You might like to look at this..
It tries to explain likely twinning possibilities in P21.
If you get C2222 and P21, then probably a~=c - then Beta can have any
value.
C222 axes are then always possible with a* +c* , a*-c*, b* all
having angles ~ 90
Without twinning you wont get 222 symmetry though. Pointless helps here.
Eleanor
http://www.ccp4.ac.uk/dist/html/twinning.html
On 09/27/2011 08:23 PM, Linda Schuldt wrote:
Dear Yuri,
in a monoclinic space group an orthorhombic lattice metric can be
simulated when one of the following conditions is fulfilled:
i) a = c [e.g. in Wittmann& Rudolph (2007) Acta Cryst. D63, 744-749],
ii) the beta angle is close to 90° [e.g. in Larsen et al. (2002) Acta
Cryst. D58, 2055-2059 ] or
iii) c cos beta is about -a/2 [e.g. in Declercq& Evrard, (2002) Acta
Cryst. D57, 1829-1835]. The a and b axes of the orthorhombic cell are
identical to the monoclinic a and c axes, respectively. The length of the
orthorhombic b-axis can also be calculated by "c(monoclinic) cos(beta-90°)
= 1/2b(orthorhomic)".
I would assume that you have the case iii with a quite high twin fraction.
If I recall correctly, Declercq and Evrard have a nice figure in their
paper showing the geometric relationship. If not, let me know and I can
sent you a figure.
Good luck!
Linda
Yuri Pompeu schrieb:
Hello everyone,
I have a 2.3A data set that could be scaled in C 2 2 21 and P 1 21 1
Intensity statistics tests indicate twinning (pseudo-merohedral h,-k,-h-l
in P 1 21 1)
I find a good MR solution and when I try to refine it with the twin law I
get fairly good maps and decent Rs 21-28%. I can see features tha were not
in the search model
Which leads me to think that this a valid solution. The one thing that
bothers me however is the fact that my beta angle in P 1 21 1 is 104 (not
close to 90) and that the geometry gets worse after refinement?
Any suggestions?
cheers
*******************************
Dr. Linda Schuldt
Department of Molecular Biology
University of Aarhus
Science Park
Gustav Wieds Vej 10c
DK-8000 Århus C
Denmark
