Avisek, Besides the advice that Bernhard and Remy have given you, beware of strange packing arrangements with low symmetry space groups and higher order oligomers. We have had several cases in P21 and C2 where tetramers pack with 6 or 8 molecules in the asymmetric unit (ASU), but could not be solved by MR using a tetramer model. Why? The true packing element was a dimer portion of the tetramer. Hence, the ASU with 6 monomers contained a tetramer and a half; the full unit cell then contained 3 tetramers, with the 2-fold axis of one tetramer sitting on a crystallographic axis. Likewise, the ASU with 2 monomers contained 1 tetramer and 2 "half" tetramers; the full unit cell then contained 2 tetramers, with the 2-fold axes of 2 tetramers sitting on a crystallographic axis.
In each case, using the tetramer as a probe failed, but monomers and the appropriate dimers worked. The take-home message is don't get fixated on the tetramer as the only possible MR probe. Good luck, Michael **************************************************************** R. Michael Garavito, Ph.D. Professor of Biochemistry & Molecular Biology 603 Wilson Rd., Rm. 513 Michigan State University East Lansing, MI 48824-1319 Office: (517) 355-9724 Lab: (517) 353-9125 FAX: (517) 353-9334 Email: rmgarav...@gmail.com **************************************************************** On Aug 17, 2014, at 2:54 AM, Avisek Mondal <avisekmonda...@gmail.com> wrote: > Hello everyone, i am struggling with a problem.. My crystal was diffracted at > 1.9A in P21 spacegoup with unit cell parameter a=87.7 > > b=93.9, c=111.78 ,beta=94.98 which contains 16 molecules per assymmetric > unit (Molecular weight of the Protein+DNA =56 KDa. > > actually it is a complex of 40Kda tetramer and 23bpDNA ) .In solution, it > shows tetrameric in nature The crystal structure of its > > homologous structure has been reported earlier (50%identical in amino acid > seq.) and it was also a tetramer and its unit cell (also P21) > > was approximately 4 times less than mine. It showed 4 molecules per > assymmetric unit. I didn't get any molecular replacement. All > > the programms are takings very long time to do it. Although the single > crystal is untwinned i think it is a case of pseudosymmetry. > > Please help me if you have any good suggestion regarding molecular > replacement other than experimental phasing. >
