“Sticky hard spheres” model of proteins near crystallization: A test based on the osmotic compressibility of lysozyme solutions

In order to establish a simple model of protein interparticle interaction near crystallization conditions, we have measured the osmotic compressibility of the metastable fluid phase of hen egg-white lysozyme within the fluid-solid coexistence region. Light scattering measurements were performed in an extended volume fraction range at $\mathrm{pH}=4.7\mathrm{}$ as a function of temperature. A sufficient amount of NaCl has been added in order to severely screen the electrostatic interactions. Below 0.2 M NaCl, the system can still be kept in a metastable fluid phase for a sufficiently long time before nucleation of the crystal phase takes place. The experimental compressibility up to particle volume fractions $\Phi \approx 0.23$ is compared to the theoretical expression for adhesive (“sticky”) hard spheres, and the agreement is found satisfactory. We also determined the values of the Baxter τ adhesiveness parameter, which shows an apparent linear temperature dependence within the investigated temperature range. These results suggest that the main temperature dependence of the interparticle interaction potential is simply through the Boltzmann $k_{B}T$ factor.