Bismuth pyrostannate, Bi2Sn2O7, from the first structurally characterized heterobimetallic Bi:Sn alkoxides

The first heterobimetallic Bi:Sn alkoxide complexes [Bi(2)SnO(OCH(CF(3))(2))(5)(O(t)Bu)(3)(THF)] (1) and [BiSnO(OCH(CF(3))(2))(3)(O(t)Bu)(2)](2) (2) are described. The complexes were obtained through mixing and heating equimolar quantities of the component alkoxides, Bi(OCH(CF(3))(2))(3) and Sn(O(t)Bu)(4), under solvent-free conditions (1) and in THF (2). The solid-state structures were determined by single crystal X-ray diffraction showing ligand redistribution from Bi(III) to Sn(IV) in the two molecular species. Compound 2 behaves as a single-source precursor for the thermolytic formation of bismuth pyrostannate, Bi(2)Sn(2)O(7).     

Experimental and Computational Study of a New Wheel-Shaped {[W(5)O(21)](3)[(U(VI)O(2))(2)(μ-O(2))](3)}(30-) Polyoxometalate

A new wheel-shaped polyoxometalate {[W(5)O(21)](3)[(U(VI)O(2))(2)(μ-O(2))](3)}(30-) has been synthesized and structurally characterized. The calculated electrostatic potential reveals the protonation of several μ-oxo bridges reducing the polyoxometalate total charge. A protonated structure computed at the density functional level of theory (DFT) is in good agreement with the experimental fit. This species presents a classical polyoxometalate electronic structure with well-defined metal and oxo bands belonging to its U/W and oxo/peroxo constituents, respectively. Furthermore, fragment calculations indicate that the electronic structures of the uranyl-peroxide and polyoxotugstate fragments are little affected by the nanowheel assembly.     

An integrated workflow for extraction and solubilization of intermediate filaments from colorectal biopsies for proteomic analysis

We report a technique for isolation and solubilization of intermediate filament (IF) proteins from colonic biopsies compatible with both gel electrophoresis and liquid chromatography "shotgun" proteomics using mass spectrometry (MS). This is important because changes in the IF proteome, particularly in keratin expression and modification, are noted in colonic mucosa of patients with colorectal cancer. Though keratins have traditionally been dissolved in high concentration of urea, the latter solvent precludes efficient proteolytic digestion by trypsin prior to gel-free LC-MS/MS approaches. The extraction of cytoskeletal proteins was initially evaluated using MCF-7 cancer cell lines using a published, differential detergent solubilization protocol. IF proteins were extracted from colonic biopsies using a combination of homogenization and sonication. Since comparable efficiency of solubilization was noted on the extracted IF from cell lines between urea and guanidine hydrochloride (GuHCl) in triethylammonium bicarbonate buffer, isolated proteins from endoscopic biopsies were solubilized in GuHCl. Using immunoblotting techniques, we successfully demonstrated isolation of keratins and preservation of posttranslational modifications (phosphorylation, acetylation). Dissolved proteins were tryptically digested and peptides analyzed by MS, showing the functionality of the workflow in shotgun proteomic applications, specifically compatibility of the workflow for isobaric tagging relative and absolute quantification based quantitation approaches.     

From lin-benzoguanines to lin-benzohypoxanthines as ligands for Zymomonas mobilis tRNA-guanine transglycosylase: replacement of protein-ligand hydrogen bonding by importing water clusters

The foodborne illness shigellosis is caused by Shigella bacteria that secrete the highly cytotoxic Shiga toxin, which is also formed by the closely related enterohemorrhagic Escherichia coli (EHEC). It has been shown that tRNA-guanine transglycosylase (TGT) is essential for the pathogenicity of Shigella flexneri. Herein, the molecular recognition properties of a guanine binding pocket in Zymomonas mobilis TGT are investigated with a series of lin-benzohypoxanthine- and lin-benzoguanine-based inhibitors that bear substituents to occupy either the ribose-33 or the ribose-34 pocket. The three inhibitor scaffolds differ by the substituent at C(6) being H, NH(2), or NH-alkyl. These differences lead to major changes in the inhibition constants, pK(a) values, and binding modes. Compared to the lin-benzoguanines, with an exocyclic NH(2) at C(6), the lin-benzohypoxanthines without an exocyclic NH(2) group have a weaker affinity as several ionic protein-ligand hydrogen bonds are lost. X-ray cocrystal structure analysis reveals that a new water cluster is imported into the space vacated by the lacking NH(2) group and by a conformational shift of the side chain of catalytic Asp102. In the presence of an N-alkyl group at C(6) in lin-benzoguanine ligands, this water cluster is largely maintained but replacement of one of the water molecules in the cluster leads to a substantial loss in binding affinity. This study provides new insight into the role of water clusters at enzyme active sites and their challenging substitution by ligand parts, a topic of general interest in contemporary structure-based drug design.     

Hydrogen/deuterium exchange of hydrophobic peptides in model membranes by electrospray ionization mass spectrometry

We demonstrate here that the hydrogen/deuterium solvent exchange (HDX) properties of the transmembrane fragment of the M2 protein of Influenza A (M2-TM) incorporated into lipid vesicles or detergent micelles can be studied with straightforward electrospray (ESI) and nanospray mass spectrometry (MS) configurations provided that key factors, including sample preparation techniques, are optimized. Small unilamellar vesicle preparations were obtained by solubilizing dimyristoyl phosphatidylcholine (DMPC) and the M2-TM peptide in aqueous solution with n-octyl-β-D-glycopyranoside, followed by dialysis to remove the detergent. Electron microscopy experiments revealed that subsequent concentration by centrifugation introduced large multilamellar aggregates that were not compatible with ESI-MS. By contrast, a lyophilization-based concentration procedure, followed by thawing above the liquid crystal transition temperature of the lipid component, maintained the liposome size profile and yielded excellent ion fluxes in both ESI-MS and nano-ESI-MS. Using these methods the global HDX profile of M2-TM in aqueous DMPC vesicles was compared with that in methanol, demonstrating that several amide sites were protected from exchange by the lipid membrane. We also show that hydrophobic peptides can be detected by ESI-MS in the presence of a large molar excess of the detergent Triton X-100. The rate of HDX of M2-TM in Triton X-100 micelles was faster than that in DMPC vesicles but slower than when the peptide had been denatured in methanol. These results indicate that the accessibility of backbone amide sites to the solvent can be profoundly affected by membrane protein structure and dynamics, as well as the properties of model bilayer systems.     

Biosynthesis of structurally novel carotenoids in Escherichia coli

Previously, we utilized in vitro evolution to alter the catalytic functions of several carotenoid enzymes and produce the novel carotenoids tetradehydrolycopene and torulene in Escherichia coli. Here we report on the successful extension of these pathways and the C(30) carotenoid diaponeurosporene pathway with additional carotenoid genes. Extension of the known acyclic C(30) pathway with C(40) carotenoid enzymes-spheroidene monooxygenase and lycopene cyclase-yielded new oxygenated acylic products and the unnatural cyclic C(30) diapotorulene, respectively. Extension of acyclic C(40) pathways with spheroidene monooxygenase generated novel oxygenated carotenoids including the violet phillipsiaxanthin. Extension of the torulene biosynthetic pathway with carotene hydroxylase, desaturase, glucosylase, and ketolase yielded new torulene derivatives. These results demonstrate the utility of extending an in vitro evolved central metabolic pathway with catalytically promiscuous downstream enzymes in order to generate structurally novel compounds.     

Biomimetic synthesis, antibacterial activity and structure-activity properties of the pyroglutamate core of oxazolomycin

Biomimetic intramolecular aldol reactions on oxazolidine templates derived from serine may be used to generate densely functionalised pyroglutamates, which are simpler mimics of the right hand side of oxazolomycin. Some of the compounds from this sequence exhibit in vivo activity against S. aureus and E. coli, suggesting that pyroglutamate scaffolds may be useful templates for the development of novel antibacterials, and cheminformatic analysis has been used to provide some structure-activity data.     

Divergent outcomes of gold(I)-catalyzed indole additions to 3,3-disubstituted cyclopropenes

Depending on the conditions employed, gold(I)-catalyzed addition of indoles to 3,3-disubstituted cyclopropenes can be controlled to yield either 3-(E)-vinylindoles (3) or bis-indolylalkanes (4). If the cyclopropene substituents are sterically bulky, unprecedented gold-catalyzed oxidation under air occurs to yield bis-indolylalkene (5) and epoxide (6) at room temperature.     

The temperature dependence of Cr3+:YAG zero-phonon lines

This paper deals with the photoluminescence temperature dependence of the zero-phonon lines of Cr(3+) ions in an yttrium aluminium garnet (YAG) matrix. Experimental data were analysed in the framework of electron-phonon coupling in the quadratic approximation and it was found that Cr(3+) ions in the YAG matrix are strongly coupled with lattice vibrations, with a Debye temperature of about 550 K and a value of the quadratic coupling constant of 0.65. The analysis of both homogeneous and inhomogeneous contributions to the photoluminescence linewidth is performed and the results obtained are compared with previously reported data for Fe(3+):YAG and discussed with respect to the different dependences of the two transition metal ions on the crystal field.     

Effect of 3d doping on the electronic structure of BaFe2As2

The electronic structure of BaFe(2)As(2) doped with Co, Ni and Cu has been studied by a variety of experimental and theoretical methods, but a clear picture of the dopant 3d states has not yet emerged. Herein we provide experimental evidence of the distribution of Co, Ni and Cu 3d states in the valence band. We conclude that the Co and Ni 3d states provide additional free carriers to the Fermi level, while the Cu 3d states are found at the bottom of the valence band in a localized 3d(10) shell. These findings help shed light on why superconductivity can occur in BaFe(2)As(2) doped with Co and Ni but not Cu.