Dear Yuri, Since the number of reflections depends inversely on the cube of the resolution, it is easy to show that a perfectly twinned 2.4A structure will have the same data to parameter ratio as an untwinned 3.0A structure. This determines how much you can refine and which restraints are appropriate. In my experience, the 2.4A twinned difference maps will be even less informative than 3.0A difference maps. Fortunately, as is often the case with twinned structures, you also have 2-fold NCS, so you can recover part of the damage by applying tight NCS restraints.
Best wishes, George On Fri, Sep 30, 2011 at 08:01:42AM +0100, Yuri Pompeu wrote: > Hello everyone, > I am refining a structure to 2.4A with 2-fold NCS and twinned. > Maps look ok and Rfree is 0.27however as I start checking my validations I > notice that after refinemnt > my geomtry gets significantly worse. especially the rama plot. Initially I > have 2 outliers and I end up with 32 (5%)!!! > I played with the Xray weight term but alll it helped me with was rmsd > bond/angles, rama is still messing up... > Can I impose some ramachadran restraints or maybe have a reference model? > > best, > -- Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-3021 or -3068 Fax. +49-551-39-22582
