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
