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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.
Cheers,
Mischa
>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?
--
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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
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