Coulombic interaction and ion-protein macroion coupled diffusion evidencedby light scattering

Elastic and quasi-elastic light scattering experiments have been performed on previous deionized lysozyme to follow the concentration dependence of th e macroion correlation at very low ionic strength. Nernst-Hartley protein-i on coupled diffusion coefficients have been measured over a range of pH. Th e 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 scattere d intensity and the mutual diffusion coefficient which are due to the stron g, weakly screened, Coulombic coupling, to the local electroneutrality cond ition and to the low charge and size asymmetry between proteins and counter ions. At high salinity, the electrostatic repulsion vanishes and the pair p rotein potential reduces to a short-range attraction which is responsible f or a phase separation at finite concentration.