We study, using wide-angle and small-angle light scattering techniques, the stability and aggregation/gelation behaviors of denatured filamentous bovine serum albumin pre-aggregates (BSA-PAs), induced by CaCl2. It is observed that transparent filamentous gels can be formed not only at low CaCl2 concentrations but also at high CaCl2 concentrations, while turbid gels are obtained at intermediate CaCl2 concentrations. Although the filamentous gels at low CaCl2 concentrations and the turbid gels at intermediate CaCl2 concentrations are consistent with the literature observations, the filamentous gels at high CaCl2 concentrations have to be explained by different mechanisms. The latter is attributed to the repulsive hydration interactions originating from increased surface dipoles generated by counterion binding. Since such surface dipole-induced hydration is very short-range and occurs mainly on charged or polar patches of proteins (thus protected from aggregation), the aggregation of the filamentous BSA-PAs at hydrophobic patches at the two ends is still possible, leading to formation of the filamentous gels.
