[ccp4bb]: Twin problem

[Katja <wenig@biochem.mpg.de>]

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> Dear Friends,

I am currently trying to solve a structure of a twinned crystal.

I actually have 2 measured crystals, both of them appear to be
hemihedrally twinned, one (A) very much (almost perfectly) and the other
one (B) only a bit.

I first tried to solve the structure of crystal (A).

It was possible to integrate this dataset under assumption of the space
group C222 and I got an Rmerge of about 6 %.

However, when I tried to solve the structure by molecular replacement
using AMORE and a search model with 45% identity, I couldn't find a
solution.

I then used SCALEPACK to check the cumulative intensity distribution and
found, that the crystal (A) was twinned.

Measuring my second, less twinned crystal (B) yielded a dataset that I
couldn't successfully integrate using the space group C222. Instead, this
crystal appeared to belong to space group P2.

I concluded, that our crystals belong to space group P2 and that the
additional axis is an artifact caused by the twinning.

Since molecular replacement using the (admittedly not very good) search
model again gave no result, and modifying and changing my search model
didn't help either. I didn't get an AMORE solution that I could use to
determine the twin-operator.

Now, I have heard that it is theoretically possible to calculate the twin
operator and the twin fraction from the dataset first and then to use the
dataset and the twin information to solve the structure.

Does anybody know, how this works?

For any help in this matter I would be very greatful.


Katja Wenig


Max-Planck-Institut für Biochemie
Abt. Strukturforschung
Am Klopferspitz 18 A
82152 Martinsried b. München
Germany