Showing papers by "George M. Sheldrick published in 2002"
TL;DR: Iterative dual-space direct methods based on phase refinement in reciprocal space and peak picking in real space are able to locate relatively large numbers of anomalous scatterers efficiently from MAD or SAD data.
Abstract: Iterative dual-space direct methods based on phase refinement in reciprocal space and peak picking in real space are able to locate relatively large numbers of anomalous scatterers efficiently from MAD or SAD data. Truncation of the data at a particular resolution, typically in the range 3.0–3.5 A, can be critical to success. The efficiency can be improved by roughly an order of magnitude by Patterson-based seeding instead of starting from random phases or sites; Patterson superposition methods also provide useful validation. The program SHELXD implementing this approach is available as part of the SHELX package.
1,620 citations
TL;DR: In this article, the authors proposed a new sphere of influence algorithm combined with a fuzzy solvent boundary to provide a simple, fast and robust route from substructure sites found by the program SHELXE to an initial electron density map, with an indication as to which heavy-atom enantiomorph is correct.
Abstract: SHELXE was designed to provide a simple, fast and robust route from substructure sites found by the program SHELXD to an initial electron density map, if possible with an indication as to which heavy-atom enantiomorph is correct. This should be understood as a small contribution to high-throughput structural genomics. The new sphere of influence algorithm combined with a fuzzy solvent boundary enables some chemical knowledge to be incorporated into the density modification in a general and effective manner. In the special cases of high solvent content (greater than 0.6) or very high resolution data (better than 1.5 A) high quality maps can be produced. This raises the possibility of a new paradigm for atomic resolution structure refinement: instead of alternating atom parameter refinement with weighted electron density maps calculated with the phases of the current model, which inevitably leads to some model bias, all model building should be based on the model free experimental density map.
396 citations
81 citations
TL;DR: Two examples of structures of obverse/reverse twins are presented and it will be shown how these two structures can be refined with the SHELXL program.
Abstract: Two examples of structures of obverse/reverse twins are presented. Example (1) is the structure of 2,2,4,4,6,6-hexa-tert-butylcyclotrisiloxane. It crystallizes in R3 macro c. Example (2) is the structure of [[Li(Me(3)Si)(3)CAlF(3)(thf)](3)LiF(thf)], tris[lithium tetrahydrofuran (trimethylsilyl)methyltrifluoroaluminate]lithium fluoride tetrahydrofuran. It crystallizes in R3. Additional to the obverse/reverse twinning this structure shows merohedral twinning. It will be shown how these two structures can be refined with the SHELXL program.
42 citations
27 citations
TL;DR: In the pentapeptide complex, the relative positions of the peptides are close to those required for d-Ala elimination, so this structure may provide a realistic model for the prevention of the enzyme-catalyzed cell-wall crosslinking by antibiotic binding.
26 citations
TL;DR: The results confirm deficiencies of some stereochemical restraints, such as peptide planarity or the N-C(alpha)-C angle, and suggest a link between their violations and hydrogen bonding.
Abstract: CMTI-I, a small-protein trypsin inhibitor, has been crystallized as a 4:1 protein–zinc complex. The metal is coordinated in a symmetric tetrahedral fashion by glutamate/glutamic acid side chains. The structure was solved by direct methods in the absence of prior knowledge of the special position metal centre and refined with anisotropic displacement parameters using diffraction data extending to 1.03 A. In the final calculations, the main-chain atoms of low Beq values were refined without restraint control. The two molecules in the asymmetric unit have a conformation that is very similar to that reported earlier for CMTI-I in complex with trypsin, despite the Met8Leu mutation of the present variant. The only significant differences are in the enzyme-binding epitope (including the Arg5 residue) and in a higher mobility loop around Glu24. The present crystal structure contains organic solvent molecules (glycerol, MPD) that interact with the inhibitor molecules in an area that is at the enzyme–inhibitor interface in the CMTI-I–trypsin complex. A perfectly ordered residue (Ala18) has an unusual Ramachandran conformation as a result of geometrical strain introduced by the three disulfide bridges that clamp the protein fold. The results confirm deficiencies of some stereochemical restraints, such as peptide planarity or the N—Cα—C angle, and suggest a link between their violations and hydrogen bonding.
17 citations
TL;DR: In the solid state the cage formed by two triureidocalix[6]arene units contains two molecules of water These two included guest molecules do not interact either with each other or with the inner cage surface as discussed by the authors.
12 citations
TL;DR: The ternary inclusion complex terfenadine/g -cyclodextrin/tartaric acid (TFN/ g CD/TA, 2:4:1) has been prepared from a aqueous solution as discussed by the authors.
Abstract: The crystalline ternary inclusion complex terfenadine/ g -cyclodextrin/tartaric acid (TFN/ g CD/TA, 2:4:1) has been prepared from a aqueous solution (terfenadine, TFN, rac - f -[4-(1,1-dimethylethy...
9 citations
TL;DR: In this article, the anomalous signal of weak scatterers such as sulfur, present in most proteins, can be employed for phase determination, which eliminates the need to prepare selenomethionine or other derivatives.
Abstract: Recent developments in X-ray instrumentation, like high intensity sources, cryo devices and new detectors now make it possible to measure reflection intensities precisely enough so that even the anomalous signal of weak scatterers such as sulfur, present in most proteins, can be employed for phase determination. This eliminates the need to prepare selenomethionine or other derivatives. The anomalous signal of sulfur at Cu K α wavelength is 0.5 electrons, which, although on the verge of measurability, under optimal conditions can be successfully exploited to give sufficient phase information. In-house sources, especially if equipped with a goniometer (rather than a single rotation axis) enabling a much higher redundancy to be obtained, even have advantages over synchrotron sources for phasing: it is possible to measure for a longer time without significant radiation damage. For phasing purposes the highest possible resolution is not required, often a high-resolution synchrotron dataset at shorter wavelength may be used for phase extension and structure refinement. Using the knowledge accumulated from several test cases, the key steps in data acquisition will be presented and analyzed from the point of view of success in subsequent phasing. Selected examples will illustrate the solution of test and unknown structures using SHELXD and SHELXE. In favorable cases i.e. very high resolution or high solvent content, SAD phasing based on the anomalous scattering of sulfur alone can give very high quality maps.
2 citations