Controlling the Diffusive Field to Grow a Higher Quality Protein Crystal in Microgravity

Growing high quality crystals is a bottleneck in the multi-stepped process of three-dimensional structural analyses of protein. It is known that a microgravity environment may maintain ideal depletion zones of protein and impurity around a growing crystal and the filtering effect of these depletion zones may contribute to obtaining high-resolution X-ray diffracting crystals with superior internal order. The effects of these depletion zones around growing crystals are thought to be the main mechanisms for the improvement of crystal quality in microgravity. A competition between the diffusion of protein molecules in the solution (indexed by the diffusion coefficient, D) and the adsorption of those into the growing crystal (indexed by the kinetic coeffcient, β) decides the extent of depletion zones. Lower D values and higher β values indicate that these effects are more obvious in numerical analyses. Therefore we use the D/β value as an index for these effects. The most effective method of lowering the D/β value is using viscous precipitant reagents, such as a high molecular weight polyethylene glycol (PEG) to decrease the D value and using highly homogenous protein samples to increase the β value. In this report, we briefly introduce simple yet practical methods of estimating D and β values followed by a numerical analysis to understand the filtration effects, and the results of crystallization experiments in microgravity when controlling the diffusive field around the growing crystals using the D/β value as an index.