Christine, As interesting as the di-Tyrosine idea is, from the picture you provided, it looks more like the two tyrosines share a C=C bond. Given your slightly elevated R-factors and the high symmetry space group, I would thoroughly check for even the slightest sign of twinning. I once had a case of a coiled coil that seemed to be sitting on a crystallographic two-fold. R-factors were a bit too high for the resolution. Lowering the symmetry and redefining that two-fold axis as a twinning operator lead to very reasonable R-factors, and the ends of the helices (in the original model "disordered") to adopt obviously different conformations. At the time, we did not feel too confident with that interpretation (swapping crystallographic symmetry for a twinning operator and an NCS operator at the same time) and screened for a crystal that seemed not to suffer from that malady (which we found); therefore it was - in hindsight unfortunately - never published in that form. Nowadays, I feel very strongly, that reducing the symmetry and reassigning certain "crystallographic" operators to a mix of twinning operators/NCS operators to be actually a legitimate - and often probably more correct - way of interpreting certain problematic crystals.
Just 2 cents Jens On Wed, 2012-07-11 at 13:37 -0600, Lukacs, Christine wrote: > Hi all- > > > > I have a protein that crystallizes in I422, and diffracts well, > between 1.3-1.7A. Beautiful density, slightly higher final R-factors > than you might expect at this resolution (low to mid 20s). The > density is all beautiful, except that I have this one little clash, > between a few atoms from a tyrosine and its symmetry mate. In this > picture I have it modeled as an Alanine and you can see the two > tyrosine rings interlocking; and there is clearly no alternate > conformation. > > > > > > > > Since it is not near my site of interest, I have been pretty much > ignoring it, going through refinement with it as an alanine, then > changing it at the very end to a tyrosine and just minimizing B-s, no > positional. Now that I plan to publish a bunch of these, I should > probably figure out what is really going on. Any insights? > > > > Thanks > > Christine > > Christine Lukacs > Roche > > This message is intended for the use of the named recipient(s) only > and may contain confidential and/or proprietary information. If you > are not the intended recipient, please contact the sender and delete > this message. Any unauthorized use of the information contained in > this message is prohibited. > > > >
