Dear Friends,
I have some crystals of a small RNA in sg R32 that exhibit some bizzare
behaviors and fail to give phasing solutions. I am hoping someone out there
might be able to lend some insight. All crystals come out of the same condition
and look the same morphologically but, for reasons unknown and at this point
uncontrollable, have very different cell dimensions:
Xtal 1: R32 77 x 77 x 80 Ang
Xtal 2: R32 77 x 77 x 86 Ang
Xtal 3: R32 78 x 78 x 366 Ang
Xtal 4: R32 78 x 78 x 460 Ang
Xtal 5: P3(1)21 77 x 77 x 85 Ang
I've collect >95% complete datasets for each crystal form to resolutions between
2.2-3.0 Ang. Denzo/scalepack outputs look fine in each case. Overall R-sym's
are okay (range is 8-13%). The only apparent red flag is the 2nd moment of I
calculated in truncate. These values are much larger than those expected for
either twinned (1.5) or untwinned (2.0) data:
Xtal 1: 3.2
Xtal 2: 2.5
Xtal 3: 4.9
Xtal 4: 3.7
Xtal 5: 4.2
Also, use of the Yeates server to look for partial twinning tells me there are
no twin laws for R32, while the P3(1)21 data does not seem to be twinned.
I'm guessing crystals 1 & 2 are very similar but still somewhat non-isomorphous,
and crystal 5 is also very similar with a breakdown in space group symmetry
that would put 3 mols/ASU instead of 1 mol/ASU as in R32. But I do not
understand what is going on with crystals 3 & 4. Has anyone out there
experienced similar non-integer multiples of a cell dimension for a given
crystal? I have covalently attached iodine in some of these crystal forms. But
no luck finding any sites by SAD, and I can't help but wonder if this funny
c-dimension behavior and/or the high <I^2>/<I>^2 values are indicative of some
greater crystal pathology.
Thanks in advance,
Dan
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