Limitations

However, in some of the cases described above, the investigators were not able to make the comparisons needed to demonstrate that growth in microgravity was indeed the factor responsible for producing higher quality crystals. The incorporation of additional features in the analysis of the space-grown crystals, such as the use of cryogenic techniques and synchrotron radiation, makes it difficult to be certain that the improvements are due to microgravity and not to some of these additional factors (DeLucas et al 1989). Also, it is not enough to compare space-grown crystals to crystals grown on Earth in the same equipment and solution over the same time period. The microgravity-grown crystals must also be compared to the best result from all Earth-based attempts at growing the crystal regardless of crystallization conditions, equipment, or time of growth. This latter comparison is the baseline standard for defining success.

Today's high-energy synchrotron3 sources have, in general, eliminated crystal size as the key factor in increasing the diffraction resolution limit. This was not the case when the space crystallization program began in the mid-1980's. Scientists today are interested in crystallization methods that provide higher quality crystals, where quality is measured by disorder and mosaicity. Therefore, a well-ordered crystal of average dimensions (around 30 to 50 |im) is all that is needed for effective diffraction studies. Synchrotron technology continues to improve, and the target crystal size may decrease even further before the ISS is completed. Crystal quality, rather than crystal growth, is thus the primary focus of the biological macromolecular research community.

Another limitation is that all research on protein crystallization in space so far has been done under less than optimal conditions. Most of the work has been done on fairly short Space Shuttle flights, with a few experiments occurring on the Russian Mir space station. The crystallization work has been generally restricted to a matter of days, which is not enough time in most cases to complete the crystallization process, especially in space, where crystals appear to nucleate and grow more slowly. Except for Spacelab missions devoted exclusively to microgravity research, the environment has generally not been free of noise and vibration. No mechanism has been

3 A synchrotron is a ring-shaped accelerator in which charged particles are accelerated by a magnetic field and an electric field. The high-density X-rays produced by this particle accelerator are used for gathering crystallographic data for structural determination.

provided to stabilize the crystals that do grow and to protect them from the stresses of reentry. In general, the ability to visualize crystal growth in space has been extremely limited, preventing investigators from determining if flawed crystals examined after landing had failed to grow well in space or if crystals with good morphology had indeed been grown but later had been damaged during reentry. Like for the other disciplines in space biology, the irregular schedules of space missions and the long lead times have made it difficult for scientists engaged in an extremely competitive research field to seriously consider participation in space experiments. The slow and uncertain progression of space experiments has disconnected them from the even more rapid tempo of contemporary protein crystallography research (National Research Council 2000).

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