Hello

Space groups with point groups 622 and 432 merohedral twinning is not possible (they are the highest groups possible for proteins). 
If you could merge in 622 it means that Rmerge was very small. It is very likely that point group is either 622 or 6 with very strong rotational symmetry that is perpendicular to 6 fold axis. In these cases H test (sfcheck based on this) and N(z) test (truncate is based on this) will overestimate (H) or underestimate (N(z)) if twin is present. At the moment better test for these cases seems to be L-test (that is in ctruncate I think).

Solving structure using molecular replacement in the presence of twinning does not seem to be a problem (unless data are very noisy and/or model is too far). 
Pointless gives very good idea about "true" space group. 
I would solve in P6_{x} space groups and then run zanuda (from ysbl server) to correct space group.

I hope it helps

regards
Garib


On 30 Aug 2011, at 20:31, john peter wrote:

Hello All,

This is regarding twinning in a data set.

I collected a native data set  to resolution, 1.8 A.  I used XDS suite
to process and scale the data set. It scaled well in P622 and I found
systematic absence (l=6n present).

Hence thought the space group may be P6122/P6522. SFCHECK  did not
show any twinning and also it did not detect pseudo-translation.  Twin
test in http://nihserver.mbi.ucla.edu/pystats/     ( Merohedral Twin
Detector: Padilla-Yeates Algorithm ) showed  perfect twinning.

Scaled in P61/65 and  SFCHECK reported  twinning fraction 0.272  & no
pseudo-translation.
http://nihserver.mbi.ucla.edu/pystats/     ( Merohedral Twin Detector:
Padilla-Yeates Algorithm ) showed  perfect twinning.
Another server from ucla  http://nihserver.mbi.ucla.edu/Twinning/
showed partial twinning  with twin fraction 0.23 as follows.

2 along a, b, a*, b*

No. Twin Law Related Reflections = 18033 (pairs)
No. Twin Law Pairs Considered = 9016

<H> = 0.266149
<H2> = 0.095303

Twin Fraction = 0.233249 +/- 0.000602

(SHELXL Commands: TWIN 1 0 0 -1 -1 0 0 0 -1 and BASF 0.233249)


In P61/65, I got the following matthews-coeffs

mol/asym  Matthews Coeff  %solvent       P(1.73)     P(tot)
_____________________________________________________________
 1                        9.75            87.39          .00          .00
 2                        4.88            74.79          .00          .01
 3                        3.25            62.18          .06          .14
 4                         2.44            49.57          .57          .63
 5                         1.95            36.97          .36          .21
 6                         1.63            24.36          .00          .00
 7                         1.39            11.75          .00          .00
_____________________________________________________________


May I ask what could be the real twin fraction and what is the
likelihood of solving the structure by molecular replacement by models
with 25 % sequence identity and 30 % sequence similarity.

Thank you so much for reading this mail during your  busy hours and
all suggestions, comments would be gratefully welcome &  appreciated.

thank you ccp4 mailing list.

John

Garib N Murshudov 
Structural Studies Division
MRC Laboratory of Molecular Biology
Hills Road 
Cambridge 
CB2 0QH UK
Email: garib@mrc-lmb.cam.ac.uk 
Web http://www.mrc-lmb.cam.ac.uk