Showing papers by "George M. Sheldrick published in 1997"

SHELXL: high-resolution refinement.

Abstract: Publisher Summary SHELXL-93 was originally written as a replacement for the refinement part of the small-molecule program SHELX-76. The program is designed to be easy to use and general for all space groups and uses a conventional structure-factor calculation rather than a fast Fourier transform (FFT) summation. The latter would be faster but in practice involves some small approximations and is not suitable for the treatment of anomalous dispersion or anisotropic thermal motion. The price to pay for the extra precision and generality is that SHELXL is much slower than programs written specifically for macromolecules. This is compensated for, to some extent, by the better convergence properties, reducing the amount of manual intervention required. A new version, SHELXL-97, was released in May 1997; this is the version described in the chapter. The changes are primarily designed to make the program easier to use for macromolecules. Advances in cryogenic techniques, area detectors, and the use of synchrotron radiation enable macromolecular data to be collected to higher resolution than was previously possible. In practice, this tends to complicate the refinement because it is possible to resolve finer details of the structure. It is often necessary to model alternative conformations, and in a few cases, even anisotropic refinement is justified.

[37] Patterson superposition and ab initio phasing

Abstract: Publisher Summary This chapter presents the possibility of the ab initio solution of macromolecular structures, given two unusually favorable circumstances: the availability of very high-resolution data and the presence of one or more heavier atoms, such as transition metals or even sulfur. Under ab initio , the solution of a structure from the single-wavelength native data can be understood without the use of a known structure as a search model. Automated heavy-atom location via the Patterson function provides an efficient approach in small-molecule crystallography. For unequal atom structures involving atoms on special positions or pseudosymmetry, Patterson-based methods are often more effective than direct methods. In the case of macromolecules, the Patterson approach can be used to locate the heavier atoms, which then provide slightly better than random phases for partial structure expansion by direct or indirect methods.

Synthesis, Structure and Hydrolysis Studies of Dimethyltris(Trimethylsilyl)Methylmetallanes of Aluminium and Gallium†

Abstract: The reactions of [(Me3Si)3CLi2thf] with Me2MCl (M = Al, Ga) afford the mixed trialkylmetallanes [(Me3Si)3-CAlMe2·thf] (1) and [(Me3Si)3 CGaMe2·thf] (2) in high yields. The coordinated THF molecule of compound 2 can be removed by sublimation in vacuo to yield the solvent-free product [(Me3Si)3-CGaMe2] (3). Hydrolysis of compound 2 with one equivalent of water at 0°C gives the trimeric hydroxide [{(Me3Si)3CGaMe(μ-OH)}3] (4), while the reaction with two equivalents of water at room temperature yields the unusually stable gallium hydroxide-water complex [{(Me3Si)3CGaMe(OH)(μ-OH)MeGaC(SiMe3)3) H2O·2thf] (6). On heating, compound 6 is converted to the hydroxide [{(Me3Si)3C}4Ga4(μ-O)2(μ -OH)4] (5), which has a heteroadamantane-like core. The hydrolysis of compound 1 with one equivalent of water at − 25°C gives the dimeric hydroxide [{(Me3Si)3CAlMe(μ-OH)}2·2thf] (7), while the reaction with two equivalents of water results in the formation of the novel hydroxide [{(Me3Si)3C}4Al4(μ-O)2 (μ-OH)4] (8), which is isostructural to the gallium compound 6 with the adamantane-like structure. The molecular structures of compounds 1, 2, 4, 5·3THF, 6, 7 and 8·0.5 THF have been determined by X-ray structure analysis. Compound 7 is the first structurally characterised aluminium hydroxide containing methyl groups, and 8 is the smallest structurally characterised galloxane hydroxide described in literature.

The bi-loop, a new general four-stranded DNA motif

Abstract: The crystal structure of the cyclic octanucleotide d contains two independent molecules that form a novel quadruplex by means of intermolecular Watson–Crick A⋅T pairs and base stacking. A virtually identical quadruplex composed of G⋅C pairs was found by earlier x-ray analysis of the linear heptamer d(GCATGCT), when the DNA was looped in the crystal. The close correspondence between these two structures of markedly dissimilar oligonucleotides suggests that they are both examples of a previously unrecognized motif. Their nucleotide sequences have little in common except for two separated 5′-purine-pyrimidine dinucleotides forming the quadruplex, and by implication these so-called “bi-loops” could occur widely in natural DNA. Such structures provide a mechanism for noncovalent linking of polynucleotides in vivo. Their capacity to associate by base stacking, demonstrated in the crystal structure of d(GCATGCT), creates a compact molecular framework made up of four DNA chains within which strand exchange could take place.

Crystal Structure Refinement Incorporating Chemical Information

Abstract: Crystal structure refinement using the new release of the SHELX program is discussed with emphasis on the incorporation of chemical information via constraints and restraints. It would be desirable to use refinement techniques in the structure solution as well, so as to incorporate chemical information into the phasing process. One such approach that looks very promising is ARP