Tommi,

Since you ask, SHELX and other 'direct methods' run out of steam at about
1.2A and for many years very few unknown structures have been solved at
lower resolution than 1.2A. Recntly this picture was changed by the
introduction of ARCIMBOLDO, which has solved a number of structures ab
initio with 2.0-2.1A native data and no other phase information. However
ARCIMBOLDO requires at least one alpha-helix in the structure and substantial
computer power. This is still a long way from your 6A data!

If you are using SHELXD or other programs to locate heavy atoms from SAD,
MAD or SIRAS data much lower resolutions are possible, because the heavy
atoms are still well resolved from each other. You can think of the 1.2A
limit for native data as the requirement for resolving the atoms from each
other (for a more sophisticated explanation see Morris and Bricogne, Acta
Cryst. D59 (2003) 615-617). Nevertheless, for finding the heavy atoms at
very low resolution you need all the phase information you can get, so MAD
and SIRAS will be better than SAD, and it helps to combine phases from
several different sources.

George


On Thu, Jun 16, 2011 at 09:22:42AM +0300, tommi kajander wrote:
> Hi,
>
> Thanks for all the comments, i was more wondering what the state of
> the art might
> be here, we have done similar thing at 4 Å (with 3 Å data though) 10
> years or so back
> with SnB. Maybe we need to get a heavier atom derivative indeed the
> break the phases,
> but i'll keep banging for now..
>
> I have tried quite a lot, and was mainly wondering what was the
> world record
> nowadays in desperate solutions by SHELX/other direct methods...
>
> playing around with XDS, data is finally quite ok to 6 Å low res
> anom CC 100-90%,
> SigAno about 3... goes down obviously but not terribly fast there is
> signal all the way to 6Å.
>
> Inverse beam data collection would probably be a good idea indeed
> next time, native or
> second wavelength or rahter both might help i assume..
> thanks to Tim, Clemens, Poul and others for comments,
> Best,
> Tommi
>
>
> On 15.6.2011, at 17.16, Tim Gruene wrote:
>
> >Hi Tommi,
> >
> >Give it a try?
> >You give very little information, e.g.
> >- the data quality,
> >- possible radiation damage,
> >- isomorphism with a native data set, and
> >- what you have tried so far.
> >
> >If you have a native data set with an isomorphous unit cell, try
> >SIRAS instead
> >of SAD.
> >
> >Tim
> >
> >On Wed, Jun 15, 2011 at 04:36:59PM +0300, tommi kajander wrote:
> >>Dear all,
> >>
> >>Does anyone have suggestions for 6 Å resolution phasing with large
> >>number (40-50) Se sites (SAD so far)??
> >>
> >>Thanks a bunch,
> >>Tommi
> >>
> >>Tommi Kajander, Ph.D., Docent
> >>Macromolecular X-ray Crystallography
> >>Research Program in Structural Biology and Biophysics
> >>Institute of Biotechnology
> >>P.O. Box 65 (Street: Viikinkaari 1, 4th floor)
> >>University of Helsinki
> >>FIN-00014 Helsinki, Finland
> >>Tel. +358-9-191 58903
> >>Fax +358-9-191 59940
> >>
> >
> >--
> >--
> >Tim Gruene
> >Institut fuer anorganische Chemie
> >Tammannstr. 4
> >D-37077 Goettingen
> >
> >GPG Key ID = A46BEE1A
> >
>
> Tommi Kajander, Ph.D., Docent
> Macromolecular X-ray Crystallography
> Research Program in Structural Biology and Biophysics
> Institute of Biotechnology
> P.O. Box 65 (Street: Viikinkaari 1, 4th floor)
> University of Helsinki
> FIN-00014 Helsinki, Finland
> Tel. +358-9-191 58903
> Fax +358-9-191 59940
>

--
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