Closing In on the Best Way to Grow Crystals
July 7, 1997 1:30 p.m. CDT
University of California at Riverside (UCR), they are small but important steps in improving protein crystal growth (PCG) in space. (The photo at right shows HH-DTC crystals from the previous flight of MSL-1 in April, 1997)
The video images show crystals growing inside the Handheld Diffusion Test Cells (HH-DTC) developed by Dr. Alexander McPherson of UCR. The samples will be compared with crystals grown by other methods, including equipment on the ground, and will help pave the way for an advanced Observable Protein Crystal Growth Apparatus being developed for later space missions.
A steady series of small steps like these have been crucial to putting the protein crystal growth field on a mature footing since the first experiments in the mid-1980s.
|Computer model of a molecule of hen lysozyme (compared with a molecule of sugar in the) gives an idea of the complexity involved in deciphering the structure of organic molecules. Lysozyme is relatively simple compared to other organic molecules such as human serum albumin, insulin, and segments of DNA and RNA. These images were rendered by the MacMolecule program developed by the University of Arizona.|
"Experiments since then have been dogged by persistent skepticism in a substantial segment of the crystallographic research community," McPherson wrote in the June 1997 issue of Trends in Biotechnology. Early experiments could fly only a limited number of samples on board the Space Shuttle. In recent years, though, more specimens have been flown with equipment, such as the Protein Crystallization Apparatus for Microgravity (PCAM), which is on board MSL-1, and the liquid nitrogen dewar with which McPherson is growing hundreds of samples board the Mir space station.
In addition to flying more samples, scientists like McPherson are also studying the fine details of how crystals grow in microgravity.
"The goal of the second line of investigation," he wrote, "was a definition and description, in a quantitative sense, of the mechanisms by which the quality of crystals was improved in microgravity. Understanding, and in the end, controlling the physics of the process was the objective."
The results from flying more, and studying in more detail, have "significantly altered" attitudes towards space-based PCG. "Persuasive explanations" have emerged, and a strong theoretical model has emerged to explain why space-based growth is better.
Detailed analyses of crystals grown on the ground show that they have defects that are more complex and extensive than scientists had thought, partly because of how they grow on the ground.
To grow a crystal, the protein is dissolved in a liquid and the liquid is changed to force the protein molecules together. This change could be evaporating the liquid, as in PCAM and VDA-2, or adding another liquid that draws water from the solution. Either way, the concentration of the protein is raised until it is supersaturated and the molecules can't stay dissolved; they are forced together and link to become the crystal.
On the ground, The dense solution settles to the bottom where supersaturation causes the crystals to grow too fast and can also pull impurities into the crystal.
Conditions in space are self-regulating, McPherson wrote. Instead of convection making the dense solution flow down, the molecules move by diffusion, slowly pushing themselves from the concentrated areas into less concentrated areas next to the crystal. This also filters out impurities and leads to "a significant difference in ultimate crystal quality."
The proof is in the X-rays. Like light reflecting through a chandelier, X-rays are reflected off the interior of a crystal and form a pattern of dots unique to the crystal. Applying a bit of math to the dots lets scientists decode the structure of the crystal.
Space-grown crystals yield dots that are brighter and sharper than those from earth-grown crystals, McPherson wrote.
Still, much remains to be done in refining how we grow crystals in space.
Like many materials science experiments, the HH-DTC (pictured above) is one of the tools being developed to understand the optimum conditions. Like many materials experiments, the HH-DTC provides a way for the astronauts to start, then stop the movement of molecules in the sample. In this case, it's done with a small valve in the middle of the cell. Before the flight, the valve is closed, separating the chamber into three parts. In space, the astronauts crank the valves to form a continuous chamber and let the crystals grow. At the end of the mission, the crew rotates the valve again to separate the chamber.
The objective is to let the crystal grow as the precipitant solution in one chamber slowly moves into the protein chamber and changes the concentration. The narrow valve acts as a bottleneck, slowing the diffusion to a deliberate pace.
MSL-1 carries 32 HH-DTC cells, four units containing eight cells each. The cells are tiny, about the size of a shortened cartridge for a fountain pen.
Unlike the other PCG experiments on MSL-1, the HH-DTC does not address any specific disease. The specimens include lysozyme, hemoglobin, satellite tobacco mosaic virus, and other proteins that have been studied extensively in space and on the ground. This allows investigators to make true "apples and apples" comparisons of different PCG methods.
Every few days the flight crew puts the HH-DTC apparatus under the TV camera to give investigators on Earth a look at the crystals as they form.
These images - reddened by the light-emitting diodes that backlight the scenes - provide intermediate measures that will help McPherson and his team in analyzing the crystals after the mission.
For those who want to dig into more details, McPherson's article is:
"Recent advances in the microgravity crystallization of biological macromolecules." Trends in Biotechnology. June 1997, Vol. 15, No. 6 (161), pp 197-200.
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