In general, I think we should be careful about too strong statements,
while in general structures with high solvent diffract to low-res,
there are a few examples where they diffract to high res. Obviously,
high solvent content means fewer crystal contacts, but if these few
are very stable?
Similarly, there are probably a few structures with a high percentage
of Ramachandran outliers which are real and similarly for all other
structural quality indicators. However, combinations of various of
these probably do not exist and in any case, every unusual feature
like this should be described and an attempt made to explain/analyse
it, which in the case of the Nature paper that started this thread
was apparently not done, apart from the rebuttal later (and perhaps
in unpublished replies to the referees?).
With regards to our structures 1H6W (1.9A) and 1OCY (1.5A), rather
than faith, I think the structure is held together by a real
mechanism, which however I can't explain. Like in the structure Axel
Brunger mentioned, there is appreciable diffuse scatter, which imo
deserves to be analysed by someone expert in the matter (to whom, or
anyone else, I would gladly supply the images which I should still
have on a tape or CD in the cupboard...). For low-res version of one
image see
http://web.usc.es/~vanraaij/diff45kd.png
and
http://web.usc.es/~vanraaij/diff45kdzoom.png
two possibilities I have been thinking about:
1. only a few of the "tails" are ordered, rather like a stack of
identical tables in which four legs hold the table surfaces stably
together, but the few ordered tails/legs do not contribute much to
the diffraction. This raises the question why some tails should be
"stiff" and others not; perhaps traces of a metal or other small
molecule stabilise some tails (although crystal optimisation trials
did not show up such a molecule)?
2. three-fold disorder, either individually or in microdomains too
small to have been resolved by the beam used. For this I have been
told to expect better density than observed, but maybe this is not true.
we did try integrating in lower space groups P3, P2 instead of P321
with no improvement of the density, we tried a RT dataset to see if
freezing caused the disorder and we tried improving the phases by MAD
on the mercury derivative, but with no improvement in the density for
the tail.
Mark J. van Raaij
Unidad de Bioquímica Estructural
Dpto de Bioquímica, Facultad de Farmacia
and
Unidad de Rayos X, Edificio CACTUS
Universidad de Santiago
15782 Santiago de Compostela
Spain
http://web.usc.es/~vanraaij/
On 24 Aug 2007, at 03:01, Petr Leiman wrote:
----- Original Message ----- From: "Jenny Martin"
<[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Thursday, August 23, 2007 5:46 PM
Subject: Re: [ccp4bb] The importance of USING our validation tools
My question is, how could crystals with 80% or more solvent
diffract so well? The best of the three is 1.9A resolution with I/
sigI 48 (top shell 2.5). My experience is that such crystals
diffract very weakly.
You must be thinking about Mark van Raaij's T4 short tail fibre
structures. Yes, the disorder in those crystals is extreme. There
are ~100-150 A thick disordered layers between the ~200 A thick
layers of ordered structure. The diffraction pattern does not show
any anomalies (as far as I can remember from 6 years ago). The
spots are round, there are virtually no spots not covered by
predictions, and the crystals diffract to 1.5A resolution. The
disordered layers are perpendicular to the threefold axis of the
crystal. The molecule is a trimer and sits on the threefold axis.
It appears that the ordered layers somehow know how to position
themselves across the disordered layers. I agree here with Michael
Rossmann that in these crystals the ordered layers are held
together by faith.
Mark integrated the dataset in lower space groups, but the
disordered stuff was not visible anyway. He will probably add more
to the discussion.
Petr
Any thoughts?
Cheers,
Jenny
