[ccp4bb]: The Tale of Two lattices: A Rare Twinning Phenomenon?

[m.oldham]

We have what seems to be a unique twinning problem.  We have obtained crystals in different conditions and temperatures and these crystals routinely diffract to 1.4 Angstroms belonging to same C2 space group with dimensions (124.15 x 50.49 x 41.38   90, 108.84, 90).  However. these crystals show two independent lattices of equal intensities with crystal orientations of (twin1: rotx 97.4 roty 7.0 rotz  -154.3; twin2: rotx -82.6 roty -7.0 rotz 11.2) With several crystals processed with HKL2000 we observe a 180 degree difference in rotx, an opposite sign to roty and usually a 35 degree difference in rotz.  Unfortunately, this twinning causes the spots to have various degrees of overlap from easily resolvable to those which are completely superimposable.  The integrated and scaled data give Rsym values of 5-10% and we are able to use the data for molecular replacement achieving a adequate solution with reasonably elevated R and Rfree values.  Removing overlapped spots from our highly redundant and complete data results in incomplete data with the same high redundancy since the removed overlapped reflections seem to be symmetry related.  Does anyone have ideas of how we could better separate our overlapped reflections and/or what might be the cause of this twinning phenomenon.  If possible we have selenomethionine data that we would like to use with this structure. Thanks for any help.   Michael Oldham [EMAIL PROTECTED] Purdue University West Lafayette, IN