Hi Dan,
In hexagonal R32 setting there is translational crystallographic
symmetry leading to systematic absences. If you think Xtal 5 is
basically the same but with slightly different packing that break the
symmetry then you will have pseudo-translational symmetry leading to one
set of very strong intensities (those that obey the hexagonal R32
lattice rules) and a set with very weak intensities (the once that are
systematically absent in R32). That abundance of very strong and very
weak intensities would give you a bimodal intensity distribution that
would lead to the observed very large second moment in truncate. It is
not immediately clear though what is going on with the other xtals. It
may be of interest to look at the actual intensity distributions for
different resolution slices to see if it is bimodal or not.
Bart
[EMAIL PROTECTED] wrote:
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
--
==============================================================================
Bart Hazes (Assistant Professor)
Dept. of Medical Microbiology & Immunology
University of Alberta
1-15 Medical Sciences Building
Edmonton, Alberta
Canada, T6G 2H7
phone: 1-780-492-0042
fax: 1-780-492-7521
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