The formation of a protein depletion zone (PDZ) and an impurity depletion zone (IDZ) around a growing crystal due to the suppression of a convection flow were thought to be the major effects of microgravity on higher quality protein crystal growth. We studied those effects numerically using a simplified model, and found out that the combination of the diffusion coefficient of the protein molecule (D) and the kinetic coefficient for the protein molecule (β) could be an index of the extent of these depletion zones. According to this analysis, we recommended to use high-viscous crystallization reagent to decrease D on purpose and further-purified protein sample to increase β for microgravity experiment. As a result, for some protein samples, this approach may have increased the microgravity effects and grew higher quality crystals. There are still some more details to be elucidated, but when our technology is more established and can be applied to more variety of protein samples, the crystallization in microgravity will be more useful method and will contribute to the X-ray structural analysis more practically. In this article, we explain current status of JAXA PCG, and introduce the rational approach to high-quality protein crystal growth experiment in microgravity based on numerical analyses.
