Diffusivities of lysozyme in aqueous MgCl2 solutions from dynamic light-scattering data: Effect of protein and salt concentrations

Dynamic light-scattering (DLS) studies are reported for lysozyme in aqueous magnesium chloride solutions at ionic strengths 0.6, 0.8, and 1.0 M for a temperature range 10-30 degrees C at pH 4.0. The diffusion coefficient of l ysozyme was calculated as a function of protein concentration, salt concent ration, temperature, and scattering angle. A Zimm-plot analysis provided th e infinitely-dilute diffusion coefficient and the protein-concentration dep endence of the diffusion coefficient. The hydrodynamic radius of a lysozyme monomer was obtained from the Stokes-Einstein equation; it is 18.6 +/- 1.0 Angstrom. The difference (1.4 Angstrom) between the hydrodynamic and the c rystal-structure radius is attributed to binding of Mg2+ ions to the protei n surface and subsequent water structuring. The effect of protein concentra tion on the diffusion coefficient indicates that attractive interactions in crease as the temperature falls at fixed salt concentration. However, when plotted against ionic strength, attractive interactions exhibit a maximum a t ionic strength 0.84 M, probably because Mg2+-protein binding and water st ructuring become increasingly important as the concentration of magnesium i on rises. The present work suggests that inclusion of ion binding and water structuring at the protein surface in a pair-potential model is needed to achieve accurate predictions of protein-solution phase behavior.