Accuracy in Laue X-ray diffraction analysis of protein structures.

The accuracy in protein structure analysis based on Laue X-ray diffraction has been investigated for the example of two orthorhombic structures of bovine pancreatic trypsin (BPT). The precision in the Laue structure factors and the contrast in electron-density maps were used as criteria. A comparison with the results of previous analyses based on conventional crystal rotation methods showed that high resolution around 1.4 A may be reached with both monochromatic and polychromatic techniques. Electron-density maps exhibited significantly lower contrast when calculated on the basis of Laue structure amplitudes, due to inefficient exploration of reciprocal space at low resolution by the Laue method even in the case of a broad bandwidth and inclusion of exposures from several different crystal orientations. Laue data were recorded on photographic film and processed using the program LAUEMAD [Bartunik & Borchert (1989). Acta Cryst. A45, 718-726]. The empirically derived wavelength scaling factors based on a comparison of equivalent reflection intensities were in good agreement with theoretical estimates over a broad wavelength range. One BPT structure was refined on the basis of Laue structure amplitudes (current R factor 24% at 1.8 A resolution).