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Re: [ccp4bb]: Twinning/Merging

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Hi Daniel,

First of all, I don't think that an Rmerge of 16% is very high! Given 
the information you provided, I guess you have a rather low 
I/sigma(I), particularly in the higher resolution shells. You also 
merged three different crystals. All that will result in higher than 
usual Rmerge values. For weak crystals, collected with in-house 
instrumentation, 16% Rmerge is not uncommon. Best of all, tons of 
structures have been successfully solved with and/or refined against 
such data. So I wouldn't worry about it too much.

Second, freezing/cooling (whatever you want to call it) can sometimes 
introduce some kind of crystal imperfection that can be modeled by 
twinning and that is not present in unfrozen crystals.

Third, if you want to explain your "high" Rmerge at room temperature 
with twinning, then you will have to SHOW that the data are indeed 
twinned. I am afraid, simply assuming twinning without having the 
statistical descriptors for it wouldn't do it for me. Again, an 
Rmerge of 16% isn't pathologically high.



>Of one protein, I used 3 crystals to collect a data set at room
>temperature. The data set is to 96 % complete, diffracts to 2.35 A,
>and has a very high Rmerge value after scalepack: 16 % (77 % in the
>2.38-2.35 A shell).
>The twinning test (from Yeates, www.doe-mbi.ucla.edu/Services/Twinning/)
>gave no twinning at all.
>I used the very same protein to grow crystals of the very same crystal
>form. (The crystals differ in the substrate co-crystallized). These
>crystals were measured at 100 K, and I collected full data sets of
>every crystal. Tested for twinning, it came true, that all of them
>were twins of 5-15% (P3221, (-h,-k,l)).
>Now I wonder if the 3 crystals measured at room temperature have been
>twins as well, and if I may explain the high R-value with that. Is
>this true?
Mischa Machius, PhD
Assistant Professor
University of Texas Southwestern Medical Center
Department of Biochemistry
5323 Harry Hines Blvd.                                Tel: +1 214 648 9760
L4.250                                                Fax: +1 214 648 8954
Dallas, TX 75390-9038, U.S.A.     Email: Mischa.Machius@UTSouthwestern.edu