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[ Home ] [ Overview ] The Beamline Sibyls Design SAXS experiments Single Crystal X-ray Beamline 12.3.1 |
The technical challenges in solving structures of large, biologically important macromolecular complexes involve both merging a variety of traditionally independent experimental techniques and the collection of X-ray diffraction data at different beamline geometries. The design of the SIBYLS beamline allows for rapid conversion between a variety of beamline geometries required to cover the broad spectrum of data collection needs for large macromolecular complexes. The SIBYLS beamline is providing the technology to support experiments that will dock crystal structures of the smaller component proteins into low resolution molecular envelopes of the larger, multi-component biologically relevant states, some of which are less amenable to crystallization (i.e. intermediate assembly states). Source. The source for the SIBYLS beamline is an ALS 5 Tesla Superbend magnet, which provides extremely high flux (2x10+11 photons/sec/400mA through a 100 micron pinhole at 12.4 keV) with a smooth wavelength curve to cover the desired wavelength range (0.8-1.5 Angstrom). The source will provide a combination of small source size and low vertical divergence that is optimal for diffraction from crystals with extremely large unit cells. X-ray Optics. The SIBYLS optics include two mirrors: M1 is a flat internally water cooled electroless Nickel plated Invar mirror. It is held in a mechanical bender that allows it to be bent to the required parabolic shape. The M2 mirror is an uncooled silicon cylinder mirror bent to a toroid by means of a mechanical bender. Both mirrors are coated with 8 nm of Rhodium over 25 nm of Platinum and operated at 4.5 mrad grazing angle. This bilayer coating allows for suppression of the Platinum L edges in the 11-14 keV range while extending the mirror cut off ~2.5 keV beyond what one would obtain from a single Rhodium coated. The first crystal is an internally water cooled flat Si(111) crystal, the first multilayer is a side cooled flat crystal with 150 layer pairs of Mo/B4C, d spacing = 2.4 nm. The second crystal and multilayer are simple uncooled flat elements. Additionally, The SIBYLS beamline is equipped with a dual double-crystal Kohzu monochromator allowing for automated wavelength tuning without a change in the target position, eliminating the need to place the large experimental table on a swinging arm. The first set of crystals are two highly ordered Si(111) crystals, which provide high spectral resolution (delta lambda/lamba <10-4) that is necessary for MAD phasing experiments while simultaneously providing sharp reflections. Flux rates with the Si(111) crystals for PX are 2x1011 photons/sec/400mA through a 100 micron pinhole at 12.4 keV. The second set of crystals is composed of multilayer or mosaic Si(111) crystals with a much broader band pass (delta lambda/lambda <10-2) that provide substantially more flux. For SAXS the flux is up to 3x1013 photons/sec/400mA at 10 keV with all apertures open when using the multilayer monochromator elements. Although the multilayer crystals are primarily designed for SAXS experiments, the very robust flux will also be ideal for the development of microcrystal diffraction techniques. As previously mentioned the X-ray optics provide X-rays with a very low divergence angle, which allows for optimal spatial resolution of reflections from samples with large unit cells. Wavelength. The SIBYLS beamline has a highly focused and parallel beam enabling accurate MAD experiments and rapid changes of X-ray wavelength. The wavelength range of 0.8-1.5 Angstrom makes available the anomalous edges from selenium K-shell and third row transition elements LIII transitions to those of the actinides, which provide large DF's that can be very helpful for phasing large unit cells. High resolution X-ray diffraction on low molecular weight complexes is aided by a short wavelength of the incident radiation. This provides high resolution data at moderate scattering angles and allows a larger number of reflections to be collected for a given detector surface area. In contrast, longer wavelengths can benefit the data collected on crystals of large complexes, where the quality of X-ray diffraction data are aided by the greater angular dispersion. |