Hi Bernhard et al > > The lattice metric is the same for both SGs (did not understand Eleanor’s > remark)
>> Hmmm - >> well the I212121 cell and the P21221 cells you give here have different >> volumes, so they cant be just rearrangements of symmetry operators and >> non-cryst translation. >> Cell volume for the I222 cell = 87.8 * 101.18 * 123.63 ~= 1,098,280 Å^3 Cell volume for the P22121 cell = 93.34 * 105.47 * 122.98 ~=1,210,685 Å^3 Or ~10% different - so to me, anyway, Eleanor's remark made perfect sense. > > In principle, differences between NCS and XS of compatible metric become more > distinct at > higher resolution (i.e. at night, all cats are grey). > > Best, BR > > From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Florian > Schmitzberger > Sent: Montag, 13. Oktober 2014 10:47 > To: CCP4BB@JISCMAIL.AC.UK > Subject: [ccp4bb] I222 - P22121 space-group ambiguity > > Hi everybody, > > I collected a number of X-ray data sets from crystals originating from the > same cryst. drop. I solved the initial structure in P22121 space group by MR > with Phaser locating two molecules (data to ~ 2.1 Angstr.); refined R/Rfree: > 0.213/0.244. > > Processing of some of the other data sets with XDS/Aimless is consistent with > I222 space group (resolution ~ 2.6 Ang.). I can locate one molecule. The > unit-cell dimensions for I222 and the initial P22121 space groups for two of > the data sets are: > I222: a=87.8 b=101.18 c=123.63; P22121: a=93.34 b=105.47 c=122.98; > > I superposed the molecule in I222 onto one of the two located for the > initially solved P22121; the orientation of the NCS-related molecule in > P22121 differs from the crystallographic-symmetry related one in I222. Trying > to solve this P22121 data set in I222 with MR, does not result in high Z > scores, and maps do not look good. > > Some of the data sets that process in I222 to ~ 3 Angstr., I can also solve > in P22121, locating two molecules (differences may not be that clear in this > case, since the resolution is lower). > > Some other data sets process in P22121 with Aimless; with a substantial > off-origin Patterson peak, indicating translational NCS. For these, Phaser > positions two molecules that are related by crystallographic translational > NCS. These two molecules are crystallographic-symmetry related in the > original P22121 data set. I can also solve these data sets in I222 space > group, with the overall Z score higher than for the P22121 data. > > I am uncertain, what the ‘true’ space group for some of my data sets is. > Could it be that for data that process in P22121, but can be solved in I222, > reflections that would indicate I222 space group were not collected? > Alternatively, perhaps what I am seeing is that there is a (gradual) > transition of the crystal lattice (between P22121 and I222 or vice versa), > caused by variation in crystal handling/cooling or exposure to X-rays. > > It’s relevant to me, because in P22121 space group, a region of the molecule > that is of biological interest makes NCS-related crystal contacts that are > crystallographic-symmetry related in I222. > > Has anybody observed similar cases? I would appreciate comments. > > Cheers, > > Florian > > Harry -- Dr Harry Powell, MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH Chairman of International Union of Crystallography Commission on Crystallographic Computing Chairman of European Crystallographic Association SIG9 (Crystallographic Computing)
