|
-------- Original Message --------
some more precision: the crystal appears to be P622, for which no twinning is allowed, but we have tried all space groups suggested by mosflm i.e. P6122, P6522, P6, P312,P321,etc and P3: for this latter one, we obtain 24 (!!) molecules in the asymmetric unit. The relative height of the patterson vectors is 200 for (0.666, 0.333, 0.052) and 190 for (0.666, 0.333, 0.000). As to the peak in position (0.666, 0.333, 0.052), taking into account the value of the lattice parameter c (290.0 A), it is at about 15 A from the origin: could this be a ripple? Mosflm does not suggest a lattice with a2=b2=a/sqrt(3)=101 A, and when we visualize the data with HKLVIEW, we can see reflections in the (h,k,0) plane which account for the lattice parameter a=175 A. Eleanor Dodson wrote: > First - twinning checks are all very flawed and prone to be misleading > when you have conspicuous pseudo translation. You dont say what the > relative height of the vector at (0.666, 0.333, 0.000) is to the > origin., but you probably have a pseudo cell with a2 = b2= a1/sqrt(3) . > Could the peaks at (0.666, 0.333, 0.052) be ripples from the (0.666, > 0.333, 0.000) one? > > Secondly if the space group is really P622 there cant be any > twinning? See > http://www.ccp4.ac.uk/dist/html/twinning.html#lookup_tables for a > discussion of what is possible.. > > If it appears to be P622 then it could be twinned P6 or P321 or P312 > with twinning operators respectively > -h,-k,l or k,h,-l or -k,-h,-l . Or there could be more than one > twinning operator in action! > I would try reindexing in the smaller cell, processing as P3, and then > repeat the twinning tests with that data.. > > You can check Merging Rs for P3, P312 P321 P6 and P622. > > Eleanor > > PS - or best of all finding another crystal! > > Thierry Granier wrote: > >> <>We have recently solved the structure of a protein complex to 1.9 A >> resolution >> and tried different substrates. Among these, we obtained a different >> crystal form: the crystal diffracts up to 2.9A and can be indexed in >> hexagonal >> or trigonal space groups with lattice parameters a=175 b=175 c=290. >> 90,90,120°. >> All diffraction spots are indexed in the frames by mosflm. Refinement >> of the unit >> cell goes fine and final processing with scala give Rsym of 0.13-0.15 >> for the >> different space groups. However the unit cell seems very large and >> imposes >> the asymmetric unit to contain 6 molecules in the case of P622 or 12 >> in the case P321. >> Data analysis with sfcheck, detwin, and Yeates server indicates the >> crystal is twinned >> with twin fraction of ~0.39. Taking all the possible twin laws for >> each possible space group, >> the data were detwinned with program detwin but remained twinned >> afterwards with a new >> suggested twin fraction of ~0.20 when analysed by sfcheck and others. >> For each data set, >> detwinned or not, Patterson analysis yields three high peaks at >> (0.666 0.333 0.052), (0.333 0.666 0.052) and (0.666, 0.333, 0.000) >> which suggest a pseudo translation (or a smaller possible unit cell >> (?) which was not given by mosflm). >> When using untwined data, cross rotation search with Molrep yields a >> single rotation peak, >> although one would expect several solutions. The translations >> mentioned above have been used >> but did not yield any good solution so far. When using detwinned data >> we obtain several cross rotation >> solutions but with no further success. Does any one have suggestions >> concerning this problem? >> Could it be possible that the crystal exhibit a reticular merohedral >> twinning (with a smaller unit cell)? >> But in this case should the Rsym be much higher than it actually is? >> >> Any help will be welcome >> >> -- >> Thierry GRANIER >> >> Unité de Biophysique Structurale >> Bât B8 >> Avenue des Facultés >> Université Bordeaux 1 >> 33405 TALENCE Cedex >> FRANCE >> >> > -- Thierry GRANIER Unité de Biophysique Structurale Bât B8 Avenue des Facultés Université Bordeaux 1 33405 TALENCE Cedex FRANCE -- Thierry GRANIER Unité de Biophysique Structurale Bât B8 Avenue des Facultés Université Bordeaux 1 33405 TALENCE Cedex FRANCE |
