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[ccp4bb]: Twinning problems (again....).
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Dear All,
Not CCP4 I know, but ccp4bb seems to be the best forum to get an intelligent
response.
The problem is that we appear to be getting twinned crystals, but that
neither truncate nor the twinning server shows this up. We have tetragonal
crystals, apparent space group P41212 or P43212. The crystals show 100%
incorporation of Se by mass-spec and the fluorescence scan shows a Se edge.
We collected Se-SAD data sets at the peak wavelength for five crystals, all
diffracting to 2.8 - 3.0A. The data was processed with mosflm. Parts of the
scala and truncate logfiles for one are reproduced below. As you can see, the
anomalous R merge is lower than the normal R merge, indicating (as I
understand it) that there is little or no anomalous signal. This (as I also
understand it) indicates twinning and the twinning cancels out any anomalous
signal. The truncate output, though, clearly indicates an
untwinned crystal.
N 1/resol^2 dmax Run1 AllRun
1 0.0128 8.85 0.079 0.079
2 0.0255 6.26 0.082 0.082
3 0.0383 5.11 0.082 0.082
4 0.0511 4.42 0.075 0.075
5 0.0639 3.96 0.076 0.076
6 0.0766 3.61 0.078 0.078
7 0.0894 3.34 0.086 0.086
8 0.1022 3.13 0.095 0.095
9 0.1149 2.95 0.111 0.111
10 0.1275 2.80 0.137 0.137
Overall 0.082 0.082
N 1/d^2 Dmin(A) Rfac Rfull Rcum Ranom Nanom Av_I SIGMA I/sigma
1 0.0128 8.85 0.079 0.060 0.079 0.058 291 18431. 2824.4 6.5
2 0.0255 6.26 0.082 0.064 0.081 0.048 647 11072. 1877.5 5.9
3 0.0383 5.11 0.082 0.061 0.081 0.052 877 6824. 1198.3 5.7
4 0.0511 4.42 0.075 0.060 0.079 0.043 1084 8979. 1406.5 6.4
5 0.0639 3.96 0.076 0.059 0.079 0.039 1241 6881. 1032.3 6.7
6 0.0766 3.61 0.078 0.061 0.078 0.040 1383 4751. 740.5 6.4
7 0.0894 3.34 0.086 0.066 0.079 0.043 1521 2825. 453.7 6.2
8 0.1022 3.13 0.095 0.074 0.080 0.048 1648 1529. 259.4 5.9
9 0.1149 2.95 0.111 0.087 0.081 0.054 1744 954. 188.3 5.1
10 0.1275 2.80 0.137 0.111 0.082 0.061 1826 565. 125.0 4.5
Overall: 0.082 0.064 0.082 0.046 12262 4432. 955.9 4.6
Cumulative intensity distribution (Acentric and centric)
Z N(Z)Atheor N(Z)Acen N(Z)Ctheor N(Z)Cen
0.0 0.0 0.0 0.0 0.0
0.1 9.5 9.8 24.8 25.8
0.2 18.1 18.8 34.5 35.5
0.3 25.9 26.9 41.6 41.7
0.4 33.0 34.0 47.3 47.2
0.5 39.3 39.9 52.1 52.6
0.6 45.1 45.8 56.1 56.5
0.7 50.3 51.2 59.7 59.4
0.8 55.1 56.0 62.9 61.9
0.9 59.3 60.1 65.7 64.8
1.0 63.2 64.0 68.3 67.6
The questions are these:
1) Are the crystals twinned, or is there another explanation?
2) If so, why doesn't truncate or the twinning server show this?
3) Can any useful info about the twinning be gained from the above two
questions?
4) Is there any other way of showing the twinning, without the need to
collect anomalous data (because otherwise it is going to be a hard slog
screening to find untwinned crystals)?
5) Back to finding another crystal form?
Many thanks in advance for any help you can give,
Mark Roe.
--
Dr. S. M. Roe,
X-Ray Laboratory Manager,
Section of Structural Biology,
Chester Beatty Laboratories,
The Institute of Cancer Research,
237 Fulham Road,
London.
SW3 6JB.
Tel. (+44) 020 7970 6047
Fax. (+44) 020 7970 6051
E-mail roe@icr.ac.uk
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