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Hi,
Ive collected multiple datasets (MAD and native) on crystals with apparent
trigonal symmetry and am having trouble refining my model.
The data scales reasonably well in both P31 and P3121. The data are highly
mosaic (typically 1.5-1.8), and though the # rejects is reasonable, the
rejections of some reflections look suspicious (below). I was eventually
able to phase my MAD data using the protein-bound 4Fe4S cluster (in P31,
2molec/ASU=40%solvent; in P3121, 1molec/ASU=40% solvent, both consistent
with the experimental electron density) and with the exception of a
~10-residue disordered loop, built most of the protein backbone and ~40% of
the sidechains.
Refinement of my model in CNS and SHELXL yields highly divergent or very
high (>40) Rfactors in both space groups.
The obvious possibility is twinning. I ran the data (processed in P31)
through the Yeates server, which showed an intensity distribution that did
not indicate twinning, but gave results to the partial twinning test that
suggested near-perfect twinning with the operator h, -h-k, -l. I think this
may be an artifact of actual higher symmetry, but when I refined in SHELXL
with the suggested operator and twin fraction, in P31 with two molecules, my
Rs dropped to 28 and 34 (at best), but won't improve further.
Im not quite sure how to proceed from here. Ive tried improving the data
quality by reprocessing my natives, annealing / improving the crystals to
get one with lower mosaicity, refining in different space groups. Ive had
trouble obtaining other crystals forms that diffract, so any input on how to
salvage this data would be much appreciated.
Thanks,
Jess
In P3121:
8 -2 18 102.4 1342.3 4.75
8 -2 18 a+ 35 0.8 827.5 0 168.1
-2 -6 18 a+ 109 11.2 2072.8 100 170.7
-2 8 -18 a+ 77 6.1 1447.6 100 167.9
-6 -2 -18 a+ 4 -1.1 656.7 0 165.4
-8 2 -18 a- 23 0.5 792.0 0 166.3
2 6 -18 a- 88 8.9 1784.6 100 169.9
2 -8 18 a- 92 8.6 1752.6 100 170.0
6 2 18 a- 16 -0.1 739.9 0 166.1
-8 6 18 a- 110 11.7 2106.2 100 169.1
In P31:
11 -6 13 42.2 1506.3 4.54
11 -6 13 a+ 53 0.0 939.7 0 190.2
-5 11 13 a+ 126 5.5 1843.7 100 199.7
-11 6 -13 a- 41 -0.1 929.4 0 188.8
6 5 -13 a- 127 9.1 2411.9 100 196.8
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