The molecular structure of biological macromolecules is important in
understanding how these molecules work and has direct application to rational drug design for new
medicines and for the improvement and development of industrial enzymes. In order to obtain the
molecular structure, large, well formed, single macromolecule crystals are required. The growth of
macromolecule crystals is a difficult task and is often hampered on the ground by fluid flows
that result from the interaction of gravity with the crystal growth process.
One such effect is the bulk movement of the crystal through the fluid due to
sedimentation. A second is buoyancy driven convection close to the crystal surface. On the ground the crystallization process itself induces both of these flows. Buoyancy driven convection results from density differences between the bulk solution and fluid close to the crystal surface which has been depleted of macromolecules due to crystal growth. Figure 1 is a Schlieren photograph of a growing lysozyme crystal illustrating a ‘growth plume’ resulting from buoyancy driven convection. Both sedimentation and buoyancy driven convection have a negative effect on crystal growth and microgravity is seen as a way to both greatly reduce sedimentation and provide greater stability for ‘depletion zones’ around growing crystals. Some current crystal growth hardware however such as those based on a vapor diffusion techniques, may also be introducing unwanted
Marangoni convection which becomes more pronounced in microgravity (Chayen et al.1997).
Negative effects of g-jitter on crystal growth have also been observed (Snell et al.
1997).
To study the magnitude of fluid flows around growing crystals we have attached a
number of different fluorescent probes to lysozyme molecules. At low concentrations, <40%
of the total protein, the probes do not appear to effect the crystal growth process (Figure
2). By using these probes we expect to determine not only the effect of induced flows due to
crystal growth hardware design but also hope to optimize crystallization hardware so that
destructive flows are minimized both on the ground and in microgravity.
Pusey, M., Snell, E., Judge, R., Chayen, N., Boggon, T., Helliwell, J., Fluid Physics and Macromolecular Crystal Growth in Microgravity, Fifth Microgravity Fluids Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1260-1263, August 9, 2000.