Dear Rhys, I was not aware that twinning is not possible in P6322 however I agree with Randy’s advise to check the lower symmetry P63 etc. We were just collecting data which was apparently P6322 (according to automated DLS processing: pointless) but the cell dimensions and our knowledge of the protein composition was telling us that this SG is wrong. Pointless/Aimless were however suggesting that twinning could be there and lower symmetry could be an option. As Randy mentioned we used the unmerged data which was luckily in P3 Laue group (original choice by EDNA and probably XDS) to merge the data in P63. That worked and we could then solve (and refine) the structure fine however we had to apply the twinned refinement (in Refmac) to get the reasonable R factors, R factors were much higher without the twinning. This surely means that twinning is possible in P63 SG which then produces data in over-symmetrical SG P6322.
But try also Phaser with the option of the subgroups as Randy suggested, it might work. Cheers, Aleks > On 4 Oct 2016, at 00:26, Rhys Grinter <rhys.grin...@monash.edu> wrote: > > Dear All, > > As I have approached my crystallography from a biological perspective, > sometimes so of the more mathematical/geometrical aspects sometimes perplex > me. I was wondering if anyone would be able to clarify what is going on with > some problematic crystals I'm working on. > > I've grown crystals of a protein which forms a concentration dependent > oligomer. This is almost certainly a physiological oligomer and probably is a > hexamer at maximum oligomerisation (although maybe a trimer). These crystals > diffract poorly, however after some optimisation I managed to collect data to > around 3.6 A, with a predicted space group of P6322 with unit cell dimensions > of 177, 177, 150 . In order to improve diffraction I performed dehydration on > these crystals. This seemed to improve diffraction to around 3A (although as > the crystals are quite variable attribution of effect is a little difficult), > however the best space group I can find for indexing is C2221 with a unit > cell of 177, 310, 151, XDS doesn't process the data when I force the previous > P6322 SG. It seems also that the C2221 space group isn't the correct choice > as the merging stats are worse than I would expect from looking at the > diffraction pattern. > > Additionally, the intensity statistics from both space groups suggests > twinning. Although for the P6322 space group it says twinning is not > possible. If this is the case what is causing these abnormal intensities and > is this related to my SG ambiguity? > > Also what is the best what to proceed with processing in this case? > > Cheers, > > Rhys > > -- > Dr Rhys Grinter > Sir Henry Wellcome Fellow > Monash University > +61 (0)3 9902 9213 > +61 (0)403 896 767
