Salt-induced association of beta-lactoglobulin by light and X-ray scattering

beta-Lactoglobulin, a protein belonging to the lipocaline family, is studied by Photon correlation spectroscopy and small angle X-ray scattering in acidic solutions (pH = 2.3) and at ionic strengths in the range 7-507 mM. Both experiments give a clear evidence of a protein monomer-dimer equilibrium affected by the ionic strength of the solution. The interparticle interactions are determined by means of Photon correlation spectroscopy by following the trend of the protein diffusion coefficient versus concentration (from 0.2 to 25 g/L) at ionic strengths of 7 and 107 mM. From a detailed analysis, estimates of the protein charge, hydrodynamic radius, and dimeritazion fraction have been obtained. Small angle X-ray scattering measurements performed versus ionic strength (from 7 to 507 mM) in dilute solutions (10 g/L) allow an independent estimate of the dimerization fraction.;A global fit of the SAXS scattered intensities at different salt concentrations using a simple electrostatic model for the aggregation mechanism is presented. As a result, the dissociation free energy has been obtained as a function of the ionic strength, yielding an excellent agreement with photon correlation spectroscopy data.