on the right. On the left, the ring is joined to a carbon that carries a -COOH group and a -CH₃ group. The -CH₃ group is on a hashed wedge, so it points away from you, behind the page. O H O
(S)-⁠ibuprofen, the hand that eases pain. The stereocentre is the carbon next to the -⁠COOH group; its -⁠CH₃ sits on a hashed wedge, behind the page.

Handed molecules are everywhere in living things. Your proteins are built from L amino acids, the sugar in your DNA is one hand too, and your nose, your enzymes and many medicines all work hand in glove.

Quick check

Ibuprofen tablets contain equal amounts of both hands. What happens to the (R)-⁠ibuprofen in your body?

That's the end of lesson 7

In this lesson you found out:

  • What makes a molecule left- or right-handed
  • How to spot a carbon with four different groups
  • Why mirror-image molecules can smell and act differently
Start Lesson 8: How reactions happen All lessons