local scaling

Patrick J. Loll loll at pharm.med.upenn.edu
Mon Jan 3 16:42:32 EST 2000


Hi,

	I'm trying to measure a weak anomalous signal at a relatively long
wavelength.  I have a high symmetry spacegroup (#96), and was able to take
LOTS of data--a continuous phi scan of 1440 deg (4 complete 360 degree
rotations).  Scaling all of the data together (or even selected subsets,
such as data from one 360 deg rotation) gives high R values, not surprising
in light of the expected significant absorption effects (data from the same
crystal taken at 12 keV scale beautifully with low R's).
	Because I have so much data, I can scale in P1 to obtain a data set
with reasonable redundancy with decent agreement statistics.  I'd like to
take the scaled, unmerged data in P1 and apply a sliding box type of local
scaling, averaging all the Bijvoet pairs down to the asymmetric unit in the
process.   I think this should give the best treatment of the absorption
effects.  I can't use the Matthews & Czerwinski protocol, since their
derivation assumes a comparison of only two quantities (eg, F+ and F- or
Fnati and Fderi).  This yielded a nice simple non-iterative algorithm for
calculation of the scale factors, but is not appropriate for this case,
since I would like to average over all 16 of the symmetry-related
reflections.

Questions:

1.  Does this sound nuts?  It seems to me that if one chooses a
sufficiently large box, the simultaneous estimation of even a large number
of local scale factors might remain robust.

2.  Can anyone recommend a program to do this? Or toss some snippets of
code my way?

Thanks for any bright ideas.

Pat

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Patrick J. Loll, Ph. D.                             (215) 898-1294
Department of Pharmacology                     FAX: (215) 573-2236
University of Pennsylvania School of Medicine
3620 Hamilton Walk
Philadelphia, PA  19104-6084  USA

loll at pharm.med.upenn.edu
http://pharm.med.upenn.edu/Loll/loll_lab.html


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