Dear all,
one comment on the subject of white lines and anomalous phasing
power
- one has to be extremely careful to jump to conclusions
about the effect that a chemical treatment of the heavy atom (say
oxydation/reduction of Se) has
on the shape of the edge and the associated anomalous signal:
there are crystals where the intrinsic
anisotropy of the dispersive and anomalous scattering combines with the
geometry
of the heavy-atom arrangement and the symmetry of the lattice
to give a very strong dependency of
the edge (and therefore the signal) on the orientation of the sample in
the
polarized beam.
In other words you may observe a wonderful white line but to check if
this improved edge comes from oxydising/reducing the sample that went
into the crystal
you would need to check
it by reorienting the crystal in a number of orientations. And of course
one would
want to do the same on a crystal comign from the protein before the
treatment.
And I would not be surprised if in some cases
the phenomenon would make the difference between succeeding in
determining
the structure or not - even when non chemical treatment is involved -
just a series
of crystals of the same kind but mounted and measured in crucially
different orientations.
See
Exploiting the anisotropy of anomalous scattering boosts the phasing
power of SAD and MAD experiments.
Schiltz M, Bricogne G.
Acta Crystallogr D Biol Crystallogr. 2008 Jul;D64(Pt 7):711-29. Epub
2008 Jun 18.
and references cited therein.
Regards
Pietro
--
Pietro Roversi
EP Abraham Fellow in Biochemistry - Lincoln College - Oxford
Sir William Dunn School of Pathology, Oxford University
South Parks Road, Oxford OX1 3RE, England UK
Tel. 0044-1865-275385
