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.
So I'd say while everything is theoretically possible, for very large
proteins the probability of getting submicron resolution is exceedingly
small.
Cheers,
Ed.
--
Oh, suddenly throwing a giraffe into a volcano to make water is crazy?
Julian, King of Lemurs
