On 3/15/11 1:18 PM, Ed Pozharski wrote:
On Tue, 2011-03-15 at 12:12 -0400, Phil Jeffrey wrote:
That having been said, I want to raise the unfortunate possibility that you're really in P3 and doubly twinned.
As in P3 with dimer sitting on the axis and resulting in apparent 6-fold symmetry (120+180-360=-60)?
In this particular case I'd envision it as potentially four crystallographic trimers formed by crystallographic 3-folds, giving 4 unique monomers per asymmetric unit, with two two-fold NCS elements, both perpendicular to the 3-fold, and spaced 60 degrees apart (doesn't this beget 622?). Actually you could probably do this with 3 monomers, but then you might get some apparent statistical disorder of domains when treating the data as either P321 or P63 where the asymmetric unit volume is halved. In my case I saw something that was at first blush "P321" and pseudo P622, had one "good" domain and the other half of the asymmetric unit exhibited statistical disorder that was the superposition of two domains overlapping on themselves related by a twin operator. This interpretation was made possible by looking at the SeMet peaks in the MAD map - it's not something that would be easy to come by from looking at a model-phased map. I felt it was better interpreted as P3 -> P321 -> P622 doubly twinned, because attempts at single twin refinement and handling the statistical disorder did not yield a more refinable structure. But I'm not sure how to prove it one way or the other.