Proteins in solution : from X-ray scattering intensities to interaction potentials

Biological macromolecules in solution interact with each other through medium-range (from a few Å to a few nm) interaction potentials. These potentials control the macromolecular distribution in solution, the macromolecular phase diagram and the crystallization process. We have previously shown that small angle X-ray scattering (SAXS) is a convenient tool to characterize the resulting potential, either attractive or repulsive, and to follow the changes induced by the crystallizing agents. In the present paper SAXS and simulation methods derived from statistical mechanics are coupled to determine the best fit potentials from the comparison of experimental and theoretical intensity curves. The currently used models in the colloid field are derived from the DLVO (Derjaguin, Landau, Verwey, Overbeek) potential where three types of interactions play a major role: hard sphere and electrostatic are repulsive, van der Waals are attractive. A combination of a short-range attractive potential and a coulombic repulsive indeed correctly accounts at low ionic strength for the phase diagram as a function of pH and salt concentration. The origin of the ion specificities at high ionic strength associated with the so-called “Hofmeister series” remain, however, unclear. The whole of the data demonstrates that the colloidal approach may be applied with success to protein crystallization.