Synchrotron radiation is ideally suited for measuring the highest resolution data from protein crystals. Such data have utility in accurately defining the structural parameters of macromolecules. It is generally believed that resolution can be improved over conventional sources by a few tenths or even as much as 1.0 _ by using synchrotron radiation, especially when combined with crystal freezing. During the past year, MacCHESS has begun to explore ultra-high resolution (better than 1.2_) data collection. We have identified almost 50 proteins that diffract to true atomic resolution. Ultra-high resolution data have utility in refining protein structures without introducing geometrical restraints and in the direct phasing (without heavy atoms or anomalous scatterers) of macromolecular X-ray data. Thus far we have collected data for celluase E2 (1.0 _), concanvalin A (0.9_), xylan esterase (0.8_) and monoclinic lysozyme (1.0 _). In the case of xylan esterase, the data will be used for direct phasing in collaboration with Herb Hauptman and coworkers in Buffalo. The other data is being used for high resolution refinements. The data sets mentioned above were collected using mostly routine procedures. In the future, we will explore modified data collection schemes, such as phi slicing, to improve the signal to noise ratio. We will also develop new data processing procedures including improved profile fitting methods, improved background estimations and improved scaling procedures. We will also explore the effects of beam size, crystal size, X-ray energy and others in producing the best high resolution data.
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