On Monday, January 09, 2012 11:37:23 am Ed Pozharski wrote:
> On Mon, 2012-01-09 at 18:15 +0000, Theresa H. Hsu wrote:
> > Dear crystallographers
> >
> > A theoretical question - can sub-angstrom resolution structures only be
> > obtained for a limited set of proteins? Is it impossible to achieve for
> > membrane proteins and large complexes?
> >
> > Theresa
>
> On the matter of large proteins.
>
> Let's say your molecule is so big, the unit cell parameters are
> 300x300x300 A. To obtain 1A data, you need reflections with miller
> indices of ~300. For these to be measurable, you need, I presume, ~300
> unit cells in each direction (otherwise you don't even have a formed
> Bragg plane). 300A x 300 ~ 10^5 A, or 10 micron. So it seems to me
> that with large molecules you would essentially hit the crystal size
> limit. In reality, to get any decent data one would need maybe 3000
> unit cells, or 100 micron crystal. While such crystals could
> theoretically grow (maybe in microgravity), it is highly unlikely that
> the whole crystal will be essentially a single mosaic block. Simply
> because large proteins are always multi-domain, and thus too flexible.
The ground-breaking work by Chapman et al. using the Stanford FEL to
record diffraction from nanocrystals of Photosystem II would seem to
constitute an encouraging counter-example
Nature [2011] doi:10.1038/Nature09750
> So I'd say while everything is theoretically possible, for very large
> proteins the probability of getting submicron resolution is exceedingly
> small.
It remains to be seen what resolution might ultimately be achieved by
nanocrystal experiments. As I understand it, the resolution of the
work to date has been limited by the apparatus rather than by the crystals.
Ethan
--
Ethan A Merritt
Biomolecular Structure Center, K-428 Health Sciences Bldg
University of Washington, Seattle 98195-7742
