[ccp4bb]: Monoclinic twinning?

["matthew.c.clifton.1" <cliftomc@purdue.edu>]

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Hi all,
I may have come across an example of monoclinic twinning in my data that
appears to be orthorhombic.

The story:
I am working on a MAD data set that appears to be in the space
group P21221. However, upon running the data through SOLVE, the electron
density maps are very difficult if not impossible to follow even though
the data set was to 2.4 angstroms. I thought that I had the wrong space
group and I proceeded to process the data as monoclinic.

During the indexing of the data as P2, a strange problem occurred. DENZO
had problems with the identification of the correct lattice (they varied
between data sets taken from the same crystal in the exact same
orientation).

Example indexing in P2:
Peak_minus data set 57.38 91.51 112.37 90 90.24 90
Inflection_minus data set 91.53 57.4 112.36 90 90.24 90

The data from the inflection wavelength could be "corrected" such that it
found the same lattice as the Peak_minus data set by adding ~200 more
peaks to the indexing, but it changed the unit cell parameters to:
57.15 91.36 111.06 90 91.04 90

Following this, I decided to investigate the possibility of twinning. I
ran the data through the CNS program detect_twinning where it output the
following information:
<|I|^2> / (<|I|>)^2 = 1.6711 (average value over all bins, where 2.0 is
untwinned and 1.5 is for twinned).
(<|F|>)^2 / <|F|^2> = 0.8602 (average value over all bins, where 0.785 is
untwinned and 0.865 is twinned).

When the data was processed through TRUNCATE, the Cumulative Intensity
Distribution looked almost exactly like the example given on the CCP4
website for a twinned crystal. The data appears to be clean with no large
streaked reflections or the presence of a secondary lattice.

Is the data truly twinned? If so, how is this happening geometrically? (I
am having a hard time drawing a picture of this). The only example I can
find that is similar is an abstract from the bioinformatics-structure
conference in Jerusalem, Israel from November 17-21, 1996 (HiPIP II
structure to 1.7 angstroms resolution from a twinned monoclinic crystal).
Any suggestions on how to proceed from here would also be helpful.

If not, why is all the data indicating that it may be twinned?

Any suggestions or input would be appreciated.

Thanks in advance,
Matt