From: Napoleão Valadares
Date: 17 October 2011 18:09
Hi there!
I got crystals from some synthetic peptides I bought, they are 30 residues long and are supposed to form a coiled coil. I collected various data sets (home source, Brookhaven and Diamond), including some at the resolution of 1.65 A, for which the space group appears to be C222 or C2221. The unit cell is small, 22.67, 88.06, 26.13, and the Matheus Coefficient indicates that's there's only one helix in the asymmetric unit and a 25% solvent content.
I have tried A LOT of Molecular Replacement using Phaser and Phenix AutoMR. I'm using a 80% identity coiled coil helix as search model. The programs give me solutions with "reasonable" maps, but it is never possible to refine to achieve Rvalues below 0.40. Additionally, maps from different solutions look reasonable, so I'm thinking these are all bias.
I have 5 other synthetic 30 residues peptides (that crystallize in different space groups and diffract to lower resolutions), including a SelenoMethionine (SM) derivative (but it does not have enough anomalous signal, ASU is too big, it is possible that the SM are disordered). I'm stuck on this since March.
Regarding the search model, I already tried trimming some or all side chains and removing 2, 3 or 5 residues on each/both sides. I also tried other search models. Maybe some "magic" combination of parameters on Phaser or other programs can help me.
What is your advice regarding how to proceed with MR? Is there some program, procedure, parameter, pray or human sacrifice that could help me?
Thank you.
Regards,
Napo
----------
From: Bernhard Rupp (Hofkristallrat a.D.
If for example your presumed coil is differently bent than your search
probe, overall rmsd is high and MR will likely fail. But maybe ARCIMBOLDO
with helix fragments might do at 1.65A? George or Isabel will have a better
answer.
Good luck, BR
PS: And may San Matheus de las Coefficientes listen to your prayers... :-)
----------
From: Ed Pozharski
Did you check for twinning? The most radical approach is to reprocess
in P1 and see what R-values you get
--
Edwin Pozharski, PhD, Assistant Professor
University of Maryland, Baltimore
----------------------------------------------
When the Way is forgotten duty and justice appear;
Then knowledge and wisdom are born along with hypocrisy.
When harmonious relationships dissolve then respect and devotion arise;
When a nation falls to chaos then loyalty and patriotism are born.
------------------------------ / Lao Tse /
----------
From: Nat Echols
----------
From: James Stroud
This strongly suggests a space group issue.
If the systematic absences are compelling, you should try to drop it down to P2 first. Look at your packing in the C222(1). If you change the direction of one of the coils relative to the other, will it break the orthorhombic symmetry and drop it to monoclinic? This may provide a clue to the space group you should try.
James
----------
From: James Stroud
I should have said "not compelling".
James
----------
From: Randy Read
Hi,
I agree that Arcimboldo is a good idea, especially at such good resolution (assuming your space group is correct, an issue that others have commented on). But you could also try, directly, something that Isabel Uson has pointed out about molecular replacement with helices in Phaser. For such asymmetric objects, the rotational sampling tends to be too coarse to sample the orientations rotated around an axis perpendicular to the helix axis. So you could look at the rotational sampling chosen by Phaser and override it with something, say, 1/2 to 2/3 as large.
Another issue is that, with a long helix, there will be many nearly-correct solutions offset by partial turns. So the sequence register could be wrong.
Good luck!
Randy Read
------
Randy J. Read
Department of Haematology, University of Cambridge
----------
From: M T
Dear Napoleão, we solved this type of problem in a paper of 2004 (J. Mol. Biol. (2004) 342, 275–287) with the using of EPMR which use a evolutionary search algorithm, I will send you the paper later. It was 36aa dimeric coiled-coil and we had a lot of molecular replacement problems with other tested molecular replacement programs.
http://www.epmr.info/UsersGuide.html
Good luck.
----------
From: Vellieux Frederic
Just to add to what has been "said" (written) before:
coiled coiled or simply helices can be very problematic for M.R.. Human sacrifice has never given any positive result (as reported in the literature as far as I know), but "heavy atom sacrifice" could be attempted ("heavy atom" includes in my view atoms that are used for SAD, MAD - such as S, Se... - in addition to Au, Pt...) in parallel to your M.R. searches which may never provide you with an acceptable solution.
Fred.
----------
From: Sergei Strelkov
From: Filip Van Petegem
Hello,
--
Filip Van Petegem, PhD
Assistant Professor
The University of British Columbia
Dept. of Biochemistry and Molecular Biology
2350 Health Sciences Mall - Rm 2.356
Vancouver, V6T 1Z3
----------
From: Napoleão Valadares
Thank you all for the replies.
Sorry for taking so long to reply, I was actually trying some of your interesting ideas (and I'm still trying).
I tried using the low resolution data sets for the molecular replacement (thanks to Yuriy Patskovsky), I also improved and increased my coiled coil database and employed it in many approaches using EPRM (interesting program I was not aware of), which I found to produce lots of data, hopefully addressing at some extent the helixes bent (thanks to Bernhard Rupp). I also tried some more tweaking in Phaser, although not sure if did it properly (thanks to Randy Read).
There is no twinning as far as I can tell (thanks to Ed Pozharski for the tip). Using a data set with enough completeness (360 degrees @ Brookhaven) and processing in P1 did not help me because in this space group there is most likely 2-3 helixes in the asymmetric unit, which complicates the problem (and it takes a lot of time for Phaser to run). Automated approaches also did not yield a better result (as far as I can tell). I'm convinced that the space group is C2221, but I may be wrong.
Thanks to Sergei Strelkov for the numerous useful suggestions on how to approach the problem.
One of the big issues for me is to discriminate between a lot of similarly good density maps. For example:
http://www.fullonline.org/coils/coil1.jpg
http://www.fullonline.org/coils/coil2.jpg
I have hundreds of solutions like these and I think they are all wrong.
I couldn't manage to run Arcimboldo, could not find a tutorial on it either. It was highly recommended here (and elsewhere), so I'm definitely willing to give it a try (thanks Isabel Uson).
You guys opened my eyes about a series of issues that I should learn about and approach, I'm most thankful for that.
Best regards,
Napo
----------
From: Yuri Pompeu
Hi Napo,
i am sorry if you already answered this, why are so sure your solutions are wrong?
Your maps do not look that bad. What kind of R's do you get?
Are you not happy with the packing of your coils?
----------
From: Sergei Strelkov
----------
From: George M. Sheldrick
We have unintentionally discovered a very simple way of telling whether
an MR solution is correct or not, provided that (as in this case) native
data have been measured to about 2.1A or better. This uses the current
beta-test of SHELXE that does autotracing (available on email request).
First rename the PDB file from MR to name.pda and generate a SHELX
format file name.hkl, e.g. using Tim Gruene's mtz2hkl, where 'name' may
be chosen freely but should be the same for both input files. Then run
SHELXE with a large number of autotracing cycles (here 50), e.g.
shelxe name.pda -a50 -s0.5 -y2
-s sets the solvent content and -y a resolution limit for generating
starting phases. If the .hkl file contains F rather than intensity the
-f switch is also required.
If the model is wrong the CC value for the trace will gradually
decrease as the model disintegrates. If the model is good the CC will
increase, and if it reaches 30% or better the structure is solved. In
cases with a poor but not entirely wrong starting fragment, the CC may
vary erratically for 10-30 cycles before it locks in to the correct
solution and the CC increases over three or four cycles to the value
for a solved structure (25 to 50%). The solution with the best CC is
written to name.pdb and its phases to name.phs for input to e.g. Coot.
George
--
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
----------
From: Napoleão Valadares
Date: 17 October 2011 18:09
Hi there!
I got crystals from some synthetic peptides I bought, they are 30 residues long and are supposed to form a coiled coil. I collected various data sets (home source, Brookhaven and Diamond), including some at the resolution of 1.65 A, for which the space group appears to be C222 or C2221. The unit cell is small, 22.67, 88.06, 26.13, and the Matheus Coefficient indicates that's there's only one helix in the asymmetric unit and a 25% solvent content.
I have tried A LOT of Molecular Replacement using Phaser and Phenix AutoMR. I'm using a 80% identity coiled coil helix as search model. The programs give me solutions with "reasonable" maps, but it is never possible to refine to achieve Rvalues below 0.40. Additionally, maps from different solutions look reasonable, so I'm thinking these are all bias.
I have 5 other synthetic 30 residues peptides (that crystallize in different space groups and diffract to lower resolutions), including a SelenoMethionine (SM) derivative (but it does not have enough anomalous signal, ASU is too big, it is possible that the SM are disordered). I'm stuck on this since March.
Regarding the search model, I already tried trimming some or all side chains and removing 2, 3 or 5 residues on each/both sides. I also tried other search models. Maybe some "magic" combination of parameters on Phaser or other programs can help me.
What is your advice regarding how to proceed with MR? Is there some program, procedure, parameter, pray or human sacrifice that could help me?
Thank you.
Regards,
Napo
----------
From: Bernhard Rupp (Hofkristallrat a.D.
If for example your presumed coil is differently bent than your search
probe, overall rmsd is high and MR will likely fail. But maybe ARCIMBOLDO
with helix fragments might do at 1.65A? George or Isabel will have a better
answer.
Good luck, BR
PS: And may San Matheus de las Coefficientes listen to your prayers... :-)
----------
From: Ed Pozharski
Did you check for twinning? The most radical approach is to reprocess
in P1 and see what R-values you get
--
Edwin Pozharski, PhD, Assistant Professor
University of Maryland, Baltimore
----------------------------------------------
When the Way is forgotten duty and justice appear;
Then knowledge and wisdom are born along with hypocrisy.
When harmonious relationships dissolve then respect and devotion arise;
When a nation falls to chaos then loyalty and patriotism are born.
------------------------------ / Lao Tse /
----------
From: Nat Echols
Did you try running an automated building program (phenix.autobuild or ARP/wARP)? If the solutions are real, the software should do a very good job, much better than refinement alone. Otherwise, I agree ARCIMBOLDO sounds like a good choice here.
-Nat
----------
From: James Stroud
This strongly suggests a space group issue.
If the systematic absences are compelling, you should try to drop it down to P2 first. Look at your packing in the C222(1). If you change the direction of one of the coils relative to the other, will it break the orthorhombic symmetry and drop it to monoclinic? This may provide a clue to the space group you should try.
James
----------
From: James Stroud
I should have said "not compelling".
James
----------
From: Randy Read
Hi,
I agree that Arcimboldo is a good idea, especially at such good resolution (assuming your space group is correct, an issue that others have commented on). But you could also try, directly, something that Isabel Uson has pointed out about molecular replacement with helices in Phaser. For such asymmetric objects, the rotational sampling tends to be too coarse to sample the orientations rotated around an axis perpendicular to the helix axis. So you could look at the rotational sampling chosen by Phaser and override it with something, say, 1/2 to 2/3 as large.
Another issue is that, with a long helix, there will be many nearly-correct solutions offset by partial turns. So the sequence register could be wrong.
Good luck!
Randy Read
------
Randy J. Read
Department of Haematology, University of Cambridge
----------
From: M T
Dear Napoleão, we solved this type of problem in a paper of 2004 (J. Mol. Biol. (2004) 342, 275–287) with the using of EPMR which use a evolutionary search algorithm, I will send you the paper later. It was 36aa dimeric coiled-coil and we had a lot of molecular replacement problems with other tested molecular replacement programs.
http://www.epmr.info/UsersGuide.html
Good luck.
----------
From: Vellieux Frederic
Just to add to what has been "said" (written) before:
coiled coiled or simply helices can be very problematic for M.R.. Human sacrifice has never given any positive result (as reported in the literature as far as I know), but "heavy atom sacrifice" could be attempted ("heavy atom" includes in my view atoms that are used for SAD, MAD - such as S, Se... - in addition to Au, Pt...) in parallel to your M.R. searches which may never provide you with an acceptable solution.
Fred.
----------
From: Sergei Strelkov
Dear Napoleão,
I will try to summarize our experience and give some suggestions.
Few reasons why MR with coiled coils can be very tricky, such as
their asymmetric shape and their ability to overlap onto themselves
upon a shift and rotation (for a heptad-based coiled coil, this would be a
shift by 7 residues) have been already mentioned.
And I can add two more.
First, we very often see that coiled coils get bent due to
crystal contacts. This means that the conformation may be
quite different compared to your search model.
Second, many coiled coil sequences were seen to
assemble into structures different to what
they were supposed to be. Examples I know include
a trimer when a dimer was expected, an antiparallel
coiled coil when a parallel one was expected,
a monomer when a a dimer was expected,
chains offset with respect to each other, etc etc
We were able to phase several short (40-60 residues)
coiled coils by MR in the past. The paper
Strelkov SV et al (2002) EMBO Journal describes
two such cases (please look at the Methods section
which provides a fairly detailed explanation of what
we did). Both were done with Molrep.
I do have to say that we also failed on MR
miserably in many other cases, whatever we tried.
One recent example of a coiled coil fragment
that assembled to something entirely different
than we expected it to, is in Nicolet S et al (2010)
J. Struct. Biol. There, we really had to use heavy atoms. You have collected many data sets and you are still not sure
about the space group - ? Have you looked at the systematic absences
carefully? If you are still missing the (00l) axis then you
probably could try collecting it again while considering the
orientation of your crystal.
Knowing the correct space group is a valuable
information. Yes there are cases when you can not
distinguish between two space groups from the diffraction pattern
(e.g. P61 and P65) and then you have to run your MR search
twice in both groups. Yes modern programs will do
it almost by default for you. But if you do know your correct space
group (for instance C2221 and not C222) then when you
do the MR search in the two space groups you may (with some
luck) see better results with the first than with the second.
This may be a hint that your C2221 solution is correct. As mentioned already, is really advisable to try other MR programs (MolRep, epmr etc),
as in fact they are all based on different algorithms.
From your description I am guessing that you use a monomeric helix
currently - ?
Indeed you really should try different models, anything you can
get your hands on. In fact this is a number one factor to
get MR to work.
On one hand, you should indeed try shortening your
model (even systematically trimming one residue at a time).
Do not hesitate to use much shorter models (less than
half of your full helix). You can trim the wrong side chains
but I would not advise chopping off all of them.
With a short helix, once you have the first candidate
solution, try searching for a second copy with the packing penalty
switched off. If you get another helix overlapping with your first
solution (with some offset) this may be a sign of success.
(see the EMBO J paper)
On the other hand, our experience shows that in many cases
you can only get a solution if you use a full coiled coil
(dimer, trimer,...) and not a monomeric helix.
If your real structure is a non-crystallographic dimer etc,
then you should definitely search with a dimer etc.
If your oligomer is due to a crystallographic axis, then
you may try this as well, after switching off the
packing penalty. But beware that the oligomer
will almost certainly land on the axis, even for a wrong solution.
Hope this will help, and best of luck with your MR
searches -- which are fun, since they still require
some thinking!
----------I will try to summarize our experience and give some suggestions.
Few reasons why MR with coiled coils can be very tricky, such as
their asymmetric shape and their ability to overlap onto themselves
upon a shift and rotation (for a heptad-based coiled coil, this would be a
shift by 7 residues) have been already mentioned.
And I can add two more.
First, we very often see that coiled coils get bent due to
crystal contacts. This means that the conformation may be
quite different compared to your search model.
Second, many coiled coil sequences were seen to
assemble into structures different to what
they were supposed to be. Examples I know include
a trimer when a dimer was expected, an antiparallel
coiled coil when a parallel one was expected,
a monomer when a a dimer was expected,
chains offset with respect to each other, etc etc
We were able to phase several short (40-60 residues)
coiled coils by MR in the past. The paper
Strelkov SV et al (2002) EMBO Journal describes
two such cases (please look at the Methods section
which provides a fairly detailed explanation of what
we did). Both were done with Molrep.
I do have to say that we also failed on MR
miserably in many other cases, whatever we tried.
One recent example of a coiled coil fragment
that assembled to something entirely different
than we expected it to, is in Nicolet S et al (2010)
J. Struct. Biol. There, we really had to use heavy atoms. You have collected many data sets and you are still not sure
about the space group - ? Have you looked at the systematic absences
carefully? If you are still missing the (00l) axis then you
probably could try collecting it again while considering the
orientation of your crystal.
Knowing the correct space group is a valuable
information. Yes there are cases when you can not
distinguish between two space groups from the diffraction pattern
(e.g. P61 and P65) and then you have to run your MR search
twice in both groups. Yes modern programs will do
it almost by default for you. But if you do know your correct space
group (for instance C2221 and not C222) then when you
do the MR search in the two space groups you may (with some
luck) see better results with the first than with the second.
This may be a hint that your C2221 solution is correct. As mentioned already, is really advisable to try other MR programs (MolRep, epmr etc),
as in fact they are all based on different algorithms.
Regarding the search model, I already tried trimming some or all side chains and removing 2, 3 or 5 residues on each/both sides.
From your description I am guessing that you use a monomeric helix
currently - ?
Indeed you really should try different models, anything you can
get your hands on. In fact this is a number one factor to
get MR to work.
On one hand, you should indeed try shortening your
model (even systematically trimming one residue at a time).
Do not hesitate to use much shorter models (less than
half of your full helix). You can trim the wrong side chains
but I would not advise chopping off all of them.
With a short helix, once you have the first candidate
solution, try searching for a second copy with the packing penalty
switched off. If you get another helix overlapping with your first
solution (with some offset) this may be a sign of success.
(see the EMBO J paper)
On the other hand, our experience shows that in many cases
you can only get a solution if you use a full coiled coil
(dimer, trimer,...) and not a monomeric helix.
If your real structure is a non-crystallographic dimer etc,
then you should definitely search with a dimer etc.
If your oligomer is due to a crystallographic axis, then
you may try this as well, after switching off the
packing penalty. But beware that the oligomer
will almost certainly land on the axis, even for a wrong solution.
Hope this will help, and best of luck with your MR
searches -- which are fun, since they still require
some thinking!
Sergei
-- Prof. Sergei V. Strelkov Laboratory for Biocrystallography Department of Pharmaceutical Sciences, Katholieke Universiteit Leuven O&N2, Campus Gasthuisberg, Herestraat 49 bus 822, 3000 Leuven, Belgium
From: Filip Van Petegem
Hello,
James Holton can probably tell more about this, but it is possible to create a library of potential coiled coil structures with differences in number of residues, superhelical radius, and residues per superhelical turn. A library of 300 theoretical coiled coils was generated and, in conjunction with EPMR, was used successfully to solve the structure of a KCNQ tetrameric coiled coil.
See: Howard et al (2007) Neuron 53(5),663-675.
And I second Sergei Strelkov's comment: what should be an expected tetramer could show up as a trimer, etc... so you may want to check via ultracentrifugation that you have the expected stoichiometry.
Regards,
Filip
Filip Van Petegem, PhD
Assistant Professor
The University of British Columbia
Dept. of Biochemistry and Molecular Biology
2350 Health Sciences Mall - Rm 2.356
Vancouver, V6T 1Z3
----------
From: Napoleão Valadares
Thank you all for the replies.
Sorry for taking so long to reply, I was actually trying some of your interesting ideas (and I'm still trying).
I tried using the low resolution data sets for the molecular replacement (thanks to Yuriy Patskovsky), I also improved and increased my coiled coil database and employed it in many approaches using EPRM (interesting program I was not aware of), which I found to produce lots of data, hopefully addressing at some extent the helixes bent (thanks to Bernhard Rupp). I also tried some more tweaking in Phaser, although not sure if did it properly (thanks to Randy Read).
There is no twinning as far as I can tell (thanks to Ed Pozharski for the tip). Using a data set with enough completeness (360 degrees @ Brookhaven) and processing in P1 did not help me because in this space group there is most likely 2-3 helixes in the asymmetric unit, which complicates the problem (and it takes a lot of time for Phaser to run). Automated approaches also did not yield a better result (as far as I can tell). I'm convinced that the space group is C2221, but I may be wrong.
Thanks to Sergei Strelkov for the numerous useful suggestions on how to approach the problem.
One of the big issues for me is to discriminate between a lot of similarly good density maps. For example:
http://www.fullonline.org/coils/coil1.jpg
http://www.fullonline.org/coils/coil2.jpg
I have hundreds of solutions like these and I think they are all wrong.
I couldn't manage to run Arcimboldo, could not find a tutorial on it either. It was highly recommended here (and elsewhere), so I'm definitely willing to give it a try (thanks Isabel Uson).
You guys opened my eyes about a series of issues that I should learn about and approach, I'm most thankful for that.
Best regards,
Napo
----------
From: Yuri Pompeu
Hi Napo,
i am sorry if you already answered this, why are so sure your solutions are wrong?
Your maps do not look that bad. What kind of R's do you get?
Are you not happy with the packing of your coils?
----------
From: Sergei Strelkov
Dear Napoleão,
Thank you for updating everyone on your
efforts, and also acknowledging the advice.
I wanted to respond to your question regarding maps.
I know that many people who try to figure out
whether or not their MR solution is the right one
would ask the same question.
So first of all if you wonder why you actually get very
decently looking maps the answer is a classical one:
because 'the phases are more important than amplitudes'.
The appearance of your map is defined by
your model phases, and hence a good match between the
model and the map may not be taken as
a sign of a correct solution. Once again: never ever!
On the contrary, at least in your coil1.jpg
image I clearly see that the density exactly follows
the model which is almost entirely poly-Ala.
Unless your protein is really poly-Ala this should be
alarming. If you had a correct solution they you
would hope to see the (difference)density for
at least some missing side chains.
And a second point. Unless your model contains
the complete chain (which is rarely the case, especially
for the coiled coils, as discussed already)
a sign of the correct solution would be the appearance
of extra density near the N- and/or C-terminus of
the model. If it is not there, it is almost certainly not
a solution.
And you should not be worried about the R-factors being
very high at this stage. If the solution is correct then
you should see at least some extra features in the map.
Kind regards,
Sergei
Thank you for updating everyone on your
efforts, and also acknowledging the advice.
I wanted to respond to your question regarding maps.
I know that many people who try to figure out
whether or not their MR solution is the right one
would ask the same question.
So first of all if you wonder why you actually get very
decently looking maps the answer is a classical one:
because 'the phases are more important than amplitudes'.
The appearance of your map is defined by
your model phases, and hence a good match between the
model and the map may not be taken as
a sign of a correct solution. Once again: never ever!
On the contrary, at least in your coil1.jpg
image I clearly see that the density exactly follows
the model which is almost entirely poly-Ala.
Unless your protein is really poly-Ala this should be
alarming. If you had a correct solution they you
would hope to see the (difference)density for
at least some missing side chains.
And a second point. Unless your model contains
the complete chain (which is rarely the case, especially
for the coiled coils, as discussed already)
a sign of the correct solution would be the appearance
of extra density near the N- and/or C-terminus of
the model. If it is not there, it is almost certainly not
a solution.
And you should not be worried about the R-factors being
very high at this stage. If the solution is correct then
you should see at least some extra features in the map.
Kind regards,
Sergei
----------
From: George M. Sheldrick
We have unintentionally discovered a very simple way of telling whether
an MR solution is correct or not, provided that (as in this case) native
data have been measured to about 2.1A or better. This uses the current
beta-test of SHELXE that does autotracing (available on email request).
First rename the PDB file from MR to name.pda and generate a SHELX
format file name.hkl, e.g. using Tim Gruene's mtz2hkl, where 'name' may
be chosen freely but should be the same for both input files. Then run
SHELXE with a large number of autotracing cycles (here 50), e.g.
shelxe name.pda -a50 -s0.5 -y2
-s sets the solvent content and -y a resolution limit for generating
starting phases. If the .hkl file contains F rather than intensity the
-f switch is also required.
If the model is wrong the CC value for the trace will gradually
decrease as the model disintegrates. If the model is good the CC will
increase, and if it reaches 30% or better the structure is solved. In
cases with a poor but not entirely wrong starting fragment, the CC may
vary erratically for 10-30 cycles before it locks in to the correct
solution and the CC increases over three or four cycles to the value
for a solved structure (25 to 50%). The solution with the best CC is
written to name.pdb and its phases to name.phs for input to e.g. Coot.
George
--
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
----------
From: Napoleão Valadares
The problem was resolved by Isabel Uson and Giovanna Petrillo using ARCIMBOLDO!
The real spacegroup is P21, twinned to look like C2221. I would probably take years to solve that on my own, they did it very fast. This was a difficult problem, and still is, since if I try MR using the correct solution and space group I cannot resolve it. I definitely recommend everybody with difficulties to give Arcimboldo a try. Thank you Isabel!
Thank you Sergei Strelkov, your message was very informative as usual. ;)
Thank you George M. Sheldrick for the information on how to discrimitate between correct and wrong MR solutions using SHELXE.
Best regards,
Napo
The real spacegroup is P21, twinned to look like C2221. I would probably take years to solve that on my own, they did it very fast. This was a difficult problem, and still is, since if I try MR using the correct solution and space group I cannot resolve it. I definitely recommend everybody with difficulties to give Arcimboldo a try. Thank you Isabel!
Thank you Sergei Strelkov, your message was very informative as usual. ;)
Thank you George M. Sheldrick for the information on how to discrimitate between correct and wrong MR solutions using SHELXE.
Best regards,
Napo
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