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 β-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 β-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.