Dear All,
I have a strange potential twinning issue that I cannot understand. I've
searched high and low on all the internets to find an answer but have
come up empty-handed, so I look to the wisdom of The Board to enlighten me.
I have a 4'ish Å dataset that processes nicely in P 4 21 2 (#90).
However intensity distributions indicate possible almost perfect
twinning (eg. <I^2>/<I>^2 : 1.592 ). So I speculate that the real space
group might be P 4 (#75).
Recently we collected a new fairly low resolution (7.5Å) dataset, from
the same type of crystals (same purification, same conditions).
But the space group in XDS and aimless now comes out very clearly as
either I422 (#97) or I4212 (#98) (screw-axis is unclear given the data).
The unit-cell parameters are exactly the same as in sg #90, which btw.
means that in the body-centered lattice there is no way the protein can
fit in the asym. unit.
So I guess what I don't understand is: Is it possible to go from a
primitive lattice to a body-centered lattice by twinning. Is this just a
low-resolution artifact? Or is this a P4 unitcell that can appear like
P4212 or I422 depending on small variations (weak dehydration or similar).
Has anyone experienced something similar? Am I missing a basic facet of
how twinning works, or is something else at play here?
Thanks for any insights or suggestions!
All the best,
/Bjørn
--
Bjørn Panyella Pedersen
Macromolecular Structure Group
University of California, San Francisco
