Scott Classen: October 2008 Archives

An excellent paper came out today in the Oct 3rd issue of Cell detailing structural, biochemical, and genetic studies of the Mre11-DNA complex and its role in detecting and repairing double-strand breaks in DNA. Both the SAXS and crystallography data were collected at the SIBYLS beamline. There is also a nice writeup by Paul Preuss which appears in the todays Berkeley Labs News Release.

Mre-11-pic.png

Mre11 dimers coordinate DNA end-bridging and nuclease processing in double-strand break repair” by R. Scott Williams, Gabriel Moncalian, Jessica S. Williams, Yoshiki Yamada, Oliver Limbo, David S. Shin, Lynda M. Groocock, Dana Cahill, Chiharu Hitomi, Grant Guenther, Davide Moiani, James P. Carney, Paul Russell, and John A. Tainer, appears in the 3 October 2008 issue of Cell.

The SIBYLS beamline Kohzu monochromator has often exhibited hysteresis when moving the theta 2 motor. This manifests itself in a decaying beam and suboptimal intensity. In order to fully understand the problem it helps to have a general understanding of the motors in the monochromator. The figure below illustrates the various motors and their relative motions.

mono-motors.png

The axis of the Theta motor lies on the surface of the first crystal and the angle of Theta determines the wavelength of X-rays that will be selected for a particular experiment. The second crystal is designed to take that monochromatic X-ray beam coming from the first crystal and reflect it down the beam pipe to the SAXS or MX endstation. The second crystal is ~30 meters from the crystal sample when doing a crystallography experiment and it is critical that the um-sized X-ray beam be aimed directly at the small crystalline samples. We use the Chi 2 motor on the second crystal to steer the beam from left to right (horizontal beam steering) and we use the M2 mirror to move the beam up and down (vertical beam steering).  The axis of the Theta 2 motor lies on the surface of the second crystal, and it is used to align the surface of the second crystal so that it is perfectly parallel to the first crystal thus maximizing the flux of the X-rays that exits the monochromator. Typically we tuneup the beamline before a user starts their shift and during this tuneup procedure we optimize the angle of the theta 2 motor by monitoring an ion gauge positioned at the exit tube of the mono (Imono Out) and maximize the current. If we then monitor the Imono Out gauge for several hours after the tuneup procedure is complete we sometimes see that the intensity of X-rays falls precipitously.

Imono_Out_NORMAL.png Imono_Out_BROKEN.png

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