Re: [ccp4bb]: Selenium Edge

[Gwyndaf Evans <gwyndaf@mrc-lmb.cam.ac.uk>]

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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
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