Angewandte Chemie International Edition

Direct observation of the real-time kinetics at a single enzyme level is possible. The cover picture background shows a confocal fluorescent microscope image obtained after addition of a profluorescent substrate to a CALB-enzyme-decorated surface. The fluorescence emission of a single enzyme molecule as a function of time is shown in red. As described by K. Velonia, J. Hofkens, A. E. Rowan et al. in their Communication on page 560 ff., this method reveals aspects of the catalytic behavior which are hidden in ensemble measurements.

Bye bye multiple-bond rule: Recent successes in silicon chemistry are the silicon-containing compounds 1, which has a SiSi triple bond, and the [(Me₅C₅)Si]⁺ ion 2, which is a stable derivative of the [HSi]⁺ ion. Both species are also notable from a bond-theory point of view and can be regarded as valuable starting materials for synthetic organosilicon chemistry.

Finger on the pulse: Diffusion NMR spectroscopy, based on pulse gradient spin echo NMR experiments, is a valuable method for mapping molecular interactions at the molecular level in supramolecular and combinatorial chemistry. The decay of the NMR signals also enables, for example, information on the intermolecular interactions in host–guest systems to be obtained (see picture; G is the pulsed gradient strength).

Metal incorporation and synthesis of MFI zeolites with microwave irradiation result in crystal stacking in one direction to form a fibrous morphology (see images of the Ti-MFI zeolite). These materials display several interesting physicochemical properties including lower packing density and higher hydrophobicity than their nonfibrous counterparts, and as a result, enhanced catalytic properties and increased shape-selective adsorption.

Real-time measurement of the catalysis and substrate kinetics of a single-enzyme hydrolysis reaction is demonstrated with confocal fluorescence microscopy (CFM; see picture, green=CFM beam). A single lipase is shown to have a broad range of conformations; each conformation contributes to the overall enzymatic activity, an observation that is often masked by ensemble measurements.

Excellent thermal stability and mechanical properties are exhibited by high-proton-conductive membranes synthesized by a facile copolymerization of tetrafluoroethylene, ethylene, and CH₂CHCF₂CF₂OCF₂CF₂SO₂F (see scheme and AFM image). These membranes have been successfully used in a direct MeOH fuel cell.

Unlike any other known crystalline silicate, the reduced nitridosilicate SrSi₆N₈ contains SiSi single bonds (see picture). The SiSi distance is 235 pm, which precisely matches that in diamond-type silicon. The SiSi unit is embedded in a framework of corner-sharing SiN₄ tetrahedra. Thus, Si⁴⁺ and Si³⁺ coexist within the same framework structure.

Specificity in weak biomolecular interactions: A novel technique, cold-spray ionization, can detect the weak but specific noncovalent binding of carbohydrates to proteins, water, and to other carbohydrate molecules. It allows the determination of dissociation constants and can assist in computer simulations of putative receptor–ligand binding environments (see picture).

A strong blue emission is observed for hybrid Ce³⁺/Ce⁴⁺ cerium phosphate nanotubes, whereas Ce³⁺ phosphate nanowires exhibit a characteristic double-peak UV luminescence (see picture). These cerium phosphate systems were obtained by annealing novel cerium(IV) phosphate nanotubes at different temperatures.

Orthogonal and modular templating is an effective basis for the preparation of the biologically active annonaceous acetogenin 10-hydroxyasimicin (1). The versatile tartrate-derived 2,3-butanediacetal building block together with a highly diastereoselective hetero-Diels–Alder approach to the butenolide unit were usefully employed in this novel synthetic route.

Anti-push: The axial thiolate ligand stabilizes high-spin Fe^III-OOR species (see picture) such as those found in superoxide reductase. This situation is in contrast to the “push effect” observed in structurally similar low-spin Fe^III-OOR systems, including cytochrome P450, which promote OO bond cleavage. These results show how very similar enzyme-active sites can carry out two very different functions.

The key step in a formal total synthesis of eleutherobin was an unprecedented kinetically controlled RCM reaction of a densely functionalized diene 1 bearing two PMP-protected allylic alcohols in the presence of a second-generation Grubbs catalyst. Subsequent isomerization of the E 10-membered enedione 2 to the more stable Z isomer and cleavage of the MOM protecting group led to the known precursor 3. MOM=methoxymethyl; PMP=p-methoxyphenyl.

Dramatic differences are found in the temperatures at which Bergman cyclizations occur when Mo^IV–enedithiolate–enediyne complexes are used (see scheme; S: yellow, Mo: red), and this can be directly attributed to a long-range electronic polarization effect of the metal center. Ligand-to-metal charge transfer within the C₂S₄ substructure and differential charge repulsion in the transition state lead to a lowering of the barrier to cyclization.

Out with the old, in with the new: Novel 1,8-naphthyridine C-nucleosides, Na-N^O and Na-O^N (see formulae), were synthesized through Heck coupling reactions. Oligodeoxynucleotides containing the Na-N^O and Na-O^N nucleosides formed extremely stable duplexes by the base-pairing motifs Na-N^O:Im-O^N and Na-O^N:Im-N^O.

Through the reduction of micrometer-sized silver phosphate crystals with a perfect rhombododecahedral shape by a reaction/diffusion process to hierarchical, rhombododecahedral silver cages (see picture): The controlled self-assembly of silver particles around the precursor crystal surfaces leads to the formation of morphology-preserved single- or double-walled silver cages constructed from different building units.

The deuteration of water produces the conformational change shown in the figure in the anisole⋅⋅⋅water complex. The value of the θ angle decreases from 138 to 128°, and the secondary interaction O⋅⋅⋅H_Me is replaced by O⋅⋅⋅H_Ph.

A concise route to the heptacyclic indole alkaloid stephacidin A (1) includes a simple method for the gram-scale synthesis of substituted tryptophans, a remarkable indole annulation, and the first enolate coupling of an amide to an ester. This synthesis secures the relative configuration of the natural product and paves a potential path to several of its congeners.

An odd couple: The union of pyrroles and carbonyl compounds (ketones, amides, esters, lactones, lactams, see scheme) is described, and by the use of an intramolecular variant of this new method, the nonsteroidal, anti-inflammatory drug (S)-ketorolac has been prepared in a short enantioselective synthesis. Mechanistic underpinnings of this reaction, which couples unfunctionalized C(sp²) and C(sp³) atoms, are also discussed.

An environmentally friendly reaction that excludes the potential contamination of the products by metal impurities is reported. In this reaction, optically active aldoximes and nitriles are prepared from chiral nitroalkanes (see scheme). The methodology described expands the possibilities available for the conversion of nitroalkanes into valuable targets.

Concentrated efforts: A zeolite-nanocrystal-mediated enrichment procedure that doubles as a desalting step allows low-abundance proteins to be readily detected with MALDI-TOF MS (see picture). Mass spectral signal-to-noise ratios are greatly improved for a wide range of peptides, which underscores the potential of this procedure to boost proteomics research.

A high-yielding preparation of homoallylic sulfones 2 entails the treatment of allylic tosylacetate esters 1 with stoichiometric or substoichiometric quantities of BSA and substoichiometric quantities of KOAc under conventional thermal or microwave conditions. A possible mechanism involves silyl ketene acetal formation, [3,3]-sigmatropic rearrangement, and desilylation-decarboxylation. BSA=N,O-bis(trimethylsilyl)acetamide; Ts=p-toluenesulfonyl.

Oligonucleotides immobilized in SAMs (self-assembled monolayers) with a passifying surface layer, prepared from thermally smoothed gold surfaces and disulfides, allow the direct detection of bound strands by MALDI-TOF mass spectrometry. The binding of perfectly matched (PM) and mismatched (MM) DNA targets is observed (see picture). The extent to which cross-hybridization occurs depends on the type and position of the mismatch.

The intervention of crown ethers or achiral quaternary ammonium salts as achiral phase-transfer catalysts (PTCs, see scheme) in a chiral PTC system leads to dramatic rate enhancements in the asymmetric alkylation of glycine derivatives.

Size matters: The adsorption of trans-2-pentene on Pd/Al₂O₃ model catalysts exhibits site-specific behavior, which results in a strong increase in hydrogenation activity within the 1–5-nm particle size range, in contrast to ethene hydrogenation (see figure). The size effects are explained by the reactions proceeding via di-σ-bonded pentene, which is favored on the terrace sites of large particles, and π-bonded ethene.

Away from the fold: Insertion of the terminal alkyne PhCH₂C₂H (2) into the folded C₃B₂ heterocycle (R²=CH₂SiMe₃, Me) of the formal 16-valence-electron complexes 1 gives the 18-valence-electron boron-containing ruthenocene analogues 3 with a 4-borataborepine as 6π-electron ligand. Side products 4 and C₆H₃(CH₂Ph)₃ are detected by mass spectrometry. Compound 4 is formed directly as the only product when 1 is treated with R₂C₂ (R=Et, p-tolyl).

Lots, fast: Benzo[c]phenanthridine derivatives are prepared efficiently by the reaction of aldehydes and 2-methylbenzonitrile in the presence of potassium tert-butanolate, followed by dehydration (see scheme). Some of these compounds demonstrate excellent cytostatic activity, and it is possible to introduce substituents directly into the basic molecular ring system.

A striking similarity to the structure of mother-of-pearl (nacre) is displayed by the crystal architecture of highly oriented laminated CaCO₃ coatings (see scanning electron microscope image ), which were prepared by a multistep procedure. A novel retardance imaging system (LC-PolScope) was employed to characterize the crystal texture of the polycrystalline thin films.