The effect of temperature and ionic strength on the dimerisation of beta-lactoglobulin

Bovine beta-lactoglobulin is a globular whey protein that associates partly and reversibly in dimers in the native state. Static and dynamic light scattering techniques have been used to determine the relative amount of monomers and dimers, and to estimate their size and shape in different conditions. The effect of the ionic strength on the dimerisation has been studied at pH 2, where the protein is highly charged. A simple model, taking into account the monomer-dimer equilibrium and virial interactions has been used. The strength of the interactions depends on the amount of added salt, i.e. ionic strength, and influences the dimer dissociation. When the ionic strength decreases, the equilibrium is shifted towards the monomeric form. The dissociation, however, is only complete at very low concentration. The effect of temperature has been studied at pH 7 and low concentration where virial interactions are negligible. The dissociation increases with increasing temperature (5 degrees C - 76 degrees C). At high temperatures, protein-protein aggregation is fast, even at low concentration. The temperature dependence can be described using a simple Van't Hoff model, even at temperatures where aggregation occurs. The ionic strength and temperature dependence both indicate that beta-lactoglobulin solutions have to be considered as a mixture of monomers and dimers.