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Hi Juergen & other experts

Thanks for the suggestions. I was under the impression that the twin laws/operators are to be used if the twinning is merohedral. In my case, it appears as if the twinning is non-merohedral and more over the data i have is processed as P422 which does not have twin operators ( probably because all the axial pairs are same anyway). Probably because of the same reason, xtriage did not give any operators for me to use. In such cases, do people usually try to process the data in another space group of lower symmetry which has a twin law and is there a definite way to distinguish twinning by merohedry vs non-merohedry other than just looking at the diffraction pattern ( is it crucial to make that differentiation ?) ?

Thanks

Mahesh

On Sun, Aug 25, 2013 at 10:24 PM, Bosch, Juergen <[log in to unmask]> wrote:

Hi Mahesh,

if you use Refmac, then you can tell it to refine the twin fraction, no need to tell it the twin law as Refmac will figure it out. If you use phenix, you explicitly tell it the twin law and refine then with it. You can get the possible twin laws by running phenix.xtriage and looking at the log file.

For a 1.7 Å dataset you should see excellent holes in the Phe and Tyr, even though your Rfactors are high. If that is the case then you are likely correct with the twin (if nothing else is wrong, Cbeta, Ramas etc). And you did add some waters to your structure already right ? if not the go water picking via Coot.

Have you been converted to XDS now ? Welcome to the club.

Jürgen

On Aug 25, 2013, at 8:35 PM, Mahesh Lingaraju wrote:

Hello everyone,

I collected a dataset which looked like it is twinned ( or a really long axis in the cell) and did not process in HKL2000 and MOSFLM but with some of help and suggestions from CCP4BB, XDS was able to process it. The data looks good upto 1.7 Å. However, the rfree is stuck at 0.34 even though my model is almost complete. I am beginning to wonder if the data is really twinned as it has the characteristics of non- merohedral twinning:

In the images some of the reflections are sharp while some are split and one of the axis in the cell is usually long ( the cell is a= 46.78 b= 46.78 c= 400.34; 90 90 90)

is there anyway to work around this ? or collecting better data is the only solution ?

Any help is deeply appreciated

Thanks

Mahesh

......................
Jürgen Bosch
Johns Hopkins University
Bloomberg School of Public Health
Department of Biochemistry & Molecular Biology
Johns Hopkins Malaria Research Institute
615 North Wolfe Street, W8708
Baltimore, MD 21205
Office: +1-410-614-4742
Lab: +1-410-614-4894
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http://lupo.jhsph.edu