I seem to recall a discussion at some Study Weekend where someone
reported a needle crystal in which the twin fraction appeared to vary
continuously from 0 at one end of the needle to 0.5 at the other! You
just had to make sure you picked the right end to collect the data!
-- Ian
> -----Original Message-----
> From: [log in to unmask]
> [mailto:[log in to unmask]] On Behalf Of Eleanor Dodson
> Sent: 18 April 2007 09:27
> To: Mark Mayer
> Cc: [log in to unmask]
> Subject: Re: [ccp4bb] twin fraction varies between crystals?
>
> This is a very common thing - twinning is a sort of
> accidental overlap
> of crystal fragments and thus will be different for diferent
> "crystals". ( Should a twinned object be called a crystal? Discuss)
>
> More serious - it is probably different for different parts
> of the data
> collection if your crystal is smaller than the beam - I dont know if
> anyone has ever tried to analyse for this.
> In our experience smaller is often better..
> Eleanor
>
> Mark Mayer wrote:
> > For cases where people have had merohedral twinning, did
> the twin fraction vary substantially
> > between individual crystals grown under indentical
> conditions? I have no prior experience with
> > merohedral twinning, and was surprised to see that the twin
> fraction varied substantially as detailed
> > below, and that by screening we were able to get untwinned xtals.
> >
> > The project started with a weak home data set for which the
> twin fraction was 0.478, and which
> > scaled in both H3 and H32. We just came back from APS with
> data sets from another three crystals,
> > for which the ML twin fraction, estimated using
> phenix.xtriage with scalepack merged intensities as
> > input, varied from 0.335, 0.219 and 0.02. The latter is
> refining very nicely, in H3 and will not scale in
> > H32.
> >
> > Thanks - Mark
> >
> >
> >
> >
>
>
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