Binding selectivity

In chemistry, binding selectivity is defined with respect to the binding of ligands to a substrate forming a complex. Binding selectivity describes how a ligand may bind more preferentially to one receptor than another, or how a receptor may bind more preferentially one ligand than another. A selectivity coefficient is the equilibrium constant for the reaction of displacement by one ligand of another ligand in a complex with the substrate. Binding selectivity is of major importance in biochemistry and in chemical separation processes.

In chemistry, binding selectivity is defined with respect to the binding of ligands to a substrate forming a complex. Binding selectivity describes how a ligand may bind more preferentially to one receptor than another, or how a receptor may bind more preferentially one ligand than another. A selectivity coefficient is the equilibrium constant for the reaction of displacement by one ligand of another ligand in a complex with the substrate. Binding selectivity is of major importance in biochemistry and in chemical separation processes. \begin{align} \ce{A + B AB};& \quad K_{\rm AB} = \frac{[\ce{AB}]}{\ce{[A][B]}} \\ \ce{A + C AC};& \quad K_{\rm AC} = \frac{\ce{[AC]}}{\ce{[A][C]}} \end{align} where [X] represents the concentration of substance X (A, B, C, …). A selectivity coefficient is defined as the ratio of the two equilibrium constants. K_{\rm B,C} = \frac{K_{\rm AC}}{K_{\rm AB}} This selectivity coefficient is in fact the equilibrium constant for the displacement reaction \ce{AB + C AC + B}; \quad K_{\rm B,C} = \frac{\ce{[AC][B]}}{\ce{[AB][C]}} = \frac{K_{\rm AC}\ce{[A][B][C]}}{K_{\rm AB}\ce{[A][B][C]}} = \frac{K_{\rm AC}}{K_{\rm AB}} It is easy to show that the same definition applies to complexes of a different stoichiometry, A p B q and A p C q . The greater the selectivity coefficient, the more the ligand C will displace the ligand B from the complex formed with the substrate A. An alternative interpretation is that the greater the selectivity coefficient, the lower the concentration of C that is needed to displace B from AB. Selectivity coefficients are determined experimentally by measuring the two equilibrium constants, and . R_S=\frac{\sqrt N}{4}\left(\frac{\alpha-1}{\alpha}\right)\left(\frac{k_B}{1+k_B}\right) where α is selectivity factor, N is the number of theoretical plates k A and k B are the retention factors of the two analytes. Retention factors are proportional to distribution coefficients. In practice substances with a selectivity factor very close to 1 can be separated. This is particularly true in gas-liquid chromatography where column lengths up to 60 m are possible, providing a very large number of theoretical plates. In ion-exchange chromatography the selectivity coefficient is defined in a slightly different way

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