Diffraction protein-single-crystal

Werner (21) has made a detailed study of the errors associated with the measurement of intensities from a protein single-crystal X-ray diffraction pattern. He has demonstrated that when a two-dimensional scanning microdensitometer is used, an individual intensity measurement differs by approximately 4% from the mean value of the four symmetry-related reflections. In general, it will not be possible to measure a fibre pattern to this degree of precision because of the various factors which complicate the analysis. If Werner s criteria are applied to a series of intensity measurements made from fibre patterns using AXIS, the precision attained lies between 7% and 10%. While we do not have enough data to draw any firm conclusions at present, this result is sufficiently close to 4% to be encouraging. 

The three-dimensional structure of bovine pancreatic DNase I was first determined at 2.5 A resolution by x-ray diffraction from single crystals [23]. DNase I proved to be a compact a,P-protein with two six-stranded j) sheets packed against each other forming the core of a sandwich -type structure. The two mainly antiparallel P sheets are flanked by three longer and five short a-helices and extensive loop regions (Fig. 3). 

Obtaining large single crystals that diffract to high resolution remains the primary bottleneck of protein crystallography. The most widely used... 

Figure 3.4 X-ray beam passing through the Ewald sphere and diffracted by planes in a single crystal produces reflection spots. (Adapted with permission from Figure 1.13 of Drenth, J. Principles of Protein X-ray Crystallography, 2nd ed., Springer-Verlag, New York, 1999. Copyright 1999 Springer-Verlag, New York.)...

Given the very small amount of protein present in a single crystal relative to the concentrations of DCL components, it is clear that amplification in the usual sense cannot occur. It is conceivable, however, that the protein structure influences equilibrium distribution of hydrazones in microcosm within the crystal. Conversely, it is possible that any DCC equilibrium is irrelevant, and that the hydrazone binding being observed by X-ray diffraction is due to a diffusion equilibrium of essentially static components between solution and solid state. 

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