On top of that very good explanation by Zbyszek, I find it also
interesting that
* in the tetragonal dataset the Dn symmetry indications are exactly
on the diagonal (45 degree) with the peaks a bit smeared out
* in the orthorhombic dataset the Dn symmetry 2-folds are not on the
diagonal, but rather at 55 degree - with much clearer peaks.
You didn't give us the cell dimensions, but I wonder what the relation
between those two crystal forms is - hopefully the tetragonal is truly
different and it's not a case of pseudo-tetragonal orthorhombic (with
processing sometimes going for one or the other SG).
Cheers
Clemens
On Wed, Jul 28, 2021 at 04:37:22PM -0500, zbyszek wrote:
> It's a very interesting crystallographic and likely biological problem.
>
> These patterns--a series of peaks along the axes of the Chi=180 plot, with
> the peaks corresponding to a great circle in polar (3D) space--indicate 2
> back-to-back rings. On each great circle (each line of peaks on the Chi=180
> plot) there are 16 distinct peaks, so you have likely D16 molecular symmetry
> i.e. you have two 16-mers forming a two-ring structure.
>
> This indicates that ASU has megadalton complex inside:
> - in P2221 where only one crystallographic 2-fold can be used as axis of D16
> assembly there will be likely 16 molecules in ASU.
> - in tetragonal form, if this P42 21 2 or P4 21 2, two crystallographic
> 2-folds can coincide with 2-folds of D16 assembly so there will be 8
> molecules in ASU. I would expect for such symmetry (two perpendicular
> assemblies, each 32 sub-units, in the unit cell) to result in very high
> solvent content e.g. 75-80%.
>
> If in a tetragonal space group you get ~70-80% of solvent for 8 molecules in
> ASU, then there is a very high chance that the entire assembly sits on two
> perpendicular 2-fold axes and 8 NCS operators are determined by the
> orientation of these axes. One of the 2-folds will have to coincide with 42
> screw axis or 4-fold axis, while the second 2-fold is the diagonal along ab
> directions.
>
> In both cases, the NCS operators will be represented by rotations along
> diagonal 2-fold crystallographic axis by angles 0, 22.5, 45, 67.5, 90,
> 112.5, 135, 157.5. These axes will intersect at the origin of
> crystallographic system in both space groups.
>
> If you don't have such situation, your best bet is cryoEM. It should work
> for such big assembly.
>
> Zbyszek
>
>
>
>
> On 2021-07-28 13:48, Jorg Stetefeld wrote:
> > Hi,
> >
> > we are seeking advice interpreting the attached selfrotation patterns
> > in terms of defining the NCS symmetry and eventually NCS-masks for
> > proper averaging/DM.
> >
> > We are working with a muliti-domain transmembrane receptor composed
> > of several Ig -and FN-domains. We collected data in P4x212 and in
> > P2221 (see attachments).
> >
> > Thx in advance
> > js
> >
> > __
> >
> > [1] [2]
> >
> > Jörg Stetefeld
> >
> > https://stetefeldlab.ca/ [2]
> >
> > Find a job you like- You never work a day in your life!
> >
> > [1] [2]
> >
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*--------------------------------------------------------------
* Clemens Vonrhein, Ph.D. vonrhein AT GlobalPhasing DOT com
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