It's very possible with any overlap of lattice points, even in the lower symmetry space groups. To extend Eleanor's list: For example, I once had a structure with the unit cell relationship 3c cos(beta) = -a. In cases like that, it's not really a clean diffraction, but looks very much like a single lattice of points. This is so-called "reticular pseudomerohedry across the ab-plane with a twin index of 3." The h,k,l reflections of the parent lattice are superimposed upon h, k, (-l-2h)/3 reflections of the twin lattice. So twinning can be frightenly common.
On Wed, February 21, 2007 10:49 am, Eleanor Dodson wrote: > There are many and various reasons for unpleasantly high R values, and > twinning is undoubtedly one of them. > > But you can only have twinning with apparently good diffraction if the > cell dimensions allow it. > > It is always a possibility in trigonal, tetragonal and cubic cells. > > Certain other combinations of cell dimensions allow it ;- eg monoclinic > with a ~= c or beta ~= 90 > > But you can usually detect it from the intensity statistics - see the > plots from TRUNCATE for Moments and Cumulative intensities. > > Or the output of SFCHECK which suggests possible twinning baed on > interpretation of these tests. > > So it should be relatively easy to spot those cases where twinning is a > likely cause of high Rfactor.. > > Surprisingly some degree of twinning doesnt seem to degrade the map > quality very much.. > > Just an aside - it is really puzzling why from two sets of apparently > similar data, one set gives Rfacros of 24+ while others refine to R of > 18% .. > Eleanor > > > > > Kay Diederichs wrote: >> Sue Roberts wrote: >>> Hello >>> >>> A partially philosophical, partially pragmatic question. >>> >>> I've noticed a trend, both on ccp4bb and locally, to jump to twinning >>> as an explanation for data sets which do not refine well - that is >>> data sets with R and Rfree stuck above whatever the person's >>> pre-conceived idea of an acceptable R and Rfree are. This usually >>> leads to a mad chase through all possible space groups, twinning >>> refinements, etc. and, in my experience, often results in a lot of >>> time being spent for no significant improvements. >>> >>> Just out of curiosity, does anyone have a feel for what fraction of >>> stuck data sets are actually twinned? (I presume this will vary >>> somewhat with the type of problem being worked on). >>> >>> And a sorta-hypothetical question, given nice-looking crystals; >>> images with no visible split spots, extra reflections, or streaks; >>> good predictions; nice integration profiles; good scaling with >>> reasonable systematic absences; a normal solvent content; and a >>> plausible structure solution, and R/Rf somewhat highish (lets say >>> .25/.3 for 1.8 A data), how often would you expect the Stuck R/Rf to >>> be caused by twinning (or would you not consider this a failed >>> refinement). (My bias is that such data sets are almost never >>> twinned and one should look elsewhere for the problem, but perhaps >>> others know better.) >>> >>> Sue >>> Sue Roberts >>> Biochemistry & Biopphysics >>> University of Arizona >>> >>> [EMAIL PROTECTED] >> >> Sue, >> >> I seem to be in the other camp: - "nice-looking crystals; images >> with no visible split spots, extra reflections, or streaks; good >> predictions; nice integration profiles; good scaling with reasonable >> systematic absences; a normal solvent content; and a plausible structure >> solution, and R/Rf somewhat highish (lets say .25/.3 for 1.8 A data)" >> >> - all of this may happen with merohedrally twinned crystals. I believe >> it would be good to teach students to always devote some thought to >> the possibility of merohedral twinning in case of a trigonal/ >> hexagonal/ tetragonal crystal, to avoid a rather common pitfall. I >> don't have the percentage at hand, but I believe I saw a paper by >> George Sheldrick giving a high percentage (like 20% or so) of >> merohedral twinned structures in the above crystal systems for >> small-molecule structures - why should that percentage be different >> for protein crystals? >> >> It is of course true that twinning refinement is painful, and a lot of >> additional work! But "man twinning" is always enlightening reading. >> >> Kay >
