Binding of cellulose sulphates to the 12 S globulin and the low molecular mass basic protein fraction from rape seed...

The binding of two differently substituted cellulose sulphates (CS) with DS 0.50 and 0.33 to two main rapeseed proteins, the high molecular mass neutral 12 S globulin and the low molecular mass basic protein fraction ("albumin") in insoluble complexes at pH less than Pi (protein) has been studied using turbidimetric titration and chemical analysis of the supernatant after coprecipitation. The binding of both types of CS to the globulin at pH 3.0-2.5 at the precipitation occurs at a substoichiometric CS-protein mass ratio. This result has been obtained both by turbidimetric titration and chemical analysis. Contrary to that, the CS binding to the albumin is substoichiometric according to the turbidimetric titration and stoichiometric according to the chemical analysis. The CS-protein mass ratio in the coprecipitates obtained of pH 2.5 and 3.0 is nearly independent on the CS concentration applied for the precipitation of the albumin. There is a typical dependence on the CS concentration, however, for the globulin at pH 3.0, which becomes less pronounced at pH 2.5. The CS-globulin complexes form sharp turbidimetric titration curves at pH less than Pi (pH 2.5-5.5), the maximum position of which shifts to lower pH with increasing percentage of CS. The analogous titration curves for the CS-albumin complexes are broader, owing to the heterogeneity of the albumin fraction. Both polyanions exert a solubilizing effect at a molar excess on both proteins. Regarding the weight part necessary for precipitation (W insol.), forming stoichiometric complexes (W stoich.) and solubilization (W crit.) of the proteins, the following range can be written: W insol. less than W stoich. less than W crit.