Hi Andreas -
My two cents, from having tried this a couple of times on small molecule
crystals that crossed my path:
- You can get an answer using only standard macromolecular
crystallography programs. It may not be up to the standards of the
small molecule community, but that's probably not what your student needs.
- Denzo can handle small molecule diffraction - the key is to collect
enough spots on each image. Use big oscillation ranges (10-20 degrees)
and push the detector in as far as possible, with 2theta offset as well.
- Use very short exposure times to avoid overloads. I also turned my
generator down to minimum power. If your optics permit it, you might
also slit down the beam.
- Even with small molecules, crystal quality is important. Make sure
you have good-looking diffraction - no streaky spots, multiple lattices,
etc. The great thing about small molecule crystals is that you can
always break off small pieces if the big chunk isn't clean enough - you
have diffraction intensity to spare!
- If your molecule of interest is chiral, you won't have to worry about
the centric space groups that protein crystallographers never think
about. If it's not (or there's a chance of a racemic mixture), then you
should probably process in P1 and use Pointless to tell you the space group.
- You can use SHELXS for direct methods structure determination if
you're able to collect high enough resolution data. I believe this
should work with 1.2 A data, and it ought to work well with 1.0 A or
better. I found these webpages useful in helping me interpret the SHELX
output:
http://strucbio.biologie.uni-konstanz.de/ccp4wiki/index.php/Solve_a_small-molecule_structure
http://shelx.uni-ac.gwdg.de/tutorial/english/shelxs.htm
- If you have a heavy atom (iodine, iron, etc.) in the small molecule,
you can probably get a phasing solution by SAD even with Cu Kalpha. You
could probably even solve it by looking at the Patterson maps!
Good luck! If it works, I think you'll be done in an afternoon. If it
doesn't work, I'm not sure how to troubleshoot things.
- Matt
On 3/24/14 2:04 PM, Andreas Förster wrote:
Dear all,
I've been approached by a materials student with a petri dish full of
big, sturdy, salty, yellow crystals. He claims I have the best kit
for single-crystal diffraction on campus.
I would very much appreciate advice on how to deal with this, anything
in the range from "won't work" to "use software X to analyze data in
space group P-43N" would be welcome.
Thanks.
Andreas
--
Matthew Franklin, Ph. D.
Senior Scientist
New York Structural Biology Center
89 Convent Avenue, New York, NY 10027
(212) 939-0660 ext. 9374
