A protein crystallography MAD beamline to be installed on the 7 T wiggler at the LSU/CAMD synchrotron.
The Gulf Coast Protein Crystallography Consortium (GCPCC) has received funding to build a protein crystallography MAD beamline. The consortium formed out of the needs of local users for greater access to synchrotron radiation sources for the determination of protein crystallographic structures. After several years of discussion, which grew out of a regional, annual protein crystallography meeting, members of the consortium began to evaluate beamline designs for the CAMD synchrotron at LSU. This site has the advantage of close proximity to the consortium members. The installation of an energy-shifting wiggler makes theCAMD source characteristics well suited to the demands of the user base. Specifically, the consortium members wanted increased access to synchrotron beam time with MAD phasing capabilities. Many projects are currently delayed while waiting for available beam time at national facilities. MAD phasing is becoming an increasingly important method of solving protein structures in general and specifically for consortium members. The regionally of the facility is important in that it reduces travel expenses and allows groups to bring graduate students along on data collection trips as part of their training.
The GCPCC beamline will be capable of standard macromolecular MAD phasing experiments over an energy range of 7-17.5 keV. Rather than design a system de novo, the GCPCC has taken the information and experience learned at the national labs over many years to implement an effective protein crystallographic beamline design that will deliver the desired flux.
The primary optical elements of the system are a vertical collimating mirror, a Si (111) channel-cut monochromator, and a focusing mirror (Figure 1). The first mirror is a meriodinal cylinder with a tangential radius of ~4.2 km. This mirror serves to collimate the beam vertically, which improves the energy resolution of the monochromator. The Si (111) channel-cut monochromator selects the desired wavelength. The channel-cut design reduces the number of degrees of freedom and simplifies alignment o the beamline. The second mirror is a cylinder that is bent to allow focusing of the beam in both the vertical and horizontal directions (Figure 3). The endstation will have a CCD based X-ray detector with accompanying goniostat, diagnostic, collimation, and cryocooling devices typical of current MAD protein crystallography beamlines. The GCPCC beamline is allocated 3 mrad of the wiggler fan. The resulting flux through a 200 µm sample aperture should be comparable to an NSLS bending magnet beamline.
Figure 1. Schematic anamorphic diagram of the GCPCC beamline showing the position of the vertical collimating mirror (M1) the channel-cut monochromator and focusing toroidal mirror (M2) in relation to the source and focus locations.
Figure 2. Schematic of the channel-cut monochromator for the GCPCC beamline. “White” light enters from the left and “monochromatic” light exits to the right. Click on the image for an animated illustration.
The resulting foucus looks as follows.
Figure 3. GCPCC beamline focused spot results from ray-tracing the beamline in SHADOW. A 200 µm yellow square is shown on the close-up view for reference.
The beamline delivery and commissioning timetable calls for full 24/7 user operations by October 2001. The GCPCC beamline serves as a regional research and training facility. It will accelerate the solution of new structures by providing improved access to a MAD capable synchrotron radiation beamline. Located close to the consortium laboratories, the beamline will facilitate the participation of new students in the solution of protein and nucleic acid structures. The GCPCC beamline will allocate 25 % of the beamline time to general users. We are also following the development of the “FedEx data collection” program at the NSLS as a method for meeting some of this 25 % commitment.
The CAMD Facility
The Center for Advanced Microstructures and Devices (CAMD) at Louisiana State University (LSU) operates an electron storage ring for the production of synchrotron radiation. This second generation source with a Chasman-Green lattice has been operational since 1992. The ring currently operates between 1.3-1.5 GeV. The installation of a super-conducting, energy-shifting wigglermakes this source well suited to protein crystallographic MAD phasing applications. The critical wavelength for the wiggler is 1.18 Å (10.5 keV) when operated at 7 Tesla with a 1.5 GeV electron beam. The X-rays produced by the CAMD wiggler at 1.5 GeV are harder than those from an NSLS bending magnet at 2.58 GeV (Figure 4). This results in a higher flux/mA at the Se K edge for the CAMD source.
Figure 4. Synchrotron radiation spectra for an NSLS bending magenet at 2.58 GeV and a CAMD bend and the CAMD wiggler at 1.5 GeV.
Figure 5. Location of the GCPCC beamline at CAMD.
Support for the beamline is provided by the NSF through the biological infrastructure award DBI-9871464 with interagency matching funds from the NIGMS at the NIH and the GCPCC member institutions.