Showing papers by "George M. Sheldrick published in 1998"

Refinement of twinned structures with SHELXL97

Abstract: Four examples of the refinement of twinned structures are discussed. These structures illustrate three different types of twins: twinning by merohedry, pseudo-merohedry and non-merohedral twins. How the twinning was detected, how the structures were solved and how they were refined are shown in this paper. The difference between a refinement as a disordered model and a twinned model is illustrated. Sometimes the twin law was necessary to solve the structure, while in the other examples the twinning was first recognized during refinement of the structures. Finally, a list of `characteristic warning signs' is presented which are indicative of possible twinning.

SHELX Applications to Macromolecules

Abstract: Direct methods inspired by Shake & Bake involving iteration between real and reciprocal space are able to solve structures with several hundred independent atoms, but still require data to atomic resolution (about 1.2A). It is planned to add a new program, SHELXD, incorporating such algorithms in a future release of the SHELX system. Initial trials of the preliminary test version of SHELXD have been reasonably successful, and include the ab initio solution of several large ‘small-molecules’ that had resisted attempts using previous version of SHELXS, as well as the ab initio solution of three unknown (and several known) proteins from the native data alone.

Octanuclear Bis(triple-helical) Metal(II) Complexes

Abstract: The reaction of H2L1 and H2L2 with divalent metal ions leads to octanuclear bis(triple-helical) metal(II) complexes of the general composition [Zn8O2L26] (3) and [M8O2L16] (4: M = Cd2+; 5: M = Mn2+). NMR studies of the diamagnetic cadmium complex 4c show six equivalent ligands. Unambiguous characterisation of 5b was achieved by X-ray crystallographic analysis.

Crystal Structures of Actinomycin D and Actinomycin Z3.

Abstract: Untwinned single crystals of the actinomycins D and Z3 that diffracted to atomic resolution could be obtained for the first time. Low-temperature data collection and a new ab initio method for solving the structures led to precise crystal structures which showed, for example, that the unit cell of actinomycin D contains three molecules, two of which are present in the form of a hydrogen-bridged dimer related by a pseudo-twofold axis (see picture).

Structure of Balhimycin and its Complex with Solvent Molecules

Abstract: Balhimycin is a naturally occurring glycopeptide antibiotic, related to vancomycin which acts by binding nascent bacterial cell-wall peptide ending in the sequence d-Ala-d-Ala. Crystals of balhimycin are monoclinic, space group P21, a = 20.48 (10), b = 43.93 (21), c = 27.76 (14) A, β = 100.5 (5)° with four independent antibiotic molecules, three molecules of 2-methyl-2,4-pentanediol, two citrate ions, three acetate ions and 127.5 water molecules in the asymmetric unit. With an asymmetric unit larger than those of the smallest proteins and a solvent content of about 32%, the crystals have similar diffraction properties to those of small proteins. 27 387 unique reflections were collected using synchrotron radiation. The structure was solved by a standard protein technique, the molecular-replacement method, using ureido-balhimycin as search model. The anisotropic refinement against all F2 data between 0.96 and 45 A converged to a conventional R value of 11.27% with R1=\sum\big| |F_o|-|F_c| \big|/\sum|F_o| for the 24 623 data with I > 2σ(I) and 12.58% for all 27 387 data. The four monomers possess fairly similar conformations (r.m.s. deviation 0.7 A). Two antibiotic molecules form a tight dimer with antiparallel hydrogen bonds between the peptide backbone as well as between the vancosamine residues and the peptide backbone. In each of the two dimers, one binding pocket is occupied by a citrate ion and the other by an acetate ion. The dimer units are linked in the crystal by hydrogen bonds to form infinite chains.

Location of Heavy Atoms by Automated Patterson Interpretation

Abstract: Automated computer interpretation of the Patterson function using superposition techniques provides an effective way of locating heavy atoms in small moiety structures, and — normally from isomorphous or anomalous difference data — also in macromolecules. Another promising approach is to locate potential heavier atoms by random start real/reciprocal space iteration, and then to use the Patterson superposition function as a figure of merit to identify the correct solutions.. Under particularly favorable circumstances, the location of heavier atoms with the help of the Patterson function, combined with automated E-Fourier recycling, provides an ab initio method of solving the phase problem for small proteins from the native data alone, without the use of heavy-atom derivatives, anomalous dispersion, or chemical information in the form of molecular fragments of known geometry.

Kristallstrukturen von actinomycin d und actinomycin z3

Abstract: Unverzwillingte, bis zu atomarer Auflosung streuende Einkristalle konnten erstmals von den Actinomycinen D und Z3 erhalten werden. Tieftemperaturmessungen und die Nutzung neuer Ab-initio-Strukturlosungsmethoden lieferten prazise Kristallstrukturen und ergaben, das z. B. die Elementarzelle von Actinomycin D drei Molekule enthalt, von denen zwei als H-Brucken-verknupfte Dimere vorliegen, die uber eine pseudo-zweizahlige Achse in Beziehung stehen (siehe Bild).