Hi Hari,
I have seen a poster last year's DGK by Ina Dix and Madhumati Sevanna
from George Sheldricks' group. Not sure it was published yet.
They collected data on non-merohedrally twinned crystals and processed
and scaled the domains separately and merged them afterwards, thus
increasing the redundancy. Indeed they got better results with S-SAD
phasing when using the combined data. This was on Insulin and Glucose
isomerase, which do diffract better then the average crystal and since
the data came from the same crystal, albeit twinned, the were no
problems of non-isomorphism.
On the other hand, to come back to the original question: You could also
use a kappa goniostat for your P1 crystal to get complete and redundant
data.
Best wishes,
Bram
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Jacob Keller wrote:
Shouldn't all of the "crystal-to-crystal" differences be taken out
automatically by scaling, and is there not the same proportional
anomalous signal in every isomorphous crystal, regardless of the
background? I would think that using multiple crystals would give a
better idea of "the truth," as if taking many snapshots of the same
object, and putting them together to form a three-dimensional object.
In Hazes' language, don't all isomorphous crystals "draw from the same
[underlying] distribution?"
Jacob Keller
ps admittedly if there is radiation damage or other non-isomorphisms,
this reasoning does not apply.
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Jacob Pearson Keller
Northwestern University
Medical Scientist Training Program
Dallos Laboratory
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2240 Campus Drive
Evanston IL 60208
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----- Original Message ----- From: "Bart Hazes" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Wednesday, July 23, 2008 10:05 AM
Subject: Re: [ccp4bb] Using multiple crystals for structure solution
in P1 using MAD/SAS/SAD
Increasing redundancy only helps if all data draw from the same
distribution so you get a more accurate estimate of the mean of the
distribution. When dealing with different crystals,
crystal-to-crystal variation is likely larger than the anomalous
signal you are looking for and I'm therefore not convinced that
merging of data is a good idea (never hurts to try though).
I wonder if it would work better to derive anomalous differences for
the individual data sets first and then merge those anomalous
differences. This may allow the subtraction between F+ and F- to
remove some of the systematic differences there may be between
crystal forms.
Bart
Kay Diederichs wrote:
hari jayaram schrieb:
...
I was wondering if anyone could comment on combining datasets from
multiple P1 crystals to increase the redundancy even further for
such heavy atom ( SAS / SAD ) or MAD experiments.
Hari,
well, my comment would be that it should be possible in principle
from what you describe, but the outcome strongly depends on the
details (size of expected and observed anomalous and isomorphous
signal, internal anomalous correlation coefficients, I/sigma and
R-factors, radiation damage, are crystals isomorphous, ...).
To increase the quality of the reduced data it would be advisable to
rotate around different axes, which is possible at some - but not
all - beamlines. This is even more true in P1.
For all of the major data reduction programs there exist specific
programs for merging data, and it does make a lot of sense to merge
your passes (but don't merge radiation-damaged data with undamaged
data)!. I would suggest to use at least two different data reduction
packages - everything depends on the quality of the data reduction,
and the programs have strengths in different areas.
HTH,
Kay
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