Small Molecule

General Procedure for Data Collection on a small molecule crystal

• • Turn on X-ray source & sign into logbook (see Startup/Shutdown)

  • Mount and align crystal 

  • Check that the beamstop is in place by issuing from the command line:

  •             sm i 1.5

  •     This stands for "single measurement intensity 1.5 (sec.)"

  •     Make sure there is a beam stop shadow, and no image of the direct beam.

  • Put CrysalisPRO into SM mode by double clicking on the CrysalisPRO icon in the lower right, and checking the SM radio button.

  • Execute a pre-experiment. In this step a few frames are collected to determine the unit cell, and space group symmetry.

  • Set up the full experiment. Target values:

    • I/sigma > 10

    • Redundancy > 4

    • Resolution better than 0.84 Å

    • Usually safer to uncheck Friedel mates equivalent, so ensure that complete anomalous data is collected even if the space group is incorrect.

Considerations for small molecule data collection.

• • Very high resolution is typically required (better than 0.84 Å for deposition). To get complete data with such high resolution sometimes requires the sample to be oriented in ways that can cause collisions of the sample pin with the collimator, knocking the crystal out of alignment.  The Hampton magnetic pins we typically use seem to be adequate in this regard. However, for foolproof operation, use a metal pin with glass fiber.

  • For proper absorption correction it is very important the crystal be well-aligned. Take your time aligning the crystal. Be sure to use the set screws (and be aware that someone else may have done so when you are adjusting the alignment of your crystal).

  • Crystal shape & size:

    • Ideally the crystal will have some thickness in all directions - avoid plates and needles if possible.

    • For crystals with hi-Z elements, smaller crystals may yield higher quality data, as the absorption correction will be less important. Of course, smaller crystals means longer exposures.

  • Below is a paper (Muller) with some general ideas for improving the quality of small molecule structures determined via X-ray crystallography.