I agree with Pavel and would add that we have frequently found that
collecting low resolution data on a home source and then merging it
with the higher resolution synchrotron data collected subsequently
from the same crystal can work wonders. The home source data tend 
to be missing fewer reflections (especially if the home system has
a kappa or three-circle goniometer) and may well be measured more 
precisely at low resolution, as well as suffering much less from
radiation damage.

George  

Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582


On Fri, 20 Feb 2009, Pavel Afonine wrote:

> Sent on behalf of Sacha Urzhumtsev:
> 
> Dear Richard,
> 
> I answer to the whole CCP4bb since the question was quite general and the
> answer may
> be interesting for other people.
> 
> As already mentioned by Bruno Klaholz, low-resolution reflections are not
> only "a few reflections more" but they contain information
> complementary to that in high-resolution reflections, in other words,
> that is different to it.
> 
> Low-resolution reflections correspond to 'electron density waves'
> of a large period. A right combination of low-resolution
> structure factors gives the molecular envelope. Inversely, excluding
> low-resolution reflections from the Fourier series calculation
> artificially increases the map values in some large regions and decreases
> it in others eventually destroying the molecular envelope in the maps.
> If the peaks of the Fourier series are relatively low, this loss
> of low-resolution data may completely kill the signal.
> 
> A map improvement by "phase extension for low-resolution" was illustrated
> by Podjarny et al. (1981) Acta Cryst., A37, 662-668, and by Schevitz et al.
> (1981) Acta Cryst., A37, 669-677. Later we studied in purpose the role
> of low-resolution data for the map quality (1991; Acta Cryst., A47, 794-801).
> In particular a practical illustration is given (EFG structure solution)
> where an addition of 29 reflections to about 3000 available drastically
> improved the map. At my knowledge, low-resolution data were crucial also
> for the ribosomal structure solution (J.Cate).
> 
> Also as Bruno noted, the same data are important for refinement of atomic
> models
> (see for example Kostrewa, 1997, CCP4 Newslet., 34; I remember he was
> presenting
> something earlier but did not find the trace of that presentation). For
> teaching
> goals, recently with our students we prepared an example when a rigid-body
> refinement of a protein model fails using 7377 reflections of the resolution
> 2-20 A and gives a perfect answer when 12 (!) reflections only are added
> at the resolution below 20 A.
> 
> Finally, the same low-resolution reflections may significantly
> reinforce molecular-replacement translation searches (see
> 1995; Acta Cryst., D51, 888-895; Fokine & Urzhumtsev, 2002, Acta Cryst, D58,
> 72-74).
> 
> Obviously, getting low-resolution data requires special efforts during
> experiment
> (see a recent discussion in the CCP4bb) but it is worthy to do and is feasible
> in
> practice at most of synchrotrons. Another feature of these data is a very
> strong
> contribution of the bulk solvent that should be taken into account as soon as
> one starts to use an atomic model.
> 
> I hope these few comments and some bibliography answer your questions and
> may help in some studies.
> 
> With best regards,
> 
> Sacha
> 
> [log in to unmask]
> 
> 
> On 2/20/2009 8:22 AM, Richard Gillilan wrote:
> > Several times I have heard that low order (small angle) reflections are more
> > important when solving low-resolution structures. I presume it is more than
> > just a question of obtaining greater number of reflections.
> >
> > Does anyone know why low-order reflections are so important in these cases?
> >
> > Richard Gillilan
> > MacCHESS
>