A New Method of Applying the Sips Equation

The Sips equation is frequently used in immunochemistry to describe the relationship between antibody-binding site concentration (n0[Ab]), antigen concentration, intrinsic affinity constant (K), and the heterogeneity index (a) of the affinity constant. Usually n0[Ab] is determined before calculating the remaining parameters (K and a). A new method is proposed which does not require knowledge of n0[Ab] nor an extensive calculation to determine K and a. The method can also be used to determine the antibody-binding site concentration without purified antibody or ligand saturation of the binding sites. This method can be applied to any antibody which binds a monovalent ligand and which can be obtained at a concentration greater than 1/K. Since the Sips equation can be applied to any ordinary chemical reaction by setting a=1, the proposed method can be used generally to determine the affinity constant and the initial concentration of one of the reactants.