I think once you start getting down to such small crystals, the spots are not really important, as the pattern starts getting continuous. Interestingly enough, I guess for single-molecule diffraction, resolution is limited only by radiation damage, and not by any sort of lattice disorder (or even by its disorder wrt itself over time, seeing as these images are collected in the fs range!) Each would seem to be a perfect, unlimited-resolution fourier transform of that particular molecule at that particular moment, and the resolution limits come only when trying to merge the images/particles. It seems, then, that if one somehow picked the images that were taken from the most similar molecules, one could get better and better models...
JPK On Tue, Jan 10, 2012 at 10:08 AM, Ed Pozharski <[email protected]> wrote: > On Tue, 2012-01-10 at 09:04 +0000, Colin Nave wrote: >> Yes, I think Ed's analysis is a bit misleading. > > I apologize if I misled anyone. Re-reading my post, I can see that it > lacked precision. Indeed, in a perfect monocrystal all the molecules > are lined up perfectly, so I should have emphasized rather that the > culprit is the decay of correlation between atomic positions. It is > still a bit counterintuitive that a crystal can diffract beyond the > resolution seemingly allowed by possible bragg planes. Shouldn't the > "crystal size" formfactor introduce something akin to sinx/x that will > drive intensity rapidly down past the "bragg limit"? Oh well. > > On a second thought, maybe the unit cell size does not matter directly. > What matters perhaps is how quickly the disorder accumulates over > distance, and that should be more pronounced for larger molecules. Thus > the problem is that larger molecules cannot pack as well as, say, > crambin. > > On empirical side, the largest molecule currently in the PDB with d<1A > is 3ju4, 0.98A and ~75kDa. > > See the distribution of sizes of "subangstrom" structures here. > > http://tinyurl.com/8yhbcvk > > BTW, the 3ju4 is reported on EDS as "unreliable". Shall comment in the > other thread. > > Cheers, > > Ed. > > -- > Oh, suddenly throwing a giraffe into a volcano to make water is crazy? > Julian, King of Lemurs -- ******************************************* Jacob Pearson Keller Northwestern University Medical Scientist Training Program email: [email protected] *******************************************
