Structure of sulfamidase provides insight into the molecular pathology of mucopolysaccharidosis IIIA

Mucopolysaccharidosis type IIIA (Sanfilippo A syndrome), a fatal childhood-onset neurodegenerative disease with mild facial, visceral and skeletal abnormalities, is caused by an inherited deficiency of the enzyme N-sulfoglucosamine sulfohydrolase (SGSH; sulfamidase). More than 100 mutations in the SGSH gene have been found to reduce or eliminate its enzymatic activity. However, the molecular understanding of the effect of these mutations has been confined by a lack of structural data for this enzyme. Here, the crystal structure of glycosylated SGSH is presented at 2Å resolution. Despite the low sequence identity between this unique N-sulfatase and the group of O-sulfatases, they share a similar overall fold and active-site architecture, including a catalytic formylglycine, a divalent metal-binding site and a sulfate-binding site. However, a highly conserved lysine in O-sulfatases is replaced in SGSH by an arginine (Arg282) that is positioned to bind the N-linked sulfate substrate. The structure also provides insight into the diverse effects of pathogenic mutations on SGSH function in mucopolysaccharidosis type IIIA and convincing evidence for the molecular consequences of many missense mutations. Further, the molecular characterization of SGSH mutations will lay the groundwork for the development of structure-based drug design for this devastating neurodegenerative disorder. © 2014 International Union of Crystallography.This work was funded by the DFG. Partial support from DFG grant No. SH 14/5-1 is gratefully acknowledged (NSS). IU is grateful to the Spanish MEC and Generalitat de Catalunya for financial support (grants BFU2012-35367, IDC-20101173 and 2009SGR-1036)Peer Reviewe

Structure solution of DNA-binding proteins and complexes with ARCIMBOLDO libraries

Protein-DNA interactions play a major role in all aspects of genetic activity within an organism, such as transcription, packaging, rearrangement, replication and repair. The molecular detail of protein-DNA interactions can be best visualized through crystallography, and structures emphasizing insight into the principles of binding and base-sequence recognition are essential to understanding the subtleties of the underlying mechanisms. An increasing number of high-quality DNA-binding protein structure determinations have been witnessed despite the fact that the crystallographic particularities of nucleic acids tend to pose specific challenges to methods primarily developed for proteins. Crystallographic structure solution of protein-DNA complexes therefore remains a challenging area that is in need of optimized experimental and computational methods. The potential of the structure-solution program ARCIMBOLDO for the solution of protein-DNA complexes has therefore been assessed. The method is based on the combination of locating small, very accurate fragments using the program Phaser and density modification with the program SHELXE. Whereas for typical proteins main-chain α-helices provide the ideal, almost ubiquitous, small fragments to start searches, in the case of DNA complexes the binding motifs and DNA double helix constitute suitable search fragments. The aim of this work is to provide an effective library of search fragments as well as to determine the optimal ARCIMBOLDO strategy for the solution of this class of structures. © 2014 International Union of Crystallography.We gratefully acknowledge financial support by the Royal Society through an International Collaboration Grant. EP is grateful to the EPSRC (EP/H051759/1) for financial support. GMS thanks the VW-Stiftung for the Niedersachsenprofessur. IU is grateful to the Spanish MEC and Generalitat de Catalunya for financial support (grants BFU2012-35367, BIO2009-10576, IDC-20101173 and 2009SGR-1036). KM thanks the Deutsche Forschungsgemeinschaft for support (ME 3679/1-1)/Juan de la CiervaPeer Reviewe

Selective functionalization of a bis-silylene

Ghadwal R, Azhakar R, Pröpper K, Dittrich B, John M. Selective functionalization of a bis-silylene. Chemical Communications. 2013;49(53): 5987.Functionalization of N-heterocyclic carbenes (NHCs) has an important influence on their stability, Lewis donor, and acceptor properties. In this study, we report on the selective functionalization of a four-membered N-heterocyclic bis-silylene (2,6-Ar2C6H3NSi:)2 (1) (Ar = 2,4,6-iPr3C6H2) with mono-oxygen sources N2O and Me3NO. Treatment of 1 with N2O results in the selective formation of mono-silylene (2,6-Ar2C6H3NSi(OH)2)(2,6-Ar2C6H3NSi:) (2) as a major product, along with a small amount of further oxidized product (2,6-Ar2C6H3NSi(OH)2)2 (3). Compound 2 is the first four-membered mono-silylene with a di-coordinate silicon atom

Crystalline guanine adducts of natural and synthetic trioxacarcins suggest a common biological mechanism and reveal a basis for the instability of trioxacarcin A

a b s t r a c t X-ray crystallographic characterization of products derived from natural and fully synthetic trioxacarcins, molecules with potent antiproliferative effects, illuminates aspects of their reactivity and mechanism of action. Incubation of the fully synthetic trioxacarcin analog 3, which lacks one of the carbohydrate residues present in the natural product trioxacarcin A (1) as well as oxygenation at C2 and C4 yet retains potent antiproliferative effects, with the self-complimentary duplex oligonucleotide d(AACCGGTT) led to production of a crystalline covalent guanine adduct (6). Adduct 6 is closely analogous to gutingimycin (2), the previously reported guanine adduct derived from incubation of natural trioxacarcin A (1) with duplex DNA, suggesting that 3 and 1 likely share a common basis of cytotoxicity. In addition, we isolated a novel, dark-red crystalline guanine adduct (7) from incubation of trioxacarcin A itself with the selfcomplimentary duplex oligonucleotide d(CGTATACG). Crystallographic analysis suggests that 7 is an anthraquinone derivative, which we propose arises by a sequence of guanosine alkylation within duplex DNA, depurination, base-catalyzed elimination of the trioxacarcinose A carbohydrate residue, and oxidative rearrangement to form an anthraquinone. We believe that this heretofore unrecognized chemical instability of natural trioxacarcins may explain why trioxacarcin analogs lacking C4 oxygenation exhibit superior chemical stabilities yet, as evidenced by structure 3, retain a capacity to form lesions with duplex DNA. Ó 2014 Elsevier Ltd. All rights reserved. The trioxacarcins are structurally complex bacterial metabolites that exhibit potent cytotoxicity in cultured human cancer cells and have demonstrated antibacterial and antimalarial activities

A Dimer of Silaisonitrile with Two-Coordinate Silicon Atoms

Ghadwal R, Roesky HW, Pröpper K, Dittrich B, Klein S, Frenking G. A Dimer of Silaisonitrile with Two-Coordinate Silicon Atoms. Angewandte Chemie International Edition. 2011;50(23):5374-5378.A specialty of silicon: A stable dimeric silaisonitrile (ArNSi:)2 (see picture; Ar=2,6-bis(2,4,6-triisopropylphenyl)phenyl) was prepared by the reduction of dichlorosilaimine IPr⋅Cl2Si NAr with KC8. The dimer is the first base-free disilylene with two-coordinate silicon atoms; reaction with trimethylsilyl azide affords the first bis(silaimine) (ArNSi NSiMe3)2 with three-coordinate silicon atoms

Donor–acceptor stabilized silaformyl chloride

Ghadwal R, Azhakar R, Roesky HW, et al. Donor–acceptor stabilized silaformyl chloride. Chemical Communications. 2012;48(66): 8186.Formyl chloride (H(Cl)C[double bond, length as m-dash]O) is unstable at room temperature and decomposes to HCl and CO. Silicon analogue of formyl chloride, silaformyl chloride IPr·SiH(Cl)[double bond, length as m-dash]O·B(C6F5)3 (3) (IPr = 1,3-bis(2,6-diisopropyl-phenyl)imidazol-2-ylidene), was stabilized by Lewis donor–acceptor ligands. Compound 3 is not only the first stable acyclic silacarbonyl compound but also the first silacarbonyl halide reported so far

Donor–Acceptor-Stabilized Silicon Analogue of an Acid Anhydride

Ghadwal R, Azhakar R, Roesky HW, et al. Donor–Acceptor-Stabilized Silicon Analogue of an Acid Anhydride. Journal of the American Chemical Society. 2011;133(44):17552-17555.A stable silicon analogue of an acid anhydride {PhC(ButN)2}Si{═O·B(C6F5)3}O–Si(H){═O·B(C6F5)3}{(NBut)(HNBut)CPh} (4) with a O═Si–O–Si═O core has been prepared by treating monochlorosilylene PhC(ButN)2SiCl (1) with H2O·B(C6F5)3 in the presence of NHC (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Compound 4 has been characterized by elemental analysis and multinuclear NMR spectroscopic investigations. The molecular structure of 4 has been established by single-crystal X-ray diffraction studies, and DFT calculations support the experimental results

Aspherical-Atom modeling of coordination compounds by single-crystal X-ray diffraction allows the correct metal atom to be identified

Single-crystal X-ray diffraction (XRD) is often considered the gold standard in analytical chemistry, as it allows element identification as well as determination of atom connectivity and the solid-state structure of completely unknown samples. Element assignment is based on the number of electrons of an atom, so that a distinction of neighboring heavier elements in the periodic table by XRD is often difficult. A computationally efficient procedure for aspherical-atom least-squares refinement of conventional diffraction data of organometallic compounds is proposed. The iterative procedure is conceptually similar to Hirshfeld-atom refinement (Acta Crystallogr. Sect. A­ 2008, 64, 383–393; IUCrJ. 2014, 1,61–79), but it relies on tabulated invariom scattering factors (Acta Crystallogr. Sect. B­ 2013, 69, 91–104) and the Hansen/Coppens multipole model; disordered structures can be handled as well. Five linear-coordinate 3d metal complexes, for which the wrong element is found if standard independent-atom model scattering factors are relied upon, are studied, and it is shown that only aspherical-atom scattering factors allow a reliable assignment. The influence of anomalous dispersion in identifying the correct element is investigated and discussed

A New Dimension in Cyclic Coinage Metal Pyrazolates: Decoration with a Second Ring of Coinage Metals by Inter-ring Metallophilic Interactions

When pyrazolate ligands with thioether chelate arms are used in cyclic coinage metal pyrazolates [Au(μ-pz)]n, the inner gold ring can be framed with an outer silver ring to give novel heterometallic double-crowned complexes [AuAg(μ-Lx)(BF4)]4. They feature short intramolecular in-plane Ag–Au interactions, are stable as octanuclear species in solution, and show promising luminescence properties