Abstract
It is said that the microgravity environment positively affects the quality of protein crystal growth. The formation of a protein depletion zone and an impurity depletion zone due to the suppression of convection flow were thought to be the major reasons. In microgravity, the incorporation of molecules into a crystal largely depends on diffusive transport, so the incorporated molecules will be allocated in an orderly manner and the impurity uptake will be suppressed, resulting in highly ordered crystals. Previously, these effects were numerically studied in a steady state using a simplified model and it was determined that the combination of the diffusion coefficient of the protein molecule (D) and the kinetic constant for the protein molecule (β) could be used as an index of the extent of these depletion zones. In this report, numerical analysis of these depletion zones around a growing crystal in a non-steady (i.e. transient) state is introduced, suggesting that this model may be used for the quantitative analysis of these depletion zones in the microgravity environment.
Highlights
Protein crystal growth experiments are a promising area in the usage of microgravity to contribute to structural biology (McPherson, 1999; Littke & John, 1986; Kundrot et al, 2001; Vergara et al, 2003)
When protein molecules are taken into a crystal, a spherical area of low protein concentration is formed around the growing crystal
A density-driven flow occurs to supply protein molecules to the low concentration area, disturbing this area. In microgravity, this density-driven flow does not occur, so the protein molecules are supplied to the crystal only by thermal diffusion caused by Brownian motion
Summary
Protein crystal growth experiments are a promising area in the usage of microgravity to contribute to structural biology (McPherson, 1999; Littke & John, 1986; Kundrot et al, 2001; Vergara et al, 2003). When protein molecules are taken into a crystal, a spherical area of low protein concentration is formed around the growing crystal. Following similar steps as the protein molecules, a low-impurity concentration area around the growing crystal (impurity depletion zone, IDZ) is formed (Chernov, 1998; Thomas et al, 2000), suppressing crystal disorder. In reality, the process of crystal growth in a conventional protein crystal growth experiment occurs in a non-steady transient state, decreasing protein concentration in the solution as the protein molecules are incorporated into the crystal. We can compare the transient and homogeneous state (terrestrial gravity) with the transient and diffusive state (microgravity), and propose that doi:10.1107/S0909049513022784 1003 diffraction structural biology this model can be applied to the examination of the process of protein crystallization both in microgravity and terrestrial environments, quantitatively
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