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,

Von: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] Im Auftrag von Chris 
Gesendet: Dienstag, 11. Oktober 2016 10:40
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.


On Tue, Oct 11, 2016 at 2:15 AM, Keller, Jacob 
<<>> 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<tel:%28571%29209-4000%20x3159>

Reply via email to