Yes, I think Ed's analysis is a bit misleading. If, as an extreme case, you have just two unit cells (in each dimension) with 300A cell dimensions, the interference function could still manifest itself out to 1A resolution. Play around with some of James Holton's simulation programs to find out or use one of the Fresnel/Fraunhofer diffraction applets such as in http://www.falstad.com/diffraction/).
The reasons spots merge and become no longer measurable are due to factors like spread of cell dimensions, "mosaicity" or wavelength spread. -----Original Message----- From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK] On Behalf Of Ethan Merritt Sent: 09 January 2012 19:47 To: ccp4bb Subject: Re: [ccp4bb] Sub-angstrom resolution 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
