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

¹ Laboratoire d'Enzymologie et de Biochimie Structurales, UPR 9063 CNRS Bâtiment 34 - 91198 Gif sur Yvette Cedex, France
² Service de Chimie Moleculaire, Groupe Colloïdes, CEA/SACLAY 91191-Gif-sur-Yvette Cedex, France
³ Laboratoire d'Ultrasons et de Fluides Complexes, URA 4, ULP-CNRS rue Blaise Pascal, 67070 Strasbourg Cedex, France

Received: 30 September 1998
Accepted: 29 January 1999

Abstract

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.