Yes, having different crystal forms in the same crystallization conditions is, 
while clearly uncommon, not unheard of. I had a case once where at least four 
different crystal forms were observed in crystals harvested from identically 
prepared drops.  It may be that there is some major set of contacts that 
promote certain arrangement of protein molecules (fiber-like), and these can 
then be packed in multiple ways as controlled by another ser of weaker 
interactions. 


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<div>-------- Original message --------</div><div>From: Florian Schmitzberger 
<schmitzber...@crystal.harvard.edu> </div><div>Date:10/13/2014  4:47 AM  
(GMT-05:00) </div><div>To: CCP4BB@JISCMAIL.AC.UK </div><div>Subject: [ccp4bb] 
I222 - P22121 space-group ambiguity </div><div>
</div>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


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