Hi Lisa,
The problem you have can be very tricky. Here my
comments:
-What do you mean by "I can not solve the structure by
molecular replacement"?. Moleculare replacement programs usually produce some
solution, even if it is wrong. Do the R-factors stay around 50%, is the electron
density very bad? If you do not get a solution at all, the solution might be
rejected because of clashes. In this case you may want to cut out some variable
loops from the model or increase the maximum number of allowed
clashes.
-What does the Matthews number tell you? If you get a
very dense crystal packing even with one molecule in the asymmetric unit, you
most likely have twinning.
-3.3 Å means rather poor data. This means that the
statistics you get on your space group and on the twinning will be tentative and
you have to test all possibilities. Also you may have pseudo crystallographic
NCS making the apparent space group look different from the true space group.
This means that in molecular replacement, you have to test all possibilities,
e.g. P622, P6122, P6222, P6322 etc. Most molecular replacement programs have an
option to automatically test them all.
-Considering twinning, you have to process the data in
the lower symmetry space group, presumably P6 and submit the data to a twin
server, as Fred suggested. I would also run molecular replacement on the lower
symmetry data. Even with perfect twinning, one usuallly gets two solutions,
corresponding to the two twin domains.
Good luck!
Herman
Hi all,
Does anyone know how to check a data is twin and how to
detwin?
My data is 3.3 A with space group P6222 or P6422? But I can
not solve this structue by molecular replacement with the model has 42%
sequence identity. I am wondering my data maybe
twin.
Thanks
Lisa
