[ccp4bb]: Twinning in C2

["Scapin, Giovanna" <giovanna_scapin@merck.com>]

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Greetings!

I hope somebody can help me!
I have a data set (not fantastic) that was processed and reduced in C2,
a=186, b=154, c=64 A, beta = 94.5 degrees; the data are OK enough that I
could solve the structure (4 mol/au) by molecular replacement, and refine
it.  But I got stuck with R in the mid-twenties and Rfree in the low
thirties.  I decided to take a closer look at the data, and intensities
analysis with TRUNCATE and DETWIN (and CNX) show that the data are twinned,
with a twinning fraction less than 0.1. Easy to detwin, but of course, there
are no twinning operators in monoclinic for merohedral twinning, so I cannot
detwin it. I checked to see if I got the wrong space group, but (at least in
HKL2000 and XGEN) whatever I got is the only possible choice (by far).
It could be a case of  non-merohedral twins (from the CCP4
documentation:"... where the reciprocal lattices do not overlap exactly and
only some of the reflections are affected by the twinning") but I still
don't know what to do.  The same document states that: " Monoclinic with na
+ nc ~ a or na + nc ~ c can be twinned. See HIPIP examples. ", very
confusing to me

Any idea, suggestion, comment will be greatly appreciated!!!!!

Giovanna

Giovanna Scapin
E&CB
Merck Research Labs.
(732)594-8429