We have studied the dimerization of α-chymotrypsin as an example of a typical self-associating system in order to show how such a reaction may be characterized. α-Chymotrypsin has also been suggested as a model system to ensure the proper operation of the analytical ultracentrifuge. An ideal model system should be a well-characterized association where a single equilibrium reaction is being observed with little effect from solution conditions such as pH, ionic strength and temperature. Such a system probably does not exist. However, α-chymotrypsin has been well-characterized and has been used to test the consistancy of instrument measurements over time. At about pH 4, α-chymotrypsin exhibits a monomer-dimer equilibrium, but the association constant can vary with different lots and different buffer conditions. Therefore, it is a good idea to have a standard lot number and buffer for repeated examination. In 0.01 M acetate, 0.2 M KCl, pH 4.4 at 20°C, the dimerization constant has been reported to vary about two-fold between lots of α-chymotrypsin purchased from Worthington Biochemicals with a maximum of 44×103 L/mol. The variation appears to be a function of lot number and not of experimental error since the same lot showed less than 10% variation over a period of four years.

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