How Does Buoyancy-driven Convection Affect Biological Macromolecular Crystallization? An Analysis of Microgravity and Hypergravity Effects by Means of Magnetic Field Gradients

Abstract

The production of crystals of adequate size and high quality is the "bottleneck'' for three-dimensional structure analysis of protein crystals. In this work, in order to shed additional light on the (still controversial) beneficial effect of microgravity on crystal growth, we focus on recent advanced experimental and theoretical research about the effects of buoyancy-driven convection on protein crystallization. In the light of the numerical studies the following major outcomes can be highlighted: (1) when the crystal size exceeds several dozens of µm, buoyancy-driven convection dominates solute transport near the growing crystal and the crystal growth rate becomes larger than that under zero gravity. (2) The ratio of the side-surface growth rate to the top-surface growth rate increases with crystal size because of convection and the ratio is about three when the crystal size is 100 µm. The ratio of the side-surface growth rate to the top-surface growth rate measured experimentally confirms these results (the averaged value for 127 protein crystals was determined to be about two). Thus, both numerical and experimental studies provide a solid basis to the idea that convection strongly affects the crystal growth rate. Moreover, since according to experiments about the dependence of crystal quality on effective gravity (hypergravity or microgravity obtained by means of magnetic field gradients), protein crystals (e.g., orthorhombic lysozyme or snake muscle fructose-1,6-bisphosphatase) exhibit better quality with decreasing the gravity level, buoyancy-driven convection may be thought of as also affecting crystal quality in a detrimental way.