The effects of microgravity on protein crystallization: evidence for concentration gradients around growing crystals

Atomic force microscopy (AFM) investigations have revealed that macromolecu lar crystals, during their growth, incorporate an extensive array of impuri ties. These vary from individual molecules to large particles, and microcry stals in the micron size range. AFM, along with X-ray topology, has further shown that the density of defects and faults in most macromolecular crysta ls is very high in comparison with conventional crystals. The high defect d ensity is a consequence of the incorporation of impurities, misoriented nut rient molecules, and aggregates of molecules. High defect and impurity dens ity, contributes to a deterioration of both the mechanical and the diffract ion properties of crystals. In microgravity, access by impurities and aggre gates to growing crystal surfaces is restricted due to altered fluid transp ort properties. We designed, and have now constructed an instrument, the ob servable protein crystal growth apparatus (OPCGA) that employs a fused opti cs, phase shift, Mach-Zehnder interferometer to analyze the fluid environme nt around growing crystals. Using this device, which will ultimately be emp loyed on the international space station, we have, in thin cells on earth, succeeded in directly visualizing concentration gradients around growing pr otein crystals. This provides the first direct evidence that quasi-stable d epletion zones formed around growing crystals in space may explain the impr oved quality of macromolecular crystals grown in microgravity. Further appl ication of the interferometric technique will allow us to quantitatively de scribe the shapes, extent, and magnitudes of the concentration gradients an d to evaluate their degree of stability. (C) 1999 Elsevier Science B.V. All rights reserved.