Determination of the osmotic second virial coefficient and the dimerization of β-lactoglobulin in aqueous solutions with added salt at the isoelectric point

Aqueous solutions of beta-lactoglobulin (at the isoelectric point pH = 5.18) have been studied by membrane osmometry. The osmotic second virial coefficient as well as the monomer-dimer equilibrium of beta-lactoglobulin have been found to depend significantly on the salt concentration. At low salt concentration the virial coefficient becomes negative, which could be attributed to dipole-dipole interactions which become important at the isoelectric point of the protein when the salt concentration decreases.