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Mark, I suggest you start by doing MR with the twinned data to search for the individual domains to obtain an initial solution. With near perfect merohedral twinning, deconvoluting your structure factors is unreliable. To what resolution does your crystal diffract? If it is beyond ~2.5Ang., I would suggest refinement using SHELXL. Additionally, CNS has some nice refinement protocols for merohedrally twinned data. As for your suggestions, I would definitely screen your crystals (without compound if it is expensive or rare) to see if there is variation in the twin fraction. The lower the better. Best luck, Chris On Thu, 29 Mar 2007, Mark Mayer wrote: >>>Hi, >>> >>>I'd greatly appreciate advice on how to proceed with trying to solve and refine a structure with >>>nearly perfect merohedral twinning - or is this impossible? The SG is H3; most likely nmol is asu >>>is 4 from Matthews coefficient - but not sure about this. >>> >>>Here are the twinning stats from phenix xtriage >>>------------------------------------------------------------------ >>>| Operator | type | R obs. | Britton alpha | H alpha | ML alpha | >>>------------------------------------------------------------------ >>>| h,-h-k,-l | M | 0.038 | 0.452 | 0.468 | 0.478 | >>>------------------------------------------------------------------ >>> >>>The data is for a novel ligand complex for which we have previously solved structures with >>>different ligands in other space groups - the protein has two domains, and these may have moved >>>compared to other structures. >>> >>>So what should I do? Try and solve by MR using detwinned structure factors? Can Refmac refine >>>the twin fraction, or should I refine against detwinned mtz file? Or ... should we look for additional >>>xtal forms, or try additives to see if we can reduce twinning? >>> >>>Thanks !!! >>> Christopher L. Colbert, Ph.D. Assistant Instructor Phone: (214) 645 5944 University of Texas Southwestern Medical Center FAX: (214) 645 5945 6001 Forest Park Lane Dallas, TX 75390