A light-scattering study of the effect of calcium chloride on the molecular weight of busycon hemocyanin

Solutions of Busycon canaliculatum have been studied by light scattering. In 0.05 M Trizma buffer + 0.1 M NaCl at pH 7.0 at 14°, the weight-average molecular weight is 8.9 X 10⁶. In the presence of added CaCl₂ (0.02 M), the molecular weight of the protein increases to 10.7 X 10⁶, and the second virial coefficient is reduced. At pH 9.95, the molecular weights with and without 0.02 M CaCl₂, are 3.7 X 10⁶ and 1.3 X 10⁶, respectively; and the effect of Ca⁺⁺ in reducing the second virial coefficient is much greater than at pH 7.0. These results can be understood on the basis that at pH 7.0, Ca⁺⁺ increases the association of hemocyanin, by binding and intermolecular linkage through the carboxyl groups of protein side chains. At pH 9.95, amino groups are deprotonated and therefore also become available for Ca⁺⁺ binding. The relative effect of Ca⁺⁺ in enhancing the association of hemocyanin therefore becomes greater at the higher pH.