Lower Incorporation of Impurities in Ferritin Crystals by Suppression of Convection:  Modeling Results

We have developed an axi-symmetric time-dependent numerical model of the diffusive-convective transport of a crystallizing protein and an impurity, in an isothermal crystal growth system at standard and zero gravity. We model crystallization of ferritin, a protein with M w = 440 000, and its most abundant impurity, the protein's native dimer. The diffusivities of the protein and the dimer and the kinetic coefficients for crystallization and impurity incorporation are available. The model assumes the geometry of a cylindrical vessel used in protein crystallization trials on earth and in space, the DCAM. At terrestrial gravity, buoyancy-driven convection with a maximum velocity of 12 μm/s enhances the supply of both protein and impurity. In the absence of convection, e.g., at 0g, the diffusion depletion zone is wider and the interfacial concentrations drop significantly. The lower diffusivity of the larger dimer results in its incorporation at 0g lower by factors of 2−3 than on earth. The three-dimensional computational scheme used here allows direct comparisons of these results with space and laboratory experimental data. The two data sets agree quantitatively, suggesting that in some cases the improved quality of space-grown crystals as compared to the earth-grown controls may be due to the suppressed supply of larger impurities.