Crystal Growth

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Crystal Growth

Among the most productive areas of space-based research have been the investigations into growing protein crystals. Proteins are complex biological molecules in all living things, critical for a variety of functions, such as transporting oxygen and chemicals in the blood, forming the major components of muscle and skin, and fighting diseases. Research efforts are focused on understanding the structure of individual proteins, how the structure affects the protein's function, and the design of drugs to interfere with or enhance the protein's function. The human body alone contains more than 100,000 proteins. Scientists, however, know the structure of only about 1 percent of them.

Many diseases involve proteins, such as toxins secreted by invading organisms, or proteins an invading organism needs to survive and spread. Angeogenin, for example, is a protein produced by tumor cells to help lure blood vessels toward the tumor. Cells infected with the HIV virus need the protein HIV protease to replicate. Studying these proteins helps pharmaceutical researchers design drugs to fight the diseases. Protein crystal studies also benefit other areas of biotechnology, such as the development of disease-resistant food crops and basic biological research.

The first step to understanding how proteins function is to produce crystals that are big enough and uniform enough to provide useful structural information upon analysis. Protein crystals are cultivated by moving large molecules through a fluid. Gradually, the concentration of the protein solution is increased so that the growing protein molecules contact each other and form a complex crystal. Temperature, salt concentration, pH balance, and other factors all affect the protein crystal's formation.

On Earth, protein crystal growth is hampered by convective flows, as molecules diffuse from the surrounding solution and join the growing crystal structure. The solution bordering the crystal then contains a lower protein concentration than the remainder of the solution, and therefore, a lower density. This less-dense solution tends to rise, and the denser solution sinks under the influence of gravity, creating eddies next to the crystal. These convective currents are harmful because they alter the orientation of the protein molecules as they hook onto the crystal lattice .

Earth-grown crystals also are adversely affected by sedimentation. Once crystals have grown large enough, the suspending solution can no longer support its mass and the crystals fall on top of each other and grow together. Proper analysis of the protein crystals requires individual molecules.

Space-grown crystals tend to be larger and better organized than their terrestrial counterparts. The microgravity environment minimizes sedimentation and the effects of convection on the crystal, resulting in a more uniform, highly ordered molecular structure. The space-grown crystals thus have fewer defects than Earth-grown crystals.

The National Aeronautics and Space Administration (NASA) has flown dozens of protein crystal growth experiments on the space shuttles and plans to continue the investigations aboard the International Space Station. The delicate space crystals, which are about the size of a grain of salt, are returned to Earth for analysis. A process called X-ray crystallography is used to reveal the inner structure of the protein. Unlike dental X rays , this technique does not produce a shadow image, but a diffraction pattern, as the X rays bounce through the crystal structure. The scattered X rays are recorded on photographic film or electron counters. This data is then fed into a computer, which can perform precise measurements of the intensity of the X rays scattered by each crystal, helping scientists to map the probable positions of the atoms within each protein molecule.

The cleaner the structure of the protein, the more defined the diffraction patterns will be. Once the protein is mapped, researchers look for receptor sites and active areas on the protein where it will connect with other molecules, somewhat like a lock and key. From this information, drugs can be designed to aid protein interaction or block it, without affecting the rest of the body.

see also International Space Station (volumes 1 and 3); Made in Space (volume 1); Microgravity (volume 2); Space Shuttle (volume 3); Space Walks (volume 3).

Irene Brown