Binding constant

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Concepts in
Chemical Equilibria
Acid dissociation constant
Binding constant
Chemical equilibrium
Chemical stability
Dissociation constant
Distribution coefficient
Distribution ratio
Equilibrium constant
Equilibrium unfolding
Equilibrium stage
Liquid-liquid extraction
Phase diagram
Phase rule
Reaction quotient
Relative volatility
Solubility equilibrium
Stability constant
Thermodynamic equilibrium
Theoretical plate
Vapor-liquid equilibrium
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The binding constant is a special case of the equilibrium constant K. The equilibrium state of molecular binding, i.e. the balance between the binding and dissociation processes after infinite reaction time, may be formalized as the unbound compounds (reactants, A and B) transforming into a complex (product, C):

A + B \rightleftharpoons C

This directionality of reaction defines the association equilibrium constant Kass:

K_{ass} = {k_{on} \over k_{off}} = {[C] \over [A] \times [B]} (second order or bimolecular association)

Since the binding constant is related to the molar free enthalpy, it is used to quantify the affinity of binding. For this purpose, the reverse directionality is used because the resulting formalism is a reaction of first order and thus its dimension is always in mol (or mol/L). This defines the dissociation equilibrium constant Kdiss:

K_{diss} = {k_{off} \over k_{on}} = {[A] \times [B] \over [C]} (first order dissociation)
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