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Paul, The fibre-like characteristics present in some diffraction patterns of nucleic acids, which I think you are referring to, is the contribution of the helical transform of the double-stranded helix. Essentially, the length of the regular spacings in the base pairs modulates the intensities of the Bragg reflections. These then do not decay smoothly as the function of resolution which results in intensities along the helical axes at 3.4A being quite strong whilst perpendicular ones are much weaker. I have observed this in the diffraction patterns of RNA crystals but I have not seen the diffuse streaking along lattice lines that you show. One recent example of fibre-like diffraction of a nucleic acid is illustrated here: https://drive.google.com/file/d/0B-wXJqcqFfUHUVpvZDhqREdqMzA/edit?usp=sharing I do not think that what you are seeing in your crystals can be attributed solely to the above. The streaking between the spots is probably some sort of lattice anomaly. It would be interesting to hear if this had impacted in some way on phasing or refinement of the structure. Eugene On 7 April 2014 19:03, Paul Paukstelis <[log in to unmask]> wrote: > Seeing the diffuse streaking threads that have cropped up recently got me > to thinking about many of the DNA data sets we have collected on. We always > see diffuse streaking along lattice lines. > > See images: > > http://vespa.chem.umd.edu/~paul/hex_streak01.png > http://vespa.chem.umd.edu/~paul/hex_streak02.png > http://vespa.chem.umd.edu/~paul/hex_streak03.png > > My original interpretation of this as fiber diffraction from alignment of > DNA bases came after reading Doucet et al. Nature 337, 190-192 and seeing > that the streaks start at around 3.4A and orientation of the helical > portions of the structure are perpendicular to the six-fold. I didn't > really give it much more thought till seeing the threads about diffuse > streaking. These show up even in crystals that diffract to higher > resolution than the one shown in the images. I wonder if this is actually > something else given that they are along lattice lines? Opinions? > > --paul > -- Dr Eugene Valkov Room 3N049 Division of Structural Studies MRC Laboratory of Molecular Biology Francis Crick Avenue Cambridge Biomedical Campus Cambridge CB2 0QH, U.K. Email: [log in to unmask] Tel: +44 (0) 1223 267358