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 interact ion near crystallization conditions, we have measured the osmotic comp ressibility of the metastable fluid phase of hen egg-white lysozyme wi thin the fluid-solid coexistence-region. Light scattering measurements were performed in an extended volume fraction range at pH=4.7 as a fu nction of temperature. A sufficient amount of NaCl has been added in o rder to severely screen the electrostatic interactions. Below 0.2 M Na Cl, the system can still be kept in a metastable fluid phase for a suf ficiently long time before nucleation of the crystal phase takes place . The experimental compressibility up to particle volume fractions Phi approximate to 0.23 is compared to the theoretical expression for adh esive (''sticky'') hard spheres, and the agreement is found satisfacto ry. We also determined the values of the Baxter tau adhesiveness param eter, which shows an apparent linear temperature dependence within the investigated temperature range. These results suggest that the main t emperature dependence of the interparticle interaction potential is si mply through the Boltzmann k(B)T factor.