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Jay,
At an APS ID beamline, I usually get ~10% decrease in data
completeness and 0.1A in completeness after 180degree of data at 25%
beam attenuation. A useful strategy I use to collect a 3wavelength MAD
data on the SAME crystal, especially if the crystal is large enough and
is a long needle, is to confine the beam to a different section of the
crystal for each wedge or wavelength. If your crystal is as follow
(<--A--><--B--><--C-->, where A, B and C represent the different
sections), then I first take two test exposures of each section (0 and
90 degree) to make sure that they are all equivalent in quality, then
collect one wedge/wavelength, recenter the crystal at the next section,
and repeat for the next wedge/wavelength. You will no non-isomorphism
issues since you are collecting on the same crystal. Hence, for a weak
crystal and one wavelength, you can afford to overexpose each section to
get higher I/sigma, and merge your data from each section after data
collection.
thang
Jay Thompson schrieb:
Hi All,
We have crystals (0.2 x 0.2 x 0.2 mm) that belong to the spacegroup
C2, with a unit cell of 310 A x 290 A x 230 A, 90 102 90 and
diffracts rather poorly at ~4 angstroms. We're trying to collect a
Hg and Pt-SAD datasets, since a MAD dataset is likely not feasible.
So far I've collected data on a couple of crystals (~45 minutes of
total exposure time, Rsym= 0.14, I/sig(I)=7.0, redundancy=4). and I'm
having trouble detecting any anomalous peaks in the Harker sections
using the programs in CCP4. It looks like these crystals are
exhibiting radiation decay (based on unit cell length, scale factor,
and mosaicity increases), after ~10-15 minutes of exposure time on a
2nd generation synchrotron (like SSRL or ALS). If anyone has any
wonderful strategies to collect SAD data on weakly/poorly diffracting
and radiation sensitive crystals, that would be great!!
I need to decrease the exposure time on the crystal to be able to
collect a complete dataset with some anomalous signal. However, I'm
worried that if I decrease the exposure time the signal-to-noise
would suffer and I would still have trouble finding an anomalous peak
in the Harker section (although redundancy should increase
signal-to-noise). Which is better in terms of improving signal to
noise for SAD or MAD datasets? Would people recommend to keep the
exposure time short and just collect lots of images to increase
redundancy and signal-to-noise or increase exposure time but have a
less redundant dataset. So I guess the question comes down to
whether multiple weak reflections is better than one strong
reflections for SAD? Does anyone have any nice strategies for
estimating/optimizing the exposure time for SAD/MAD datasets? Also
do people still like to collect inverse beam for C2 spacegroups? Any
comments would be greatly appreciated. Thank you very much.
JT
