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
