29.4.4—Relevance of chirality to the synthetic

A chiral drug may exist as enantiomers that have the same bulk physical properties but different biological effects because receptors, enzymes and transport proteins are chiral.

Synthetic preparation may therefore aim for one enantiomer, resolve a racemate, or test the mixture carefully. The useful enantiomer can be more active, while the other may be inactive or have a different effect.

A molecule designed to fit one enzyme binding site like a hand in a glove may have a mirror-image form that binds weakly or to another target.

“One enantiomer is always harmless” is not a chemical rule. Biological effects must be measured, and a racemate is not automatically equivalent to a pure enantiomer.