Beamline

The Molecular Biology Consortium (MBC) was formed to manage the construction and operation of X-ray beamline 4.2.2 at the Advanced Light Source (ALS): a superconducting dipole magnet source (a “Superbend”). The beamline was constructed under the leadership of Dr. Edwin Westbrook, Scientific Director of the MBC. Currently, the beamline is operated under the direction of Dr. Jay Nix, Beamline Manager of the MBC.

Technically, beamline 4.2.2 is almost ideal as a macromolecular crystallography station. Its 5T dipole magnet source, in the 1.9 GeV ALS electron storage ring, provides a spectrum with a critical energy of 12 keV: essentially perfect for our discipline1. The source is small2, and its angular width is also small3, so it is possible to achieve a very tight focus of the X-ray beam with little angular divergence, with good X-ray optics. The X-ray optics of 4.2.2 were designed by Dr. Gerold Rosenbaum, and implemented by Larry Rock of L.R. Design, Scottsdale, AZ. Beamline 4.2.2 provides high flux and high flux density over a wide spectrum: 5 - 15 keV. Over this energy range, beamline 4.2.2 delivers a total flux exceeding 1012/s into a focused spot 50μm vertical, and 125μm horizontal4. The beam is essentially parallel, with a Gaussian cross-section. The energy resolution is that of its Si(111) monochromator crystals: about 0.00014, or about 1.7 eV at the selenium edge (12.66keV).

We have installed a Rigaku ACTOR crystal mounting robot. Many of our users now routinely send crystals in dewars, and collect data remotely without actually traveling to Berkeley. MBC staff mount crystal samples in pucks, and then turn over the beamline to the user who, with help from web cameras and remote presence software, can run their own experiments with minimal participation by MBC staff. So far our record for greatest productivity is 41 molecular replacement structures solved in 48 hours. 75% of beamline 4.2.2 is used by MBC member institutions. The remaining 25% is allocated to the ALS General User program, through peer-reviewed applications. Many General Users have solved their structures at 4.2.2. Almost all have been NIH sponsored research.


1 http://www-cxro.lbl.gov

2 σx=100μm, σy=10μm

3 σ'x=0.06mrad, σ'y=0.01mrad

4 Flux density exceeding 1.5 x 1014/s/mm2