Dear Fred,
May I join Phil Evans in trying to dissipate the feeling that anomalous
differences were fictional before flash-freezing and all the mod cons. I can
remember cutting my teeth as a PhD student by helping Alan Wonacott with the
experimental phasing of his B.St. GAPDH structure in 1973-74. The data were
collected at room temperature on a rotating-anode source, using film on an
Arndt-Wonacott rotation camera (the original prototype!). The films were
scanned on a precursor of the Optronics scanner, and the intensities were
integrated and scaled with the early versions of the Rotavata and Agrovata
programs (mention of which should make many ccp4 old-timers swoon with
nostalgia). Even with such primitive techniques, I can remember an HgI4
derivative in which you could safely refine the "anomalous occupancies"
(i.e. f" values) for the iodine atoms of the beautiful planar HgI3 anion to
5 electrons. This contributed very substantially to the phasing of the
structure.
In fact it would be a healthy exercise to RTFL (Read The Fascinating
Literature) in this area, in particular the beautiful 1966 papers by Brian
Matthews in Acta Cryst. vol 20, to see how seriously anomalous scattering
was already taken as a source of phase information in macromolecular
crystallography in the 1960's.
In spite of that, of course, there would always be the unhappy cases
where the anomalous differences were too noisy, or the data processing
program too unsophisticated to filter them adequately, so that only the
isomorphous differences would be useful. It was in order to carry out such
filtering that Brian Matthews made another crucial contribution in the form
of the Local Scaling method (Acta Cryst. A31, 480-487).
With best wishes,
Gerard.
--
On Wed, Jun 06, 2012 at 11:02:05AM -0400, Dyda wrote:
> >I suspect that pure MIR (without anomalous) was always a fiction. I doubt
> >that anyone has ever used it. Heavy atoms always give
> >an anomalous signal
>
> >Phil
>
> I suspect that there was a time when the anomalous signal in data sets was
> fictional.
> Before the invent of flash freezing, systematic errors due to decay and the
> need
> of scaling together many derivative data sets collected on multiple crystals
> could render
> weak anomalous signal useless. Therefore MIR was needed. Also, current
> hardware/software
> produces much better reduced data, so weak signals can become useful.
>
> Fred
>
> [32m*******************************************************************************
> Fred Dyda, Ph.D. Phone:301-402-4496
> Laboratory of Molecular Biology Fax: 301-496-0201
> DHHS/NIH/NIDDK e-mail:[email protected]
> Bldg. 5. Room 303
> Bethesda, MD 20892-0560 URGENT message e-mail: [email protected]
> Google maps coords: 39.000597, -77.102102
> http://www2.niddk.nih.gov/NIDDKLabs/IntramuralFaculty/DydaFred
> *******************************************************************************[m
--
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