Dear Crystallographers, the "saxs on crystals" thread reminded me of a question I have had for a while, and never having collected data better than ~1.6 Ang or so, cannot answer myself from experience: I would think that there might be powder-like diffraction rings at distances corresponding to the various covalent bond lengths in proteins (1.2-1.5 Ang), but have never heard of such. My thinking is that the protein itself is essentially a powder sample within the unit cell consisting of many small, randomly-oriented molecules (amino acids) with their covalent bonds. Do the rings in fact exist, and if not, why not? Maybe the electron density is not as "atomic," or discrete, as the nuclei are? I wonder whether generally data collected to beyond ~1 Ang have an intensity "bump" at those covalent bond lengths, as I believe is seen in nucleic acid-containing structures at the base-stacking distance (at the right orientation)?
Jacob -- ******************************************* Jacob Pearson Keller Northwestern University Medical Scientist Training Program email: [email protected] *******************************************
