Dear Crystallographic Colleagues,
We have developed a new Web-based server for checking protein
diffraction data for merohedral twinning. Merohedral twinning may
occur in any space group in which the lattice symmetry exceeds the
point group symmetry (e.g. P3 falls on a hexagonal lattice). The
diffraction pattern appears normal, but the observed intensities
contain weighted contributions from crystallographically distinct
reflections. The consequences vary, depending on the extent of the
twinning. In unnoticed or left untreated, twinning will usually
prevent structure determination. A recent survey of the diffraction
data in the PDB (to be reported in the Feb. issue of Structure) shows
that twinning has been overlooked in a number of cases. The new Web
server provides sensitive and specific tests for 'partial' and
'perfect' twinning, and identifies twinning in cases that have evaded
other routines for examining intensity statistics. In its present
implementation, the server supports data in the X-plor and CIF formats;
additional formats will be added shortly. More details about twinning
are available at the Web site and in referenced cited there.
(http://www.doe-mbi.ucla.edu/Services/Twinning)
Todd Yeates
UCLA-DOE-MBI
