Automatic identification of alpha-helices in Patterson maps
Giovanni Luca Cascarano, Rocco Caliandro, D. Dibenedetto, Giovanni Nico, A. M. MAZZONE · Acta Crystallographica Section A Foundations of Crystallography · 2011
Recent developments in the field of X-ray crystallography, e.g. 3 rd generation synchrotron radiation of increased intensity and improved detectors, facilitate macromolecular structure determination of biological samples at high resolution.Several protein and DNA structures are known with a resolution better then 1.0 Å. High-resolution diffraction data reveal electron density features more clearly and enable the use of non-spherical scattering factors.Such data also allow to resolve static disorder that remains undetected at lower resolution or when using data of low quality.In order illustrate the benefits of combining highresolution crystallography and non-spherical scattering factors we studied the 16-residue thiopeptide Thiostrepton and a DNA structure by invariom refinement [1].For this purpose complete and redundant Bragg data from the thiopeptide Thiostrepton were measured at the Swiss Light Source synchrotron at a temperature 100K to a resolution of 0.65 Å and compared to laboratory data to 0.81 Å. Furthermore Dauter et al. kindly provided a 0.55 Å resolution dataset from a Z-DNA structure [2].These datasets were initially evaluated with the independent atom model (IAM) and afterwards re-refined using nonspherical scattering factors of the invariom database [1],[3] which is based on the Hansen-Coppens multipole model [4].High resolution single-crystal diffraction data evaluated with invarioms provide not only detailed and accurate molecular geometries, but also information on the electron-density distribution and on properties derived from it.With a view to biological, structural and medical functionality of Thiostrepton as well as DNA, an analysis of the electrostatic potential and the molecular dipole moment is especially relevant, and both properties will be reported.