Use of dual polarization interferometry as a diagnostic tool for protein crystallization

The use of dual polarization interferometry (DPI) as a tool for probing the different possible outcomes of protein crystallization experiments is described. DPI is a surface analytical technique used for the characterization of structure and interactions of molecular layers on an optical waveguide surface for a wide range of applications, including protein-protein interactions and conformational changes. The application of this technique provides a "signature" of crystallization events, thus predicting if there will be protein crystal formation, amorphous precipitate, or clear solution. The technique was demonstrated on a number of model proteins, and it also produced meaningful results in the case of two problematic target proteins. DPI in conjunction with a dialysis setup, allows changes in the protein solution above the waveguide surface to be monitored simultaneously with continuous control of its precipitant content. DPI has the potential to be used as a powerful method for discovering crystallization conditions, for obtaining information on the crystallization process, and as an aid in crystal optimization. It has also provided what is, to the best of our knowledge, the most direct observation to date of salting-in behavior in a protein-salt solution.