Combined use of monochromatic and Laue diffraction techniques for macromolecular structure determination

A novel strategy of macromolecular structure analysis is described which combines the use of monochromatic scanning Laue (SCL) and white-beam Laue (WBL) diffraction techniques. It provides, when applied with an area detector with on-line capabilities, a means of interactively determining and optimizing experimental parameters; it further makes rapid data evaluation feasible, also with off-line detector systems. These new procedures have been applied to a protein structure, β-trypsin, using a FAST area detector (Enraf-Nonius) and image plates (Fuji) on a double-focusing synchrotron beamline at DORIS. Structure factors, which were derived from FAST Laue data, were empirically scaled by comparing equivalent reflections in different wavelength bins. A 2F_o - F_c difference Fourier map, which was calculated at 1.8 Å resolution using these structure-factor moduli together with phases from the known structural model, showed well defined electron density distribution (R = 22%). Image-plate exposures showed diffraction to 1.2 Å resolution. The effect of crystal mosaicity on the maximum wavelength bandwidth for Laue exposures has been investigated. SCL techniques, which involve rapid scanning (with a crystal or multilayer monochromator or a tunable undulator) through a defined wavelength range, extend the applicability of Laue techniques to crystals with broadened mosaic spread.