THE PROTEIN-WATER PHASE DIAGRAM AND THE GROWTH OF PROTEIN CRYSTALS FROM AQUEOUS SOLUTIONS

Cornelis Haas and Jan Drenth

Laboratory for Biophysical Chemistry, University of Groningen, Nyenborgh 4, 9747 AG Groningen, The Netherlands
E-mail: j.drenth@chem.rug.nl

Keywords: Protein crystallization / Phase diagram / Nucleation / NMR

The phase diagram of a protein - water system is described with a simple model with parameters for the interaction between the protein molecules in the crystal and in the solution. For a certain range of these parameters the phase diagram shows a metastable liquid - liquid immiscibility region. It is shown that this region corresponds to the "crystallization slot" for growing protein crystals, as proposed by George and Wilson [Acta Crystallogr. D50, 361 - 365 (1994)]. Nucleation in this region proceeds in two steps. First, small liquid droplets with a high protein concentration are formed; then small crystalline nuclei grow inside these droplets. Crystals grown in the crystalline slot conditions are covered by a thin liquid film with a high protein concentration.

We also report NMR experiments on lysozyme, which show that nucleation is a transient process with an induction time.