Detwinning without a known structure, and such a high twin factor must
be very unreliable .
If you have only IT1 and IT2, the detwin algorithm gives I1 = (TF*It1
+ (1-TF)*IT2) / (1-2*TF)
(Maybe signs wrong there!) Since (1-2*TF) is almost 0, this is pretty
inaccurate..
But if you have a structure already and can calculate I1(calc) and
I2(calc), the detwinning keeps the calculated ration of
I1(calc)/I2(calc) and gets a much better estimate of I1 and I2.
So it isnt surprising you P6 twinned map looks better than one
calculated against detwinned P3 data.
Re the gap - I dont know.
Did you test to see if there was a pseudo translation vector or any
other strange features to the data.
Eleanor
Benirschke, Robert C. wrote:
Dear All,
I had written earlier about my twinned crystal that was giving me
problems......thanks to all who responded, it was very helpful. I
have another couple questions about the same crystal that is confusing
us. The twinned crystals space group is P3. Twinning was found using
the Yeates twinning server and the twin fraction of different crystals
ranged from .32-.48. Crystals with the lower twin fraction diffracted
to a much lower resolution and the more twinned crystals diffracted to 3.
Using the data from the .48 twinned crystal I refined into the twinned
P6 data (what we believed to be the space group originally) as well as
the detwinned P3 data. The result of this is that both are about the
same as far as R/Rfree (.28/.36) and the P6 is better as far as
structure geometry and density fit. Why is this the case? Does the
detwinning process introduce more error than if one just refined into
the twinned data? Is truncate and the twinning server ever wrong in
their diagnosis of twinning?
Secondly, this structure still has an area of missing density parallel
to the a-b plane which never resolves (1/6 at top and bottom along
c). This is the case for twinned, detwinned, P6, P3, and everything
else I have tried. It is also seen as P1, so we are not averaging out
weak reflections when scaling in different groups. I have also
searched these areas with FFEAR which yields nothing substantial.
This area is where many long disordered chains point (nearly 30
residues), so one of our thoughts is that there are many different
conformations of these tails that point into this area. Is this
possible? It seems to me that this would be a situation that promotes
twinning as well. Our second idea is that there is a translation of
1/6 a unit cell along c that would allow the rest of the cell to be
resolved as long as that plane of empty density is filled with same
contents. Would this be possible? Any ideas of how to test each of
these possibilities? Can anyone think of any other ideas.?
I´m sure I´ve said things that are unclear, so if clarification is
needed please let me know.
Thanks,
Robert
