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Diffusion has a central role in protein crystal growth both in microgravity conditions and on ground. Recently several reports have been focused on the importance to use the generalized Fick's equations in n-component systems where crystals grow. In these equations the total flux of each component is produced by the own concentration gradient (main flow) and by the concentration gradient of the other components (cross-flow) present in the system. However in literature the latter effect is often neglected, and the so-called pseudo-binary approximation is used. Lin et al. (1995) proposed a mathematical model to evaluate the concentration profile of the species present around a growing protein crystal. Although the model is reliable, it suffers of the pseudo-binary approximation (neglecting cross term diffusion coefficients and using binary diffusion coefficients), probably because of the lack of multicomponent diffusion data. The present model is based on the experimental set-up proposed by Lin et al. (1995). Nevertheless we have included the coupled diffusion effects, according to the correct description of the matter transport through the generalized Fick's equations. The crystal growth rate is calculated for different gravity levels. The model has been applied to the ternary lysozyme-NaCl-water and quaternary lysozyme-poly(ethylene glycol) (PEG)-NaCl-water systems using recent diffusion data.
research papers
Single chains of the collagen model polypeptide with sequence (Pro-Pro-Gly)10, hereafter referred to as (PPG)10, aggregate to form rod-shaped triple helices. Crystals of (PPG)10 were grown in the Advanced Protein Crystallization Facility (APCF) both onboard the International Space Station (ISS) and on Earth. The experiments allow the direct comparison of four different crystallization environments for the first time: solution in microgravity (μg), agarose gel in μg, solution on earth, and gel on earth. Both on board and on ground, the crystal growth was monitored by a CCD video camera. The image analysis provided information on the spatial distribution of the crystals, their movement and their growth rate. The analysis of the distribution of crystals reveals that the crystallization process occurs as it does in batch conditions. Slow motions have been observed onboard the ISS. Different to Space-Shuttle experiment, the crystals onboard the ISS moved coherently and followed parallel trajectories. Growth rate and induction time are very similar both in gel and in solution, suggesting that the crystal growth rate is controlled by the kinetics at the interface under the used experimental conditions. These results provide the first data in the crystallogenesis of (PPG)10, which is a representative member of non-globular, rod-like proteins.
research papers
Crystals of the collagen-like polypeptide (PPG)10 were obtained within the Advanced Protein Crystallization Facility on board the International Space Station, during the STS-105/STS-108 mission. The duration of this mission was such to ensure that the crystallization process had reached its end. Crystals were grown both in the presence and in the absence of agarose gel, to compare the quality of the crystals obtained from these different environments. As a result, crystals grown in the absence of agarose on Earth as well as in microgravity showed X-ray diffraction up to 1.15 Å. The intensity/sigma ratio was slightly higher for microgravity grown crystals. Crystals grown in agarose gel, both in microgravity and on ground, showed a comparable diffraction power, with a resolution limit of 1.45 Å.
