Illustrated Glossary of Organic Chemistry

Molecular recognition: Selective interaction between ions and/or molecules due to noncovalent molecular forces, molecular size, and other factors. Molecular recognition examples include enzyme selectivity, where an enzyme binds only with a select group of molecules due to molecular size and noncovalent molecular force constraints.

Crown ethers are a series of cyclic ethers developed by Professor Donald Cram of UCLA. In a demonstration of molecular recognition, crown ethers bind selectively with certain cations, depending on the size of the crown ether's central cavity and diameter of the cation. For example, 12-crown-4 binds Li⁺ strongly but not K⁺, whereas 18-crown-6 binds K⁺ much more strongly than it binds Li⁺. Note that Li⁺ (90 pm diameter) is smaller than K⁺ (152 pm diameter), and the central cavity in 12-crown-4 is smaller than the central cavity in 18-crown-6. Professor Cram shared the 1987 Nobel Prize in Chemistry "for their development and use of molecules with structure-specific interactions of high selectivity."

Chymotrypsin, a serine protease, cleaves only peptide bonds involving the carbonyl group of phenylalanine. A hydrophobic pocket that has the right size for phenylalanine's benzyl group explains the enzyme's selectivity.