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Hi, I mentioned that R factors went down because they went down dramatically, from 33/36% to 22/24% (which shows that application of twinning does the right job even if such R-factors cannot be compared), and I have ‘seen’ such dramatic change for one of my small molecule refinements in old days as well, but the main reason in my switch from P6322 symmetry to P63 was the fact that the volume of ASU in P6322 was a half volume needed for my protein plus that the twinning was found by Aimless. Cheers, Aleks On 4 Oct 2016, at 16:03, Pavel Afonine <[log in to unmask]<mailto:[log in to unmask]>> wrote: Hi, since several people mentioned R factors in this conversation I thought I remind that R-factors are not comparable between refinements using twinning and not. For example, R-factor decrease after switching to twin refinement doesn't mean much; you can't use R factor drop as an argument for considering twining vs not considering it at all! Pavel On Tue, Oct 4, 2016 at 7:43 AM, Pankaj Chauhan <[log in to unmask]<mailto:[log in to unmask]>> wrote: As Aleks is suggesting, lower symmetry would be better. I had similar issues with one of my protein with unit cell dimensions 57, 57, 256. Xtriage suggested suggested three 2-fold merohedral twin operators (-h,-k,l; h,-h-k,-l; and –k,-h,-l). I went to lower symmetry (P31) and applied twinning law (-h,-k,l) during refinement and that reduced by R-factors. Pankaj On Tue, Oct 4, 2016 at 10:22 AM, Aleksander Roszak <[log in to unmask]<mailto:[log in to unmask]>> wrote: 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 <[log in to unmask]<mailto:[log in to unmask]>> 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<tel:%2B61%20%280%293%209902%209213> > +61 (0)403 896 767<tel:%2B61%20%280%29403%20896%20767> -- Pankaj Kumar, PhD Gyanu Lemichhane lab<http://webhost.nts.jhu.edu/gl/> Centre for Tuberculosis Research Lab Department of Infectious Disease Johns Hopkins University School of Medicine 725 N. Wolfe St. Baltimore, MD 21205-2185 Lab phone: (410) 955-3967<tel:%28410%29%20955-3967> [log in to unmask]<mailto:[log in to unmask]> [log in to unmask]<mailto:[log in to unmask]> http://pankajimtech.webs.com<http://pankajimtech.webs.com/>