There is quite a lot of background to these Qs in a variwty of text
books, and something on this website.
http://www.ccp4.ac.uk/dist/html/pxmaths/index.html
For finding heavy atoms all methods are based on Patterson searches,
direct methods or a combination of both starting from the observation
F(heavy atom) ~~ mod ( Fhkl(+) -Fhkl(-) ) (SAD)
or F(heavy atom)~~ mod ( FHP hkl-FP hkl ) (SIR)
The methods will generate positions on either hand randomly, and on any
of the acceptable origins.
When changing hand you need to consider whether this also involves a
change of spacgroup; eg P32 instead of P31 , and remember as George says
that not all changes of hand are simply a matter of changing x,y,z to
-x,-y,-z
See http://www.ccp4.ac.uk/dist/html/alternate_origins.html or
http://www.ccp4.ac.uk/dist/html/non-centro_origins.html for a list of
the alternate origins.
Eleanor
George M. Sheldrick wrote:
> Not true. For MAD and SIRAS you still have to try both hands of the heavy
> atom substructure (unless the heavy atom arrangement is itself
> centrosymmetric, then both hands are correct).
>
> Maybe I should also mention for completeness, that for the space groups
> I41, I4122 and F4122 the heavy atoms have to be inverted at a point that
> is not at the origin (e.g. x, y, z -> 1-x, 0.5-y, 0.25-z for I4122).
> Fortunately SHELXE and some other programs know this and apply the
> correct inversion automatically.
>
> George
>
> Prof. George M. Sheldrick FRS
> Dept. Structural Chemistry,
> University of Goettingen,
> Tammannstr. 4,
> D37077 Goettingen, Germany
> Tel. +49-551-39-3021 or -3068
> Fax. +49-551-39-22582
>
>
> On Fri, 20 Aug 2010, Jacob Keller wrote:
>
>>>
>>> MAD and SIRAS will in general behave like SAD. However if your isomorphous
>>> difference is large and the anomalous signal is lost in the noise, they
>>> might be dominated by it and so tend to behave more like SIR.
>>>
>>>
>> I thought that MAD and SIRAS had no hand ambiguity--not true?
>>
>> Jacob Keller
>>
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