You do know even that if the pointgroup is P422 the the SPACEgroup can by P 4 2 2 or P 41 21 2 or P43 21 2 or P41 22 or ...... Lots of possibilities!
Eleanor Although you R factors look good.. On 22 May 2015 at 08:13, <herman.schreu...@sanofi.com> wrote: > So both space groups most likely describe the same crystal packing. In > this case I would choose the highest symmetry which still gives good > refinement results. > > > > Best, Herman > > > > > > > > *Von:* dhaval patel [mailto:pateldhaval...@gmail.com] > *Gesendet:* Freitag, 22. Mai 2015 09:09 > *An:* Schreuder, Herman R&D/DE > *Cc:* ccp4bb > *Betreff:* Re: [ccp4bb] AW: [ccp4bb] Point group > > > > Dear Herman > > > > Thanks for reply. My molecule is generally found to be tetramer. In > P212121 space group cell content analysis shows 4molecules in asymmetric > unit and in P4212 it shows two molecules in asymmetric unit. > > > Dhaval Patel > > PhD Student, > > Bioinformatics & > > Structural Biology > Indian Institute of > > Advanced Research > +91-9925450504 > > > > On Fri, May 22, 2015 at 12:31 PM, <herman.schreu...@sanofi.com> wrote: > > Dear Dhaval, > > > > In principle, it does not matter how you describe the packing of the > molecules in your crystal. E.g. you can refine a P212121 structure in space > group P212121 with one molecule in the asymmetric unit, or in P1 with 4 > molecules in the asymmetric unit. In the latter case, these 4 molecules are > related by non-crystallografic symmetry operators, which are identical to > the crystallographic symmetry operators in P212121 (provided of course, > that the same origin was choosen in both cases.). > > > > So if your molecule is a tetramer with an internal 4-fold axis, the > crystal packing could be described in P4212 with one molecule in the > asymmetric unit, or in P212121 with 2 molecules in the asymmetric unit. In > the latter description, the 4-fold axis is non-crystallographic. (I did not > check though, how this works out with 2-fold versus 21 axes). > > > > In general, if there is no reason to assume lower symmetry, e.g. too high > Rfactors during refinement, one generally chooses the highest symmetry, > since there is no reason to assume that the molecules are different. > However, if there are clear differences during refinement in the different > space groups, one chooses the space group which yields the lowest Rfactors. > Beware that in lower symmetry, Rfactors might be somewhat lower, just > because the refinement program has more parameters to fiddle with. This is, > in my eyes, not significant. > > > > Best, > > Herman > > > > > > *Von:* CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] *Im Auftrag von > *dhaval patel > *Gesendet:* Freitag, 22. Mai 2015 06:21 > *An:* CCP4BB@JISCMAIL.AC.UK > *Betreff:* Re: [ccp4bb] Point group > > > > Even I have the same problem my crystals get processed in to P212121 as > well as P4212 and I am currently analysing the data but not able to > conclude the space group. Any idea? Both structure while refining is giving > around same R/Rfree. Any help will be appreciable. > > > Dhaval Patel > > PhD Student, > > Bioinformatics & > > Structural Biology > Indian Institute of > > Advanced Research > +91-9925450504 > > > > On Thu, May 21, 2015 at 10:57 PM, frazao <fra...@itqb.unl.pt> wrote: > > On 05/21/2015 05:23 PM, Keller, Jacob wrote: > > I can process my 3.8 A dataset in either P4 or P422 point groups. > > Do the scaling statistics look similar for both? If so, go with P422. > Trying to enforce an incorrect symmetry operator would blow up your stats. > > ...But in the case of twinning, lower space groups can masquerade as > higher ones. I say try out P4 with twin refinement, see how your stats go. > > JPK > > Or eventually even to further lower symmetry... I had once an example of > true P21212 crystals that processed nicely as P4212, due to a twinning > two-fold axis parallel to the a,b diagonal > > Carlos > > -- > ************************************** > Dr. Carlos Frazao > Structural Biology Laboratory - > Macromolecular Crystallography Unit > ITQB-UNL, Av Republica, Apartado 127 > 2781-901 Oeiras, Portugal > > Phone: (351)-214469666/609 > FAX: (351)-214433644 > e-mail: fra...@itqb.unl.pt > www.itqb.unl.pt > > > > >
