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The equilibrium between molecules A and B and the complex AB
is maintained by a balance between the two opposing reactions shown in (1) and (2). As shown in (3), the ratio of the
rate constants for the association and the dissociation reactions is equal to the equilibrium constant (K) for the reaction. Molecules A and B must collide in order to react, and the rate in reaction (2) is therefore proportional to the product
of their individual concentrations. As a result, the product [A]
x [B] appears in the final expression for K, where [ ] indicates concentration.As traditionally defined, the concentrations of products appear in the numerator and the concentrations
of reactants appear in the denominator of the equation for an equilibrium constant. Thus the equilibrium constant in
(3) is that for the association reaction A + B
![[right arrow]](/books/bookres.fcgi/system_default/rarr.gif)
AB. For simple binding interactions this constant is called the affinity constant or association
constant (in units of liters per mole); the larger the value of the association constant
( K a), the stronger is the binding between A and B. The reciprocal of K ais the dissociation
constant (in units of moles per liter); the smaller the value of the dissociation constant
( K d), the stronger is the binding between A and B.
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