If the twin law is k,h,-l, then your a axis must almost equal the b axis? And if the twin fraction is 0.48 then you have additional symmetry I guess?
How sure are you that the point group is P4/mmm? On 13 March 2014 20:41, Teresa Swanson <teresa.m.swan...@gmail.com> wrote: > Dear collegues, > > I'm working with a drug complexed protein structure that is having major > twinning issues. The drug has a single Br atom on a benzene ring, which I'd > like to use for orienting the drug in the binding site. I have various > anomalous data sets, ranging from 3.0A resolution, all scaled into P222 > with a Rlin of .125. > > Using MR, the twin law (k, h, -l) and NCS restraints, I can confidently > solve the structure without anomalous, and the drug density is clear in the > Fo-Fc map, with Rw/Rf at ~.26/.29 and a space group of P21221. It might be > important to note that any simulated annealing I've tried invariably > increases the Rfree by 2-3%, so I've scraped it. As you can imagine, when > using the twinned data, the anomalous maps are weak and random. > > I've used the Phenix "detwin" option in Xtriage to see if I can pull the > anomalous signal out of it. If I use the .mtz file that is output for MR > and calculate the anomalous maps, it looks promising. The twin fraction for > the one particular dataset I've been using is estimated at approx .48. Is > this too close to 50% to do the detwinning? Now I'm wondering how to > properly refine this further. I'm assuming that since I've "detwinned" the > data, I do refinement without the twin law. But that gives an initial Rf of > .38 when using it gives .31. Since I've already solved the structure > without using the anomalous flag, can I just use the "detwinned" > reflections and the refined structure to calculate an anomalous map > (without having to redo the refinement)? > > Mainly, my main question is about how to tease out and properly refine the > anomalous data from a twinned structure. Also, how much of a difference > will it make to scale into P222 versus P21212. And, if I have quite high > redundancy, should I "scale anomalous" in HKL2000 or just use the > "anomalous" flag? > > Any help on refining this twinned structure would be greatly appreciated! > Thanks, > > Teresa > PhD Student >
