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Re: [ccp4bb]: Selenium Edge
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Peter Brick wrote:
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> Hi
>
> A spread of energies for the Selenium K edge from selenomethionyl labelled
> proteins have been reported in the literature (see for example Hendrickson
> and Ogata in Methods in Enzymology Vol 276, 494, 1997). To what extent does
> this spread represent real differences rather than errors in the calibration
> of the monochromator?
This is a difficult one to answer because good energy calibration on an
absolute scale (or at least
on a common scale) is something you rarely find at crystallography beamlines.
During a MAD
experiment the energy will be selected solely based on the recorded
fluorescence spectrum. The
value of the energy recorded by the beamline hardware controllers will be on a
scale determined by
some arbitrary calibration to some 'standard' absorption edge. However this
calibration will drift
with time! It will be correct certainly to 1 part in 1000 but you may question
the accuracy at the level of
1 part in 10000 or better. i.e. be suspicious of wavelengths quoted to 5
decimal places
e.g 0.97978!!!!!!
In any case this is usually not something to worry about! As far as your
data analysis goes
and the determination of cell dimensions etc., 1 in 1000 is pretty good and
the fact that you
observe the edge position before collecting data should mean that you are
measuring at the
correct energy!
I feel that the spread of energies you refer to is more likely to be due to
'poor' (or rather
not perfect!!!!) calibration of the beamline monochromators.
> Does an oxidized selenomethionine have a
> significantly different spectrum?
>
Yes! There is a short piece by J. L. Smith and A. Thompson. Stucture 6 815-819
15th July 1998 which
touches on this subject.
Gwyndaf
--
Gwyndaf Evans | Tel: (01223) 402213
MRC Laboratory of Molecular Biology | Fax: (01223) 213556
Hills Road, Cambridge CB2 2QH, UK. | gwyndaf@mrc-lmb.cam.ac.uk
URL: http://lagrange.mrc-lmb.cam.ac.uk/doc/gwyndaf/gwyndaf.html