Hi Rhys, We had a similar problem where initial data processing suggested P6522 space group but the content of the crystal was too large to fit in the unit cell (one well known symptom for twining). I reprocessed the data in P6, P312, P321 and finally the solution was obtained in P6 (P65).
However, crystal dehydration has been previously shown to cause change of space group and twining, see the link - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2631638/pdf/d-65-00101.pdf Good luck, Sudipta. On Mon, Oct 3, 2016 at 6:26 PM, 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 >
