Pressure dissociation of β-lactoglobulin oligomers near their isoelectric point

We study pressure dissociation of aggregated states of β-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 = 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 ΔV = 101 ml mol^-1 for the octamer-dimer and ΔV = 276 ml mol^-1 for the dimer-monomer transition. The calculated free energy of association for the dimers at atmospheric pressure is ΔG = -59.3 kJ mol^-1 and for the monomers is ΔG = -59.8 kJ mol^-1. The high pressure dissociation is not emphasized by lowering the temperature. Instead, the quaternary structure of β-lactoglobulin at T = 10°C remains unchanged up to pressures of P = 180 MPa followed by aggregation at pressures P > 180 MPa.