Dear All, I had written earlier about my twinned crystal that was giving me problems......thanks to all who responded, it was very helpful. I have another couple questions about the same crystal that is confusing us. The twinned crystals space group is P3. Twinning was found using the Yeates twinning server and the twin fraction of different crystals ranged from .32-.48. Crystals with the lower twin fraction diffracted to a much lower resolution and the more twinned crystals diffracted to 3.
Using the data from the .48 twinned crystal I refined into the twinned P6 data (what we believed to be the space group originally) as well as the detwinned P3 data. The result of this is that both are about the same as far as R/Rfree (.28/.36) and the P6 is better as far as structure geometry and density fit. Why is this the case? Does the detwinning process introduce more error than if one just refined into the twinned data? Is truncate and the twinning server ever wrong in their diagnosis of twinning? Secondly, this structure still has an area of missing density parallel to the a-b plane which never resolves (1/6 at top and bottom along c). This is the case for twinned, detwinned, P6, P3, and everything else I have tried. It is also seen as P1, so we are not averaging out weak reflections when scaling in different groups. I have also searched these areas with FFEAR which yields nothing substantial. This area is where many long disordered chains point (nearly 30 residues), so one of our thoughts is that there are many different conformations of these tails that point into this area. Is this possible? It seems to me that this would be a situation that promotes twinning as well. Our second idea is that there is a translation of 1/6 a unit cell along c that would allow the rest of the cell to be resolved as long as that plane of empty density is filled with same contents. Would this be possible? Any ideas of how to test each of these possibilities? Can anyone think of any other ideas.? I´m sure I´ve said things that are unclear, so if clarification is needed please let me know. Thanks, Robert
