>For any sample, crystalline or not, a generally valid description of >diffraction intensity is it being a Fourier transform of electron density >autocorrelation function.
I thought for non-crystalline samples diffraction intensity is simply the Fourier transform of the electron density, not its autocorrelation function. Is that wrong? Anyway, regarding spot streaking, perhaps there is a different, simpler formulation for how they arise, based on the two phenomena: (1) Crystal lattice convoluted with periodic contents, e.g., protein structure in exactly the same orientation (2) Crystal lattice convoluted with aperiodic contents, e.g. n different conformations of a protein loop, randomly sprinkled in the lattice. Option (1) makes normal spots. If there is a lot of scattering material doing (2), then streaks arise due to many "super-cells" occurring, each with an integral number of unit cells, and following a Poisson distribution with regard to frequency according to the number of distinct conformations. Anyway, I thought of this because it might be related to scattering from aperiodic crystals, in which there is no concept of unit cell as far as I know (just frequent distances), which makes them really interesting for thinking about diffraction. See the images here of an aperiodic lattice and its Fourier transform, if interested: http://postimg.org/gallery/1fowdm00/ >Mosaicity is a very different phenomenon. It describes a range of angular >alignments of microcrystals with the same unit cell within the sample. It >broadens diffraction peaks by the same angle irrespective of the data >resolution, but it cannot change the length of diffraction vector for each >Bragg reflection. For this reason, the elongation of the spot on the detector >resulting from mosaicity will be always perpendicular to the diffraction >vector. This is distinct from the statistical disorder, where spot elongation >will be aligned with the crystal lattice and not the detector plane. I have been convinced by some elegant, carefully-thought-out papers that this "microcrystal" conception of the data-processing constant "mosaicity" is basically wrong, and that the primary factor responsible for observed mosaicity is discrepancies in unit cell constants, and not the "microcrystal" picture. I think maybe you are referring here to theoretical mosaicity and not the fitting parameter, so I am not contradicting you. I have seen recently an EM study of protein microcrystals which seems to show actual tilted mosaic domains just as you describe, and can find the reference if desired. >Presence of multiple, similar unit cells in the sample is completely different >and unrelated condition to statistical disorder. Agreed! Jacob
