Coulombic interaction and ion-protein macroion coupled diffusion evidenced by light scattering

Elastic and quasi-elastic light scattering experiments have been performed on previous deionized lysozyme to follow the concentration dependence of the macroion correlation at very low ionic strength. Nernst-Hartley protein-ion coupled diffusion coefficients have been measured over a range of pH. The analysis of experimental data has been derived within the primitive model of colloidal systems and using the hypernetted chain integral equation. In the absence of added salt, unusual behaviors are obtained for the scattered intensity and the mutual diffusion coefficient which are due to the strong, weakly screened, Coulombic coupling, to the local electroneutrality condition and to the low charge and size asymmetry between proteins and counterions. At high salinity, the electrostatic repulsion vanishes and the pair protein potential reduces to a short-range attraction which is responsible for a phase separation at finite concentration.