New SAXS Capabilities
on Beam Line 4-2

- H. Tsuruta

Low-angle single crystal diffraction:

We have added a single-axis goniometer to the facility in order to record single crystal diffraction data in the range of 10 Å to 800 Å in Bragg spacing. The need for good low resolution data is primarily driven by the potential of obtaining phase information by advanced phase determination methods, as well as the studies of large-unit cell systems, such as viruses. We expect that low resolution data will be very useful for phase extension in virus crystallography because there is usually very little overlap between high-resolution diffraction data and structural models deduced from electronmicroscopy, from which one would obtain initial phasing. Low resolution data also present potential for obtaining information about less-ordered components like RNA molecules packed inside the virus protein shell.

Using the new capability, H. Tsuruta (SSRL) and H. Luecke (now at UC Irvine) recorded a fully complete data set from a Hsc20 crystal (longest unit cell axis 125 Å ) down to 15 Å with very high signal-to-background ratios. The project is aimed at obtaining phase information using the ab initio phasing technique recently developed by S. Subbiah (Stanford University). In collaboration with the J.E. Johnson group (Scripps), we recorded low resolution diffraction data from several different types of non-infectious virus or virus-like particle systems, covering the 15 Å to 700 Å range. The group hopes to expand the icosahedral spherical harmonics technique they developed for virus solution scattering to low resolution single crystal diffraction. They plan on obtaining low resolution structure models, and therefore initial phase information, solely from diffraction data.

Time-resolved muscle fiber diffraction:

In collaboration with T. Irving (BioCAT, IIT) and S. Taylor (Mayo Clinic), we have conducted time-resolved fiber diffraction studies on an isolated frog muscle fiber. We wrote software that allows fast timing among the diffraction data acquisition, an electrical muscle stimulator, and the beam shutter. We plan to develop more sophisticated software and hardware not only for time-resolved fiber diffraction but also for other time-resolved experiments that involve fast synchronization of data acquisition with user-supplied sample handling equipment.

On-going and future upgrades:

We have had many user requests to have access to wider angles. This is not easily accessible with our standard camera (sample-to-detector distance about 2.3 m) without re-assembling the vacuum path, an effort that takes about a day. A new vacuum flight path (~0.9 m) is being constructed in order to have an easy access to wider scattering angles up to approximately 10 Å . The short camera will be mounted on a set of slides next to the 2.3 m camera so that users can change sample-to-detector distance easily.

We have also installed a new set of motorized in-vacuum slits, which will eliminate all the air gaps except around the specimen. We are also considering building an in-vacuum solution cell which would reduce background further. Consequently, we hope to provide a low background camera system which will facilitate studies of relatively small proteins or dilute solutions (less than 1 mg/ml).

We are planning to build a cryo crystal cooler for single crystal diffraction studies on Beam Line 4-2, and expect to have it available in early 1997. A new stopped-flow rapid mixer, funded by NIH, will be added to the system in early 1997. The new mixer will allow one to monitor structure changes in a protein solution system optically as well as with time-resolved solution x-ray scattering. Within the next year, we are also planning to make a series of upgrades to the solution scattering software in accordance with the computer and CAMAC controller upgrade.

As the user community has become more diverse, we feel strong demand for a low noise, high-count rate, two-dimensional detector. Currently we are looking into the logistics of obtaining a commercially available 2D detector for the Beam Line 4-2 SAXS facility. Furthermore, we will test two CCD x-ray detectors developed at the Photon Factory in early 1997. Those who are interested in giving input, please contact H. Tsuruta at tsuruta@ssrl.slac.stanford.edu.

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December 2, 1996