Fair enough!
The heavy atom positions are simply the "S" atoms in 3dko. There are 22
of them. Also, in this case the Met side chains (12 of those) are 32%
occupied with Se. The other 68% is sulfur. I think it is "realistic"
that one could know the extent of Se incorporation ahead of time from
something like mass spec (especially if you knew it could make-or-break
your structure determination). However, I don't think it is "realistic"
that you would know where they are before running shelx.
-James Holton
MAD Scientist
On 1/12/2013 7:46 AM, George Sheldrick wrote:
Dear James,
I agree with Pavel that your example is not very realistic. In practice
one would start from the heavy atom positions. As well as providing
starting phases, they are useful in other ways. For example. shelxe
(and probably most other tracing programs) adds them to a 'no-go'
map so it knows where NOT to trace the main-chain.
Best wishes, George
Dear James,
your challenge in its current form ignores an important source
of information for model building that is available for your
simulated data - namely, it does not allow to use anomalous
phase information in the model building. In difficult cases on
the edge of success such as this one, this typically makes
the difference between building and not building.
If you can make the F+/F- and Se substructure available, we
can test whether this is the case indeed. However, while I
expect this would push the challenge further significantly,
most likely you would be able to decrease the Se incorporation
of your simulated data further to such levels that the anomalous
signal is again no longer sufficient to build the structure. And
most likely, there would again exist an edge where a small
decrease in the Se incorporation would lead from a model built
to no model built.
Best regards,
--
Pavol Skubak
Biophysical Structural Chemistry
Gorleaus Laboratories
Einsteinweg 55
Leiden University
LEIDEN 2333CC
the Netherlands
tel: 0031715274414 <tel:0031715274414>
web: http://bsc.lic.leidenuniv.nl/people/skubak-0
--
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582
