Numerical Analysis of the Diffusive Field around a Growing Protein Crystal in Microgravity

t is believed that a microgravity environment may maintain ideal depletion zones of protein (PDZ) and impurity (IDZ) around growing crystals and may contribute to growing high-quality crystals. This can lead to an X-ray diffraction data collection of higher resolution with lower mosaicity, because of the better internal order and fewer defects in the crystals when compared to ground-grown crystals. The extent of these depletion zones are dependent on a competition between the diffusion of the molecules in the solution (indexed by the diffusion coefficient, D) and the adsorption of those into the growing crystal (indexed by the kinetic constant, β). If we use the D/β value as an index of the extent of PDZ and IDZ, a lower D/β value is ideal for maintaining PDZ and IDZ. Using experimental results, we could easily obtain the D/β value. When we combined the D/β value with the quality of protein crystals obtained in microgravity experiments provided by Japanese Space Agency (JAXA), we found that the effects of microgravity contributed to obtaining superior crystals especially if the D/β value was less than 3 mm. The numerical analysis of the PDZ and IDZ shows that the radius of the crystal (R) is also related to the PDZ and the IDZ. If the Rβ/D value is large, both the PDZ and the IDZ provide a filtration effect, but if the Rβ/D value is small, only the IDZ does.