Search for: "experimental phasing with detwinned data" and you will find some hits. You need a low degree of twinning, and a strong sub-structure signal..
Re MR solutions - it is usually possible to solve the structure providing you get the space group right, but in my experience the maps are clearer if you do twinned refinement. I cant see any reason not to use it? Eleanor On 11 October 2016 at 10:05, <herman.schreu...@sanofi.com> wrote: > Dear Jacob, > > > > I agree with Chris. In my experience for MR, twinning is just like having > a space group with one extra 2-fold (or other twin operator) so MR is > indeed largely immune to twinning. Also the maps, calculated with > uncorrected data, often look surprisingly good, just having a higher level > of random noise. > > > > I did not do the math, but my feeling is that the (greatly) inflated > measurement errors (exploding as you mention near a twin fraction of 0.5) > make direct phasing methods, that are often at the very limit of useable > signal to noise ratio, fail after detwinning. Also, an incorrectly > estimated twin fraction may introduce systematic errors, especially in case > of a varying twin fraction depending on the rotation of the crystal. > Probably random noise is less harmful than systematic errors. > > > > My 2 cts, > > Herman > > > > *Von:* CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] *Im Auftrag von > *Chris Fage > *Gesendet:* Dienstag, 11. Oktober 2016 10:40 > *An:* CCP4BB@JISCMAIL.AC.UK > *Betreff:* Re: [ccp4bb] Why Does Detwinning Not Work? > > > > Dear Jacob, > > > > I'm not an expert on the topic, but from my experiences with twinning I > can agree with you. I recently solved my second twinned structure by MR > (twin fraction of 0.43, as estimated by Xtriage). Performing twin > refinement in Refmac or phenix.refine dropped the R-factors, as expected, > but worsened the geometry considerably without a noticeable improvement in > the maps. For this reason, I opted *not* to go with the twin refinement... > I don't know if others would make the same choice, though it seemed > reasonable to me. Besides, my Rwork/Rfree is down to 0.25/0.29, which ain't > too shabby for 2.6 A resolution. > > > > Cheers, > > Chris > > > > On Tue, Oct 11, 2016 at 2:15 AM, Keller, Jacob <kell...@janelia.hhmi.org> > wrote: > > Dear Crystallographers, > > > > Based on some data sets I have looked at and anecdotal-type evidence here > and there I have gotten the impression that detwinning does not help in > structure solution. (Please let me know if you have a case where detwinning > saved the day.) Is there a clear answer to this enigma anywhere, to > anyone’s knowledge? Wouldn’t it seem that **any** detwinning would be > better than **no** detwinning? I understand that the errors explode as > one approaches 50% twins and does detwinning, but still, I don’t think one * > *loses** information by detwinning, right? Take the case of a 33% twin: > since the twin-reflections are on average about half the intensity of the > non-twin, and since they are generally not correlated in intensity, isn’t > this like having noise added at 50% of the measured intensity? So why does > detwinning make things worse generally? Is there something wrong in the > assumptions underlying the detwinning algorithm, or perhaps something about > the calculation that throws things off? > > > > A related sub-enigma: why is MR generally immune to twinning, but > anomalous methods are susceptible? > > > > All the best, > > > > Jacob Keller > > > > ******************************************* > > Jacob Pearson Keller, PhD > > Research Scientist > > HHMI Janelia Research Campus / Looger lab > > Phone: (571)209-4000 x3159 > > Email: kell...@janelia.hhmi.org > > ******************************************* > > > > >
