I'm afraid that I have to disagree with Eleanor, a very rare event. Both
pure SIR and pure SAD give you only half of the necessary phase
information. The initial map will in both cases be a double image. One
then tries to improve it by density modification. For pure SAD one image
is positive and one is negative, so just setting negative density to
zero is a good start. For pure SIR both images are positive so this does
not help, the map remains centrosymmetric. This also explains why SAD
works much better if the solvent content is high, there is less danger
of the images canceling each other. This is much less of a problem for
MAD or SiRAS, both of which could work for a bromine derivative. If you
collected Friedel opposites you may still be able to try SIRAS.
George
On 23.07.19 11:43, Eleanor Dodson wrote:
Well - there are lots of ways to proceed. It doesnt really matter for
the crystallography theory if the exptl phases are from SAD or SIR.
Just harder to handle in some software
I think I would start to refine the poor MR model with the xptl phases
as restraints.
REFMAC will do this. Then see if the maps look better - can you see
features not in the model?..
If they do you could use Buccaneer to try to rebuild the existing
model using exptl phases..
It might work!
Eleanor
On Tue, 23 Jul 2019 at 09:55, Luke Smithers
<luke.smith...@research.uwa.edu.au
<mailto:luke.smith...@research.uwa.edu.au>> wrote:
Hi all,
I have collected native data and data on a bromide derivative of
one of my crystals, but have struggled to get a second derivative.
I went through the SIR pipeline as the anomalous signal from the
Br was weak and have generated some not-so-great phases. I have
also attempted MR with the closest match in the PDB (which is only
22% sequence identity) and got some more not-so-great phases. I
have found plenty of programs that will combine SAD/MR phases, is
there anything that will do SIR/MR? I understand it's likely I
will need more and better data and ideally a second derivative but
thought it was worth putting it out there to see if there is
something like this.
Luke
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