Hi all
I have a case of a dataset that indexed, integrated, and scaled well
in P 21 21 21 (55.6410 81.6493 147.1294 90.0000 90.0000
90.0000) . The data has an Mn(i/sd) of 2.1 at 3.5 A with a Rpim of
about 0.398 at the highest resolution shell (3.49-3.58).
Analysis with phenix.xtriage warns of pseudotranslational symmetry
(26% of origin).
x y z height p-value(height)
( 0.500, 0.000, 0.233 ) : 26.344 (2.681e-03)
( 0.000, 0.338, 0.000 ) : 5.380 (8.476e-01)
If the observed pseudo translationals are crystallographic
the following spacegroups and unit cells are possible:
space group operator unit cell of reference
setting
C 2 2 21 (b-1/4,c-1/4,2*a) x+1/2, y, z+1/4 (73.64, 55.47,
81.46, 90.00, 90.00, 90.00)
From what I've read about pseudo c-centering via pseudotranslational
symmetry, the problem exhibits itself with alternating weak and strong
reflections at low resolution, but become consistent at high
resolution. Inspection of the h+k parity groups via truncate does not
show this behavior .
Despite the fact the data was collected at the anomalous peak, I do
not observe any anomalous signal (DelAnom correlation between half-
sets is 0.013 for all data).
Using a reasonably complete model (>80%) I searched for two molecules
in the ASU in space group P 21 21 21 and obtained a solution at
TFZ=22.1 for two molecules related solely by a translation. However
the electron density maps (after rigid body refinement) are not great
(or maybe my expectations are too high). I am encouraged by the fact
the density is weak for a region of the model which should have a
different conformation, while strong density is maintained for the
rest of the molecule.
Is this the proper way to approach pseudotranslation (i.e. is there
any reason to believe that the solution obtained by MR is not the
correct solution?).
Is the space group determined? (i.e. does the pseudo c-centering
affect pointless's ability to analyze the systematic absences?).
Is the lack of a pattern of alternating weak/strong reflections normal
(would observing this behavior be dependent on the crystal
orientation) ?
any advice would be greatly appreciated! (especially from those who
have had a case like this before)
F
---------------------------------------------
Francis E. Reyes M.Sc.
215 UCB
University of Colorado at Boulder
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