Dear Marina,
Mark seems to have hit the nail on the head. The left-hand picture of
your second jpg shows that you have an axis, at about 45 degrees from the
horizontal (and hence from the rotation axis), along which the spots are
very close. These spots seem to be only just separated on that picture, when
that direction is parallel to the detector plane, but for values of the
rotation angle away from that particular one, they are likely to collide or
even overlap within the same diffraction image, especially if your image
width is not especially small.
When you have a crystal with such a long axis in real space, and
therefore closely spaced spots in reciprocal space, you need to orient it in
such a way that this axis be essentially aligned with the rotation axis, and
to put the detector far enough that the close spots in that direction be
resolved. The idea is that the angular separation of spots provided by the
spatial resolution of the detector is much better than the separation you
could get from even very fine-sliced images.
To orient your crystals in this way you will benefit from using a
beamline equipped with a multi-axis goniostat. The dataset you will collect
with such an aligned axis will have a "cusp" of missing reflections around
that axis, but that is a better outcome than the alternative of having the
zillions of "angular overlaps" between distinct reflections that would be
produced with a long-axis orientation such as that shown in your pictures.
That random overlap will produce the same effect on intensity statistics as
twinning, but there would not be a consistent twinning fraction among all
your measurements that you could then put into a refinement against them.
Do your unit-cell parameters confirm this scenario?
With best wishes,
Gerard.
--
On Fri, Mar 12, 2021 at 06:29:06PM +0100, Mark J van Raaij wrote:
> Hi Marina,
> The close-together spots in the zoom inset of your figure I think are not
> split spots, but separate reflections. They are close togeter because you
> appear to have a unit cell with one axis much longer than the other two (we
> work on elongated proteins, so we have some experience with that). The same
> short distances are also clearly visible in the picture on the bottom left.
> If you index the image in MOSFLM and it puts boxes around both I'd conclude
> they are separate reflections, but there are perhaps more sophisticated ways
> to verify this.
> So I think its true that in this case the twinning is not (obviously) visible
> in the diffraction pattern - but detected through intensity statistics later.
> Best wishes,
> Mark
>
> Mark J van Raaij
> Dpto de Estructura de Macromoleculas
> Centro Nacional de Biotecnologia - CSIC
> calle Darwin 3
> E-28049 Madrid, Spain
>
>
>
> > On 12 Mar 2021, at 11:30, Marina Gárdonyi
> > <marina....@pharmazie.uni-marburg.de> wrote:
> >
> >
> > 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
> >
> > http://www.agklebe.de/
> >
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