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
