Dear Marina,
When I saw your figures I felt this tug from the deep, dark past. Four decades
ago [Fisher, R. G. & Sweet, R. M. (1980). Acta Cryst. A36] Richard Fisher and I
were wrestling with a red-algal light-harvesting protein. It made large (0.5mm)
nice-looking crystals in R3 (189Å, 189Å, 60Å). These were the BA-days (before
ADSC), so our test images were all precession photos on film. I'm attaching the
seminal figure from that article, where you can see the problem we had. I've
marked a spot where the two images clearly differ.
Anyway, the tug I felt in looking at your images, especially the fourth one, is
that with a twinned crystal, if you squint, ALL THE REFLECTIONS LOOK THE SAME.
We found this was true for us with rotation images when we were taking 3D data.
We actually discovered the twin, after we'd decided that the space group was
R32, by noticing that sometimes reflections in the hk0 zone that should be
symmetry related made a strange image -- on one side would be a strong
reflection with a little chunk taken out, and opposite a very weak reflection
with a dark little spot where the chunk was missing on the other side. The
twins were perfectly separated in the crystal. In this case the twin mechanism
had the unique axis in R3 going up in one, and down in the other twin. We
proposed a simple-minded method to detect and correct for twinning. In the
decade after this publication, Doug Rees and Todd Yeates did a nicer job of the
statistics.
Anyway, by now you understand that the answer to your question is yes.
Best wishes,
Bob
================================================
Robert M. Sweet E-Dress: sw...@bnl.gov
Scientific Advisor, CBMS: The Center for BioMolecular
Structure at NSLS-II
Photon Sciences, Brookhaven Nat'l Lab.
Upton, NY 11973 U.S.A.
Phones: 631 344 3401 (Office)
631 338 7302 (Mobile)
================================================
________________________________________
From: CCP4 bulletin board <CCP4BB@JISCMAIL.AC.UK> on behalf of Schreuder,
Herman /DE <herman.schreu...@sanofi.com>
Sent: Friday, March 12, 2021 8:11 AM
To: CCP4BB@JISCMAIL.AC.UK
Subject: [ccp4bb] AW: [ccp4bb] Can twinning be seen in the diffraction pattern?
Dear Marina,
A lot can happen with twinning andthe crystallization process does not always
adhere to rules written in text books.
The first thing I would do is to look at the predicted spots to see if the
"split" spots are predicted as single spots (and would then really be split),
or if they are predicted as two different spots (and would therefore not be
split).
I assume that the cell dimensions of the P622 and P3(2)21 cell are the same (no
doubling of some sell axis) and that the detection of twinning was based on the
intensity distribution of the data set.
What may have happened is that if the transition between the two twin domains
is not perfect, the two twin domains may have slightly different orientations,
leading to split spots.
My 2 cents,
Herman
-----Ursprüngliche Nachricht-----
Von: CCP4 bulletin board <CCP4BB@JISCMAIL.AC.UK> Im Auftrag von Marina Gárdonyi
Gesendet: Freitag, 12. März 2021 11:31
An: CCP4BB@JISCMAIL.AC.UK
Betreff: [ccp4bb] Can twinning be seen in the diffraction pattern?
Hello everyone,
I am a PhD student at the Philipps-University in Marburg and I am currently
writing my thesis.
I have problems to understand whether in my case twinning can be seen in the
diffraction pattern or not.
I know that it depends on the type of twinning wheter you can see it in the
diffraction pattern. The crystal had a resolution of 2.2 A.
During processing it seemed to have the space group P622, but in the end it was
P3(2)21. With phenix Xtriage I found out, that the data set was twinned. The
twin law was -h,-k,l. So it should be merohedral twinning.
I read in a paper, that in case of merohedral twinning you cannot see it in the
diffraction pattern. But in my case that seemed not to be the case because of
splitted reflections. Or am I wrong???
I would be very happy to hear your opinion on that. Thanks in advance!
Best regards,
Marina
--
Marina Gárdonyi
PhD Student, Research Group Professor Dr. Klebe
Department of Pharmaceutical Chemistry
Philipps-University Marburg
Marbacher Weg 6, 35032 Marburg, Germany
Phone: +49 6421 28 21392
E-Mail: marina....@pharmazie.uni-marburg.de
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