Pressure dissociation of β-lactoglobulin oligomers near their isoelectric point
We study pressure dissociation of aggregated states of b-lactoglobulin at pH 4.6 using static and dynamic light scattering. We find that octamers dissociate at 20 C into dimers at pressures P < 100 MPa and into monomers at P < 200 MPa. The dimer–monomer dissociation equilibrium at T ¼ 20 C is reversible. The dodecamer–dimer equilibrium is quasi-reversible since aggregation occurred after decompression from 30 MPa back to ambient pressure. The corresponding molar volume difference is DV¼101 ml mol1 for the octamer–dimer and DV¼276 ml mol1 for the dimer–monomer transition. The calculated free energy of association for the dimers at atmospheric pressure is DG ¼59.3 kJ 1 1 mol1 and for the monomers is DG¼59.8 kJ mol1. The high pressure dissociation is not emphasized ¼ by lowering the temperature. Instead, the quaternary structure of b-lactoglobulin at T ¼10 C remains unchanged up to pressures of P ¼ 180 MPa followed by aggregation at pressures P > 180 MPa.