Hi Yuri,
The biggest drop I've seen as the result of detwinning is 10% lower R-free.
Perhaps the detwinning has helped your refinement in othed ways. What R(-free)
do you get when you take you pdb from the twinned refinement and your input mtz
file and do 0 cycles of refinememt without detwinning. If you R(-free) is still
considerably lower than the one from the 'regular' refinememt then you are a
lucky guy and the twinned refinement just brought you to a lower minimum.
Cheers,
Robbie Joosten
> Date: Thu, 10 Feb 2011 21:36:31 +0000
> From: ga...@ysbl.york.ac.uk
> Subject: Re: [ccp4bb] very low R factor for twin refinement
> To: CCP4BB@JISCMAIL.AC.UK
>
> Maximum theoretical drop R/Rfree for perfect twin from 30% is around 25%
> (i.e. it could go down to 5%). However it could only happen only if twinning
> is perfect and there is no pseudo rotation parallel to twin operator.
> Hypothetical case it can happen if you have refined one crystal structure at
> sufficiently high resolution till (almost convergence) and another crystal is
> twinned but otherwise perfectly isomorphous to the first crystal and you take
> coordinates from the first crystal and refine against the second crystal.
>
> regards
> Garib
>
>
> On 10 Feb 2011, at 20:14, Patskovsky Yury wrote:
>
> > Dear all,
> >
> >
> > Twin refinement has yielded Rwork/Rfree values of about 0.10/0.12
> > for a nice quality 1.8A dataset (Rmerge 6%, space group I4, twin fractions
> > 0.6/04) and almost the same R/Rfree (0.095/0.115) for another 1.5A nice
> > quality data set (Rmerge 6%, space group I4, twin fractions 0.74/0.26).
> > Refinement of untwinned data resulted in Rfree of ~32% and ~22%
> > respectively. REFMAC and PHENIX both have produced the same results and
> > almost identical R factors, which are suspiciously VERY LOW for this
> > resolution of data. Twin refinement in REFMAC has produced exceptional
> > quality maps even for 1.8A data (they look rather like 1.2A maps) - I can
> > not tell the same for PHENIX - maps were looking worse (may be someone has
> > a better idea why).
> > Normally twin refinement results in lowering R-factors - say, the
> > drop in R from 30% (without twin refinement) to 20% (with twin refinement)
> > would be considered normal, however we can see the drop from 32% to 12%.
> > I wonder if anyone else has experienced similar problems and what
> > would be the most reasonable explanation for that.
> >
> >
> > Thank you
> >
> > Yury
> >
> >
> >
> >
> >
> > Yury Patskovsky, Ph.D.
> > Associate,
> > Dept of Biochemistry
> > Albert Einstein College of Medicine
> > 1300 Morris Park Ave
> > Bronx, NY 10461
> > phone 718-430-2745
> > yu...@medusa.vioc.aecom.yu.edu
