Question

Why is only one of the enantiomers of Ibuprofen effective?

Answer 1

Only one of the isomers is effective because it is the one that fits the receptor site on the enzyme involved in pain perception.

Explanation:

What ibuprofen does

Ibuprofen is works by inhibiting two enzymes called COX-1 and COX-2.

They convert arachidonic acid to prostaglandin H2 (PGH2) which, in turn, is converted by other enzymes to other prostaglandins that activate the body's response to inflammation.

How ibuprofen does it

Ibuprofen is 2-(4-isobutylphenyl)propanoic acid. Its structure is

(From scienceline.ucsb.edu)

Note the chiral centre at `"C2"` of the propanoic acid.

Why only the `S` isomer works

The (`S`)-ibuprofen has the same shape as the molecules that activate the COX enzymes to produce prostaglandins.

If we use the lock-and-key theory of enzyme action, we say that (`S`)-ibuprofen is the key that fits the lock (the receptor site) of the enzyme.

When it occupies the receptor site, it blocks access to the COX activators.

(From www.dailymail.co.uk)

The production of PGH2 ceases along with the pain and fever caused by the body's inflammatory response.