The problem of many monomers in the "ASU" is not restricted to
macromolecules. An interesting recent small molecule example is the
structure of L-tryptophan (http://dx.doi.org/10.1107/S0108768112033484)
which, amazingly, was not published until 2012. This is perhaps in part
due to difficulty in accepting 16 monomers in the ASU (they call this
Z=16), which was unprecedented.
As a beamline scientist, I have seen "high Z" macromolecular crystals on
many occasions, but they almost never get solved. Yes, they don't
diffract well, but neither does anything else in the early stages of a
project. The reason for not solving them seems more psychological than
anything else. The prospect of amplifying the building and refinement
headache by a factor of "Z" when Z > 10 is perhaps too much for an early
term graduate student to bear.
On the other hand, automated building and refinement has come a long
way, and 24-fold NCS is a great restraint if you can get it! In fact,
for virus structures, it has been shown that you can phase the structure
starting with nothing but a crude spherical envelope and lots of density
modification (http://dx.doi.org/10.1107/S0108767391013211).
but your initial problems are going to be phasing. Ideally what you'd
want is a way of folding back NCS information into the heavy atom
finding and phase refinement process, but I know of no programs that
actually do that. In fact, both molecular replacement and heavy-atom
finding are hindered by this "pseodo-translation" rather than helped by
it. Personally, I blame the fact that methods developers seldom get
their hands on "interesting" datasets like yours. And if you look in
the PDB there are very few examples of "high Z'" structures. Ahem.
Best advice I can give is to try the "usual" approach, but look very
seriously for NCS as early as you can. Then apply building/phasing
packages like shelx{cde}, phenix.autobuild, or the newly-released Crank2.
-James Holton
MAD Scientist
On 1/18/2014 11:18 PM, Felix Frolow wrote:
Francis, It can happened
We have (not yet published) P1 with 24 molecules. When we cut His-tag we get
P1 with 32 molecules.
In our case we believe it is dictated by very strong interaction between two
monomers, and strong interaction between dimers with build a flattish tetramer.
Probably such formations
is more difficult to oaks than globular oligomers.
In this moment I do not recall what we see in solution, I have to check.
Relating to structure solution, P1 is very convenient space group.
I would go for determination this structure by SAD (SHELXC/D/E pipe, PHENIX or
SHARP). For the native - molecular replacement.
In our time after tremendous developments in Refmac and Phenix and development
o DM refinement is 3-3.4 Ang. Is not very difficult.
I would use in addition to NCS restraints in refinement also multi crystal
averaging. Roumors say it is the most strongest phasing method (attributed to
Eleanor Dodson, myself never used it).
FF
Dr Felix Frolow
Professor of Structural Biology and Biotechnology, Department of Molecular
Microbiology and Biotechnology
Tel Aviv University 69978, Israel
Acta Crystallographica F, co-editor
e-mail: mbfro...@post.tau.ac.il
Tel: ++972-3640-8723
Fax: ++972-3640-9407
Cellular: 0547 459 608
On Jan 19, 2014, at 08:48 , Francis Reyes <francis.re...@colorado.edu> wrote:
You sure about this space group? 24 monomers in P1 is unusual (at least to me)
F
On Jan 18, 2014, at 9:14 AM, Chris Fage <cdf...@gmail.com> wrote:
Hello Everyone,
I am currently trying to phase a structure with an asymmetric unit predicted to
contain 20-24 monomers (space group P1). The native crystals, while beautiful
in appearance (see attached), only diffract to ~3.4-3.0 angstroms at best, and
SeMet-derived crystals grow with poor morphology (small needles). Also, based a
fluorescence scan, I know that mercury does not bind appreciably. Other than
screening for a new space group, what options might I have for phasing this
many monomers at lower resolution? Is there any real chance of solving the
structure in this space group?
Thank you in advance for any suggestions!
Regards,
Chris
<Crystals.jpg>
