An observable protein crystal growth apparatus for studying the effects of microgravity on protein crystallization

Abstract

Macromolecular crystals during their growth, incorporate an extensive array of impurities which vary from individual molecules to large particles, and even microcrystals in the micron size range. AFM (atomic force microscopy) along with X-ray topology has shown that the density of defects and faults in most macromolecular crystals is several orders of magnitude higher than in conventional crystals. High defect and impurity density contributes, in turn, to a deterioration of both the mechanical and diffraction properties of crystals, thereby lessening their value for structural biology. In microgravity, access by impurities and aggregates to growing crystal surfaces is restricted due to the elimination of convention and to altered fluid transport properties. We designed, and have now completed construction of an instrument, the OPCGA (Observable Protein Crystal Growth Apparatus) that employs a fused optics, phase shift, Mach-Zehnder interferometer, along with polarized light, time lapse video microscopy to analyze the fluid environment around growing crystals. Using this device, which will ultimately be deployed on the International Space Station, we have, in thin cells on Earth, succeeded in directly visualizing macromolecule concentration gradients around growing protein crystals. This provides the first direct evidence that quasi-stable depletion zones formed around growing crystals in space may explain the improved quality of macromolecular crystals grown in microgravity. Further application of the interferometric technique will allow us to quantitatively describe the shapes, extent, and magnitudes of the concentration gradients and to evaluate their degree of stability. The OPCGA ultimately will be used by the broad crystal growth community to study, and quantitatively describe the development of a vast range of macromolecular crystals. This will have a significant impact on our understanding of crystal growth phenomena and our ability to improve and control the process on earth.