hi all
This is regarding solving structure which has twinning .
Intially I processed the data in P222 and did structure solution but cumulative intensity distribution shows twinning of 1.5
pseudo-merohedral twinning
My data is upto 2.7A resolution and 14kDa of homodimer total of 28kDa.
But I thought that due to NCS also this type characteristic intensity distri. occurs because generally twinning doesn't occur in orthorhombic, monoclinic system.
Intially i tried solution in p212121 since I don't higher angle reflections i was notable to see h,0,0 odd absence. so i changed
spcaegroup P22121 by interchanging cell axis which I posted earlier in CCP4bb.
I got solution in p22121 but it doesn't refine it gets struck there in R 40 & rfree 44 and the geometry gets distorted completely.
Even in this case I never get sol. in Amore but only in phaser that also one molecule and using one mol again try to search for
another it gives soltuion but z= 6.2 and in case of monomer z=7.2.
Then approached Mr. Markus who has published paper in Acta D in similar.
He said following things and I did accordingly
1.My crystal is P2 but due to twinning simulates P222
2. I have pseudo-merohedral twinning,
3. The twin operator is a symmetry element of P222 that is not present in P2, i.e. I am using
-h, -k, l or h, -k, -l as twin operator.
Loggraph of the detwin file also showed twinning faction of 0.40.
Cell parameters are a= 62.3, b= 57.1, c= 83.6, 90 , 90, 90 in p222
Cell parameters in a= 62.4, b=57.4, c=83.4, 90, 89.93, 90
In Rsym in both cases are similar 11.0 (44) in P222
In case of P2 13(48)
But the probelm is that if try for solution with model having 72 % homology with my protein I am not getting solution in P21 using PHASER.
But in case of P21212 I am getting solution but it doesn't refine it gets struck with R and Rfree of 40 & 44.
so plz anyone can help me in sorting out this probelm I am sending log file of detwin and also the cumulative distri graph in p222 which shows twinning nature.
Ethayathulla
A.S.Ethayathulla,Ph.D.
Department of Biophysics
All India Institute of Medical Sciences
Ansari Nagar
New Delhi-110029
India.
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