You dont mention any twinning tests?
The L test, now part of the newest ctruncate is pretty good at detecting
twinning even with the NCS translation.
And SFCHECK does a good job too.
If these are inconclusive I would not assume twinning.
Usually you can get solutions for MR with twinned data, but I havent
much experience of the signal quality..
Can you solve it in P1 then sort out the spacegroup later?
Eleanor
Kay Diederichs wrote:
Dear all,
we have crystals that nicely diffract to 1.7 A (sharp spots), with the
following characteristics and findings:
a) the data appear as P212121, with axes 117.2 133.6 138.3 (if
reduced in P1, the largest deviation of any angle from 90° is 0.2°);
the odd screw-axis reflections are mostly indistinguishable from
noise; the data do not scale significantly better in P2/P21 (any
setting) or P1.
b) there is a good model available, with coords known from a complex
of this protein with another one; two molecules of this model would
give 64% solvent in P212121 which appears reasonable for a membrane
protein
c) the structure cannot be solved with this model in P212121, nor can
it be solved in P222, P2122, P2212, P2221, P21212, P22121, P21221
d) we conclude that the true space group must be P2 or P21 (with one
of the three possible settings), with almost-perfect twinning. Or it
is P1 with tetartohedral twinning. There are thus still six + one
possibilities.
e) MOLREP tells us
--- Check Patterson for pseudo-translation ---
PST_limit : 0.125 of origin peak
INFO: pseudo-translation was detected.
Origin Patterson peak: P,P/sig : 57.748 257.690
1 Patterson. peak : p,P/sig : 28.773 128.395
2 Patterson peak : P,P/sig : 16.551 73.856
3 Patterson peak : P,P/sig : 8.502 37.936
Peak 1: trans.vector /ort/ : 0.011 55.688 69.399
trans.vector /frac/: 0.000 0.416 0.500
Peak 2: trans.vector /ort/ : 58.554 66.863 0.000
trans.vector /frac/: 0.500 0.500 0.000
Peak 3: trans.vector /ort/ : 58.565 11.385 69.399
trans.vector /frac/: 0.500 0.085 0.500
INFO: translation vector of peak 1 will be used.
Two molecules (for the orthorhombic spacegroups) may produce only one
pseudo-translation vector. As there is more than one strong
pseudo-translation vector, I conclude that we have at least 3
molecules in the ASU (consistent with monoclinic).
f) we've calculated all seven molecular replacement searches of d) in
MOLREP. The contrast is very high in all cases. However, Refmac
rigid-body refinement of the solutions, with "Twin refinement"
activated, gives about 51% R and the same for Rfree (give or take 1
%), in all cases.
I'm wondering: how reliable is a rotation search in the presence of
perfect twinning? Is there any molecular replacement program that can
take a given twinning operator into account in the rotation and
translation search?
Any other hints what to try?
best,
Kay
