Tips and Tricks in Crystallography - Diffraction - (Click for PDF reader to view articles)
Bernard D Santarsiero: You can collect 5-10deg images, and go ahead and collect 360deg. I usually use 1-3 seconds exposures, depending on the diffraction. The detector is set as close as possible, to get data to 0.8-0.9A resolution. For a MAR CCD 300mm detector, that's around 90-100mm, with the wavelength set at 0.8A, or around 15.5KeV in energy. You can process it with HKL2000 or XDS easily.Zbigniew Dauter: I completely agree with Bernard, but I have one additional comment. The range of intensities from small molecule crystals is usually much higher than from protein crystals. It is therefore practically always necessary to collect them in multiple (sometimes even three) passes with different effective exposure time (or beam attenuation) and resolution range. Unfortunately people often forget about it and as a result very many strongest reflections are overloaded and lost.
Readings on Data Collection:
Garboczi (NIAID): Our Experiences with the Data Reduction Program
Recommended viewing: How long will my crystal last?
Mr. Jerry Alexandratos (NCI): Post-crystallization soaking. Do you have poorly diffracting crystals? Don't throw them out. Try post-crystallization soaking by adding 1.5-2 M (NH4)2SO4 with or without several percent of PEGs such as 3350. You can also try slightly dessicating drops with crystals. (I did not note down who mentioned these procedures. My apologies, as well as my thanks for the tips.)
Lothar Esser (NCI): Denzo,
XDisplayF, Scalepack. In case
you are not yet using HKL2000 which is a GUI (graphical user interface)
that controls denzo, xdisp and scalepack you should consider getting a
copy of it. Officially it is not in general distribution (current
version 0.97.638) but since the development of this program is funded
by NIH, there is no problem for NIH crystallographers to get a
pre-release free of charge. Routine and non-routine data reduction can
be done very efficiently if you (or your student) are new to data
processing but even more so if you have worked your way through the
input files for denzo and scalepack before. Indexing and integrating of
data is effortless and so is data reduction.
Is everything done by a push on a
button? Well, you still have to manually adjust the error-of-scale
factor until the overall χ2 is about 1.0. So for
straightforward data sets, i.e. data sets that do not require much
adjusting of the error bins, I have been using a utility program that
does nothing more than running scalepack until the χ2 is
1 within a user-defined limit. To get the final graphical analyses you
may run scalepack from within HKL2000 one more time with the updated
reject file and the refined error-of-scale factor. In many cases this
saved me a lot of time. (Let me know if you want to have a copy of this
Have you ever run xdisp on a remote computer and redirected it to an X display on a SGI? In that case you might have noticed that the fonts of xdisp are too large. In fact some buttons on the zoom window are missing. This problem can be solved by giving the x font server a different path to font files. Try this before you open xdisp on the SGI (the local machine):
Ed Pozharski's python script at the ccp4wiki reads a .sca file and produces a table with both <I/sigmaI> and <I>/<sigmaI> for a user-defined number of shells.
This site is maintained by Dr. Xinhua Ji (firstname.lastname@example.org) on the NCI-CCR-MCL server (http://mcl1.ncifcrf.gov).