1) You dont say whether there is a non crystallographic translation
vector - if that is so the twinning statistics can be misleading.
SFCHECK analyses this or you can just run a native patterson to 4A say
and see if there is an off-origin peak.
2) The twinning tests that use the correlation between possibly
"twinned" reflectons ( ie hkl and kh-l for tetragonal) always say your
data is twinned if the true symmetry IS 422 when hkl and kh-l are
symmetry equivalents.. (They should say - either twinned or with a
higher symmetry..)
3) I believe in the moments plotted in TRUNCATE providing there isnt a
non crystallographic translation vector..
Eleanor
Joe Smith wrote:
Hi all,
We have collected few X-ray data sets for a protein-RNA complex to
resolutions of 3.2-3.5A. While processing the data using HKL2000, we
have obtained following distortion index consistently:
primitive cubic 19.16% 127.98 74.67 130.85 74.57 85.00 73.91
111.17 111.17 111.17
90.00 90.00 90.00
I centred cubic 24.26% 132.28 189.90 127.98 120.33 85.97 120.47
150.05 150.05 150.05
90.00 90.00 90.00
F centred cubic 22.17% 177.47 190.41 189.90 46.24 90.79 91.90
185.93 185.93 185.93
90.00 90.00 90.00
primitive rhombohedral 16.32% 130.85 129.06 174.88 131.21 133.20 86.12
144.93 144.93 144.93
116.85 116.85 116.85
229.30 229.30 74.67
90.00 90.00 120.00
primitive hexagonal 15.40% 130.85 127.98 74.67 106.09 74.57 95.00
129.41 129.41 74.67
90.00 90.00 120.00
primitive tetragonal 9.40% 127.98 130.85 74.67 74.57 106.09 95.00
129.41 129.41 74.67
90.00 90.00 90.00
I centred tetragonal 0.91% 177.47 174.88 74.67 90.21 88.59 91.47
176.18 176.18 74.67
90.00 90.00 90.00
primitive orthorhombic 9.38% 74.67 127.98 130.85 85.00 105.43 106.09
74.67 127.98 130.85
90.00 90.00 90.00
C centred orthorhombic 6.53% 74.67 245.95 130.85 89.44 105.43 89.13
74.67 245.95 130.85
90.00 90.00 90.00
I centred orthorhombic 0.84% 74.67 174.88 177.47 88.53 91.41 90.21
74.67 174.88 177.47
90.00 90.00 90.00
F centred orthorhombic 0.68% 74.67 245.95 252.32 89.16 88.86 89.13
74.67 245.95 252.32
90.00 90.00 90.00
primitive monoclinic 6.52% 74.67 130.85 127.98 95.00 106.09 74.57
74.67 130.85 127.98
90.00 106.09 90.00
C centred monoclinic 0.49% 74.67 245.95 130.85 89.44 105.43 89.13
74.67 245.95 130.85
90.00 105.43 90.00
primitive triclinic 0.00% 74.67 127.98 130.85 85.00
74.57 73.91
As you see, distortion index table indicates I centered tetragonal, I
centered orthorhombic, F centered orthorhombic, C centered monoclinic
and triclinic as possible Bravais lattices.
Data processed in I centered tetragonal gives low Rmerge in all the
possible space groups namely I4, I41, I422 and even I4122. Other
space groups in lower symmetry lattices also gave low R merge values
(around 6% in most of the cases).
Since we have not been able to obtain a solution in any of the space
group from I centered tetragonal to triclinic (I4, I4122, I222, C2 and
even P1) using Se-SAD, we decided to check the data for any intrinsic
problem such as twinning.
Cumulative intensity distribution calculated using scalepack2mtz shows
no sign of twinning. However, data processed in I4 shows nearly a
perfect twin (twin fraction=0.489 with twin operator 100 0-10 00-1) in
Yeates server whereas SFcheck indicates a twin fraction of 0.431 with
twin operator –h,+k,-l. Data processed in I4122, I222, C2 and P1
doesn't show any twinning due to absence of any twin laws for these
space groups.
Now my question is:
- Are data showing low Rmerge value in I4122 due to nearly perfect
twin in space group I4?
- Why cumulative intensity distribution shows a normal pattern for
the data where as Yeates server and SFcheck indicates nearly a perfect
twin? Why Yeates server and SFCheck shows different twin fraction and
twin operator?
- Is it possible to detwin this data and use it for structure solution?
Thank you for reading till this line and I am sorry for such a long
mail. I hope I haven't made any mistake at any stage. I really need
your valuable suggestions to solve this problem.
Regards
Joe
