Dear all,
I know that some years ago a similar situation was discussed here, and I wonder
if someone has new insights about these problems.
My protein is a dimer in solution. I tried several derivatives for SAD, and all
my datasets seem to have the same problem, including the native crystals. I
processed the data with XDS and the space group determination was done with
Pointless, being a P212121.
Checking the quality of the data, I found several problematic results:
- Translational NCS is detected. There is a peak at (0.50, 0.40, 0.50)
- The L test suggests twinning (L statistic = 0.41)
- The mean acentric moments I from input data have the following values:
<I^2>/<I>^2 : 4.396
<I^3>/<I>^3: 34.478
<I^4>/<I>^4: 361.084
All these values are way higher than the expected ones for non-twinned
data.
- The twinning fraction from L-test is 0.22
This would all suggest that my space group is wrong, and that I should proceed
in a lower symmetry group, but I don’t know how should I continue. I know about
cases where a P43212 or a P41212 were suggested but in fact the correct space
group was P212121, but I would not know how to continue going down. Checking my
images, I can see some streaky spots, and the crystals grow first as needles
that eventually become plates and rarely crystals. All these would be a clear
indicative of twinning, but I would not expect to have the same results with
different derivatives. We thought about some kind of reticular twinning, and I
wonder if there is a way to solve it.
Any idea about how to proceed or to identify the problem would be welcome.
Thanks,
Carmela.
Carmela Garcia-Doval
University of Zurich
Department of Biochemistry
Winterthurerstrasse 190
CH-8057 Zurich
Switzerland
E-mail: c.gar...@bioc.uzh.ch
