Protein crystal growth--microgravity aspects.

Protein crystals, grown under reduced gravity conditions, are either superior or inferior in their structural perfection than their Earth-grown counterparts. A reduction of the crystals' quality due to low-gravity effects on the growth processes cannot be understood from existing models. In this paper we put forth a rationale which predicts either advantages or disadvantages of microgravity growth. This rationale is based on the changes in the effective solute and impurity supply rates in microgravity and their effects on the intrinsic growth rate fluctuations that arise from the coupling of bulk transport to nonlinear interfacial kinetics and cause severe inhomogeneities. Depending on the specific diffusivity and kinetic coefficient of a protein and the impurities in the solution, either transport enhancement through forced flow or transport suppression under reduced gravity can result in a reduction of the kinetic fluctuations and, thus, growth with higher structural perfection. Investigating this mechanism of microgravity effects, we first demonstrate a one-to-one correspondence between these fluctuations, that are due to the bunching of growth steps, and the formation of defects in the crystals. We have confirmed the forced flow aspects of this rationale in ground-based experiments with lysozyme utilizing flowing solutions with varying, well characterized impurity contents.