It might help to look at the images and predictions. You a have serial vs
integral extinctions
(i.e. conditions limiting reflections are:)
P 2 21 21 (Standard: P 21 21 2, btw)
0K0 : K=2N only
00L : L=2N only
I222
HKL : H+K+L=2N only
Alternatively you could process unmerged data with XPREP or so and look
at the systematic absence violations.
The lattice metric is the same for both SGs (did not understand Eleanor's
remark)
In principle, differences between NCS and XS of compatible metric become
more distinct at
higher resolution (i.e. at night, all cats are grey).
Best, BR
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of
Florian Schmitzberger
Sent: Montag, 13. Oktober 2014 10:47
To: CCP4BB@JISCMAIL.AC.UK
Subject: [ccp4bb] I222 - P22121 space-group ambiguity
Hi everybody,
I collected a number of X-ray data sets from crystals originating from the
same cryst. drop. I solved the initial structure in P22121 space group by MR
with Phaser locating two molecules (data to ~ 2.1 Angstr.); refined R/Rfree:
0.213/0.244.
Processing of some of the other data sets with XDS/Aimless is consistent
with I222 space group (resolution ~ 2.6 Ang.). I can locate one molecule.
The unit-cell dimensions for I222 and the initial P22121 space groups for
two of the data sets are:
I222: a=87.8 b=101.18 c=123.63; P22121: a=93.34 b=105.47 c=122.98;
I superposed the molecule in I222 onto one of the two located for the
initially solved P22121; the orientation of the NCS-related molecule in
P22121 differs from the crystallographic-symmetry related one in I222.
Trying to solve this P22121 data set in I222 with MR, does not result in
high Z scores, and maps do not look good.
Some of the data sets that process in I222 to ~ 3 Angstr., I can also solve
in P22121, locating two molecules (differences may not be that clear in this
case, since the resolution is lower).
Some other data sets process in P22121 with Aimless; with a substantial
off-origin Patterson peak, indicating translational NCS. For these, Phaser
positions two molecules that are related by crystallographic translational
NCS. These two molecules are crystallographic-symmetry related in the
original P22121 data set. I can also solve these data sets in I222 space
group, with the overall Z score higher than for the P22121 data.
I am uncertain, what the 'true' space group for some of my data sets is.
Could it be that for data that process in P22121, but can be solved in I222,
reflections that would indicate I222 space group were not collected?
Alternatively, perhaps what I am seeing is that there is a (gradual)
transition of the crystal lattice (between P22121 and I222 or vice versa),
caused by variation in crystal handling/cooling or exposure to X-rays.
It's relevant to me, because in P22121 space group, a region of the molecule
that is of biological interest makes NCS-related crystal contacts that are
crystallographic-symmetry related in I222.
Has anybody observed similar cases? I would appreciate comments.
Cheers,
Florian
