The main-chain conformations of 237 384 amino acids in 1042 protein subunits from the PDB were analyzed with Ramachandran plots. The populated areas of the empirical Ramachandran plot differed markedly from the classical plot in all regions. All amino acids in -helices are found within a very narrow range of 9, 2 angles. As many as 40% of all amino acids are found in this most populated region, covering only 2% of the Ramachandran plot. The -sheet region is clearly subdivided into two distinct regions. These do not arise from the parallel and antiparallel -strands, which have quite similar conformations. One region is mainly from amino acids in random coil. The third and smallest populated area of the Ramachandran plot, often denoted left-handed -helix, has a different position than that originally suggested by Ramachandran. Each of the 20 amino acids has its own very characteristic Ramachandran plot. Most of the glycines have conformations that were considered to be less favoured. These results may be useful for checking secondary-structure assignments in the PDB and for predicting protein folding.
Crystallography
of nanocrystalline materials has witnessed a true
revolution in the past 10 years, thanks to the introduction of protocols
for 3D acquisition and analysis of electron diffraction data. This
method provides single-crystal data of structure solution and refinement
quality, allowing the atomic structure determination of those materials
that remained hitherto unknown because of their limited crystallinity.
Several experimental protocols exist, which share the common idea
of sampling a sequence of diffraction patterns while the crystal is
tilted around a noncrystallographic axis, namely, the goniometer axis
of the transmission electron microscope sample stage. This Outlook
reviews most important 3D electron diffraction applications for different
kinds of samples and problematics, related with both materials and
life sciences. Structure refinement including dynamical scattering
is also briefly discussed.
Despite substantial advances in crystal structure determination methodology for polycrystalline materials, some problems have remained intractable. A case in point is the zeolite catalyst IM-5, whose structure has eluded determination for almost 10 years. Here we present a charge-flipping structure-solution algorithm, extended to facilitate the combined use of powder diffraction and electron microscopy data. With this algorithm, we have elucidated the complex structure of IM-5, with 24 topologically distinct silicon atoms and an unusual two-dimensional medium-pore channel system. This powerful approach to structure solution can be applied without modification to any type of polycrystalline material (e.g., catalysts, ceramics, pharmaceuticals, complex metal alloys) and is therefore pertinent to a diverse range of scientific disciplines.
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