(a) The white ring is a ceramic helical insert on which the screw sample holder is placed. The scale bar is 10 mm. (b) The body of the growth cell is made of quartz glass (orange). Two capillaries of quartz glass with rubber stoppers (yellow) are fixed to the body with an adhesive. Tubes of elastomer (red) are attached to each capillary. After the growth cell is filled with the growth solution (light blue), the tubes are closed with metal wires (arrows). (c) Schematic illustration of the growth cell. Credit: K. Tsukamoto et al/Tohoku University
Crystal growth rate on ISS by interferometry will show the effects of microgravity. Sometimes, distance can lend a new perspective to a problem. For Japanese researchers studying protein crystal growth, that distance was 250 miles up – the altitude at which the International Space Station (ISS) orbits the Earth. To better isolate the growth of protein crystals from the effects of gravity, Katsuo Tsukamoto and Japan Aerospace Exploration Agency grew crystals in a specially-designed chamber onboard the ISS.
The researchers monitored the very slow growth and dissolution rate – ~1cm/s of the crystals by laser interferometry. This was the first time the technique had been used onboard the ISS to measure the growth rate of the crystals at various temperatures. “We are interested in the growth mechanisms of a space-grown protein crystal – a lysozyme crystal – as a model crystal to understand why space-grown crystals sometimes do show better quality than the Earth-grown crystals,” said T. Yamazaki
The process, NanoStep, was performed in the Japanese Experimental Module (KIBO) of ISS in 2012. Tsukamoto and his colleagues had previously measured the growth rates of protein crystals under simulated microgravity by using a Russian recoverable satellite and aircraft in parabolic flights.
They took measurements of the growth rate of the lysozyme crystals vs their driving force, supersaturation (natural logarithm of protein’s concentration divided by its solubility) – with measurements of the solution’s refractive index distribution obtained through interferometry. This also yielded crucial information about the growth mechanism. They modified supersaturation of the solution by increasing/decreasing the growth cell’s temp remotely, 10 to 40C, which necessitated building a closed growth cell to withstand the stresses caused by the thermal expansion of the growth solution.
The closed, cube-like growth cell was constructed out of quartz glasses with different thickness, an essential component for laser interferometry due to its high chemical and mechanical resistances with a protein seed crystal glued to the top of the sample holder. To relieve the thermal stress on the glass, the researchers attached tubes made out of an elastomer, low-moisture-permeability thermoelastic polymer to mitigate evaporation of water in the crystal growth solution, which consisted of 30 or 35 mg/ml of lysozyme and 25 mg/ml NaCL in 50 mM sodium acetate buffer solution. They also employed a special spring tension system to reduce stress by keeping the gap between the glass cell and thermal control modules constant amid thermal expansion.
While the researchers expected growth rates of the crystal solution to be slower because of the suppression of solution convection, the results instead showed an increased growth rate. This may be due to the suppression of transport speed of impurity molecules with larger diameter to the growing crystal, as analyzed the growth rate versus supersaturation relations. This will appear in forthcoming papers.
Extended projects for the researchers using the same apparatus to test the growth of different crystals, such as glucose isomerase crystals, are currently in preparation. Â http://www.eurekalert.org/pub_releases/2016-03/aiop-ocg031516.php
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