[phenixbb] questions about twinned structure and MR

Christina Bourne bournecr at yahoo.com
Fri Apr 18 11:16:59 PDT 2008

Hello bb,
I am new to Phenix and am facing my first twinned structure.  While I wait for my copy of Yeates Methods in Enzymology (albeit from 1997) to be delivered, perhaps this bb could answer a few questions?

At what point do I detwin, or do I not do it at all?  It seems Phenix does this itself, on the fly?  Is it refining the twin fraction too?
MR (Phaser) finds solutions with twinned or detwinned data.  However, the solutions vary by 180degrees, so I suspect I'm finding solutions for both h,k,l and h,-k,-l.  These come from two different runs, but one has refined LLgain of 1010.8 (matches soln found with detwinned data, both run through ccp4), and other has refined LLgain of 1073 (through Phenix; it's rotated 180degrees from the other).

When performing AutoBuild after AutoMR, should I worry that I haven't assigned Rfree?  Does Phenix account for twinning when it does this automatically, or what keyword(s) do I need to include?

I see a section in the documentation about refining twinned structures, and am looking forward to getting to that point!  I just want to be sure I'm following a "correct" procedure for the steps leading up to that.  From the literature, it seems quite common to not account for twinning until refinement.  My concern is the two MR solns...which should I use?  If I know it's twinned, shouldn't I account for that from the beginning?

More details:
P41, data good to 2.2A, 2 molecules in the AU, twinning fraction ~0.4.  Rmerge from scaling is reasonable at ~11% overall.  MR model is 100% identical to this data, but from a different spacegroup (thus I'm running MR).

Thanks for any tips or guidance!
Christina Bourne

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