Biology: Time-Resolved Crystallography

While standard static 3-D structures of macromolecules obtained by using X-ray diffraction technique provide important insights into their function, to fully elucidate details of how these molecules perform their function, one ideally needs to capture them in action. Time-resolved crystallography is a unique tool for achieving this goal because it provides direct, detailed and global structural information as molecules in the crystal undergo structural changes.

BioCARS scientists played essential role in the early development of all aspects of time-resolved crystallography, including advances in processing and analysis of time-resolved data. BioCARS is one of only several synchrotron beamlines worldwide with infrastructure for conducting time-resolved experiments.   

Time-resolved pump-probe experiments at BioCARS utilize polychromatic, Laue X-ray diffraction technique. For comprehensive reviews of time-resolved crystallography at BioCARS we suggest the following:

 

Technical capabilities

 

  • Time-resolution
    • 100ps resolution in hybrid and 24-bunch APS storage ring mode
    • 200ns in 324-bunch APS mode
  • X-ray flux    
    • two in-line undulators (U23 and U27), optimized for 12 keV
    • polychromatic beam, 5% bandwidth typically used for time-resolved crystallography
    • 3x1010 photons/100ps pulse in APS hybrid mode
  • X-ray beam size
    • 20 (h) x 15 (v) µm2, beam focused by primary and secondary Kirkpatrick-Baez mirror systems
  • Repetition rate
    • <40Hz typical, up to 1KHz possible
  • Lasers
    • ps laser system: Spectra Physics, Ti:Sapphire Spitfire Pro 5; 780nm; 2ps; 1kHz; 5mJ/pulse; TOPAS OPA; tunable range: 350nm-2µm; pulses typically stretched to 30ps
    • ns laser: OPOTEK Opolette 355 II HE; 7ns pulse duration; 20Hz; 410-600nm: ~3-6mJ; 230-400nm:0.5-2mJ
    • a number of CW diode lasers; ms exposures possible
  • Detector
    • Rayonix MX340-HS (10-100 frames/sec)

Challenges

While time-resolved crystallographic studies of reversible reactions in naturally photosensitive proteins can be routinely done, BioCARS staff is developing methods and capabilities to meet more challenging experiments.

Sample cell for room-temperature X-ray diffraction of protein crystals under strong electric field pulses. Protein crystals are sandwiched between two glass capillaries filled with crystallization solution and containing metal wires that serve as electrodes The crystal is fixed by an electrically insulating glue to the bottom (ground) electrode and the high-voltage pulse is introduced from a top electrode through a liquid contact with the crystal  This electrode system was integrated into BioCARS  setup for time-resolved crystallography.


The micro-patterned silicon chip is raster-scanned through the X-ray beam using the goniometer at 14-ID beamline. A stream of humidified helium gas flowing over the chip prevents the crystals from drying out. Closer look illustrates short path of the direct beam between the collimator tube and the capillary beamstop behind the. By flushing the remaining free beam path with helium, extremely low background scattering levels are achieved.

 

 

 

 

 

 

 

 

 

Schematic diagram of the injector-based serial Laue crystallography at 14-ID beamline. Lipidic cubic phase (LCP) injector is used for delivering micro-crystals. Jülich chopper selects the desired number of 100ps X-ray pulses for X-ray exposure for each crystal. A Rayonix detector MX340HS records crystal diffraction. On the right, LCP is shown as mounted on the 14-ID beamline.

                          

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