You are right ; extremely accurate NCS parallel to a symmetry axis does
distort the plots, but its effect is usually resolution dependent so
expected straight lines are often not straight..
And pseudo NCS translation can be even more confusing.
Andrei Lebedev has done a lot of analysis on this - some published in
the CCP4 study weekend 2006 I think. Some to be published soon I think.
But if there is only one molecule in the asymmetric unitr the NCS cannot
be present.
Eleanor
Ian Tickle wrote:
>> -----Original Message-----
>> From: [log in to unmask] [mailto:[log in to unmask]]
>>
> On
>
>> Behalf Of Eleanor Dodson
>> Sent: 23 April 2009 15:59
>> To: Kumar
>> Cc: [log in to unmask]
>> Subject: Re: [ccp4bb] Twinning or not?
>>
>> Look at the moment plots after scalepack2mtz; if these are normal it
>> seems very unlikely you have twinning..
>> Eleanor
>>
>
> I know this subject has been beaten to death in previous BB discussions,
> but is it always as clear-cut as this? If for example you had a NCS
> 2-fold co-incident with the twinning axis wouldn't that bias the moment
> stats? The derivation of the moment stats in the twinned case assumes
> that the pairs of twin-related intensities (assuming we're talking about
> the hemihedral case) are statistically independent, but that assumption
> is clearly invalidated if there's NCS parallel to the twinning axis, and
> in that situation the moment stats would tend towards the untwinned case
> (depending of course on the exactness of the NCS and the resolution).
> It's of no consolation to someone to tell them that their situation is
> very unlikely if it actually happens to them!
>
> It seems reasonable to say that if the moment stats conform to the
> twinned case, then twinning is almost certainly present (barring data
> processing blunders); however if the moment stats conform to the
> untwinned case then you can't say for sure that it's not twinned,
> there's still a chance (maybe small) that it's twinned even if the data
> has been correctly processed.
>
> I've read on several occasions that twinning and NCS are quite likely to
> occur together, but I wonder if anyone has done a proper analysis and in
> particular looked at cases where the twinning & NCS axes coincide to see
> the effect on the moment stats?
>
> Cheers
>
> -- Ian
>
>
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