= Molecular recognition
<Molecular recognition> is selective association between <molecules> through <noncovalent interactions>, often involving complementary shape and charge. A binding partner can distinguish chemically similar molecules without permanently joining them by a new <covalent bond>.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book)>[entry 08187, molecular recognition; DOI 10.1351/goldbook.08187]{ref}
A simple one-site <chemical equilibrium> model makes selectivity quantitative. Let a free <ligand (biochemistry)> concentration be $L$, and let the <dissociation constant> of its complex with a binding partner be $K_d$. With independent sites and negligible depletion of free ligand, the bound fraction is $f=L/(K_d+L)$. For two partners with illustrative $K_d$ values of $1$ and $100\,\mathrm{\mu M}$, supplying $L=10\,\mathrm{\mu M}$ gives fractions $10/11\approx0.91$ and $10/110\approx0.091$.
Thus association can favor one partner strongly at a stated concentration. At very high ligand concentration, both fractions approach one, so selectivity in a practical measurement depends on conditions as well as binding constants. <Affinity chromatography> exploits this principle by attaching a ligand to a support and retaining selected <proteins>.<Agarose Derivatives for Purification of Protein by Affinity Chromatography by Pedro Cuatrecasas>[opening text, reversible selective interactions]{ref} Recognition alone does not imply catalytic conversion or a downstream cellular response.
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