Data collection: Tips and Tricks
From CCP4 wiki
See also cryo and Crystal growth: Tips and Tricks#Reducing the mosaicity, and Improving crystal quality.
Improvement of cryo results
- PEG3350 seems to be little too large for freezing, particularly in presence of high salt concentration. However, PEG2000 is good enough as a cryo protectant.
- Try oils for cryoprotection
- Try stepwise increase from low concentrations of cryoprotectants to the required concentration, over several minutes/hours/days. The larger the crystal, the longer should be the incubation in each step.
- Small crystals often do better in terms of mosaicity increase than big crystals (often growth faults lead to end of crystal growth)
- use a synchrotron beam (as it is less divergent than the usual home source, and the apparent mosaicity is the sum of the crystal mosaicity and the crossfire)
- shoot at parts of the crystal to find if there are good and bad parts (this requires a small beam)
Crystal with one long unit cell axis
Nukri Sanishvili wrote on April 5, 2011 on CCP4BB:
- Orient the crystal so that the long axis is along the rotation axis. It can be done during mounting the crystal on the loop or, if the goniostat is equipped with multi-axis goniometer, then after mounting your crystal on the goniometer. But try to pre-orient your crystal as much as you can when scooping it.
- Increase sample to detector distance: It can help but it will not work if your long axis is along the beam. Overlaps in that case will be more function of the frame width. In that case, see 3.
- Decrease the angular range measured in a single frame: It will alleviate the problem not solved in 2. When you do that, you may have to also decrease the exposure per image using less intense beam and/or less exposure time. Otherwise, you radiation dose per degree of data will increase killing you crystal before you collect complete data.
- Find the detector with a smaller point spread function.
- Find a synchrotron beamline with a smaller beam.
- Find a synchrotron beamline with less divergent beam. Keep in mind, that some times (but NOT ALWAYS), 5 and 6 are mutually exclusive.
4, 5, and 6 will have less effect than 1, 2 and 3.