Poly[nickel(II)‐di‐μ‐4,4′‐bipyridyl‐κ4N:N′‐μ‐dichromato‐κ2O:O′] and poly[copper(II)‐di‐μ‐4,4'‐bipyridyl‐κ4N:N′‐μ‐dichromato‐κ2O:O′]

The novel title hybrid isomorphous organic–inorganic mixed‐metal dichromates, [Ni(Cr₂O₇)(C₁₀H₈N₂)₂] and [Cu(Cr₂O₇)(C₁₀H₈N₂)₂], have been synthesized. A non‐centrosymmetric three‐dimensional (4,6)‐net is formed from a linear chain of vertex‐linked [Cr₂O]²⁻ and [MN₄O]²⁺ (M = Ni and Cu) units, which in turn are linked by the planar bidentate 4,4′‐­bipyridine ligand through the four remaining vertices of the [MN₄O]²⁺ octahedra. There are two such three‐dimensional nets that interpenetrate with inversion symmetry.     

Dynamic measurement of temperature, velocity, and density in hot jets using Rayleigh scattering

A molecular Rayleigh scattering technique is utilized to measure gas temperature, velocity, and density in unseeded gas flows at sampling rates up to 10 kHz, providing fluctuation information up to 5 kHz based on the Nyquist theorem. A high-power continuous-wave laser beam is focused at a point in an air flow field and Rayleigh scattered light is collected and fiber-optically transmitted to a Fabry–Perot interferometer for spectral analysis. Photomultiplier tubes operated in the photon counting mode allow high-frequency sampling of the total signal level and the circular interference pattern to provide dynamic density, temperature, and velocity measurements. Mean and root mean square velocity, temperature, and density, as well as power spectral density calculations, are presented for measurements in a hydrogen-combustor heated jet facility with a 50.8-mm diameter nozzle at NASA John H. Glenn Research Center at Lewis Field. The Rayleigh measurements are compared with particle image velocimetry data and computational fluid dynamics predictions. This technique is aimed at aeronautics research related to identifying noise sources in free jets, as well as applications in supersonic and hypersonic flows where measurement of flow properties, including mass flux, is required in the presence of shocks and ionization occurrence.     

Optical time lens based on four-wave mixing on a silicon chip

We propose a new technique to realize an optical time lens for ultrafast temporal processing that is based on four-wave mixing in a silicon nanowaveguide. The demonstrated time lens produces more than 100 pi of phase shift, which is not readily achievable using electro-optic phase modulators. Using this method we demonstrate 20x magnification of a signal consisting of two 3 ps pulses, which allows for temporal measurements using a detector with a 20 GHz bandwidth. Our technique offers the capability of ultrafast temporal characterization and processing in a chip-scale device.     

A single-cell analysis of myogenic dedifferentiation induced by small molecules

An important direction in chemical biology is the derivation of compounds that affect cellular differentiation or its reversal. The fragmentation of multinucleate myofibers into viable mononucleates (called cellularization) occurs during limb regeneration in urodele amphibians, and the isolation of myoseverin, a trisubstituted purine that could apparently activate this pathway of myogenic dedifferentiation in mammalian cells, generated considerable interest. We have explored the mechanism and outcome of cellularization at a single-cell level, and we report findings that significantly extend the previous work with myoseverin. Using a panel of compounds, including a triazine compound with structural similarity and comparable activity to myoseverin, we have identified microtubule disruption as critical for activation of the response. Time-lapse microscopy has enabled us to analyze the fate of identified mononucleate progeny, and directly assess the extent of dedifferentiation.     

Single-crystalline vanadium dioxide nanowires with rectangular cross sections

We report the synthesis of single-crystalline VO2 nanowires with rectangular cross sections using a vapor transport method. These nanowires have typical diameters of 60 (+/-30) nm and lengths up to >10 mum. Electron microscopy and diffraction measurements show that the VO2 nanowires are single crystalline and exhibit a monoclinic structure. Moreover, they preferentially grow along the [100] direction and are bounded by the (01) and (011) facets. These VO2 nanowires should provide promising materials for fundamental investigations of nanoscale metal-insulator transitions.     

Y-aromaticity: why is the trimethylenemethane dication more stable than the butadienyl dication?

[reactions: see text] Resonance energies of the trimethylenemethane dication (1) and the butadienyl dication (4) were evaluated using two independent computational methodologies to provide insight into the validity of Y-aromaticity. One methodology employed density functional theory calculations and examined the resonance contribution of the C=C double bond toward the double hydride abstraction enthalpies of methylpropene (6) and 2-butene (8), yielding 1 and 4, respectively. These resonance contributions by the double bond were determined by calculating the double hydride abstraction enthalpies of both the parallel and perpendicular conformations of vinylogues of 6 and 8, in which n = 1-4 vinyl units were inserted between the central carbon-carbon double bond and each of the reaction centers. Extrapolation of the resonance contribution in each vinylogue to n = 0 yielded the resonance contribution in the respective parent molecules. The second methodology employed an orbital deletion procedure (ODP), which effectively allowed us to examine the energies of individual resonance structures. The resonance energy of each dication is computed as the difference between the most stable resonance structure and that of the delocalized species. The two methodologies are in agreement, suggesting that the resonance energy of the trimethylenemethane dication is substantially greater than that of the butadienyl dication. The origin of this difference in resonance stabilization is discussed.     

Resonance Raman studies of xanthine oxidase: The reduced enzyme-product complex with violapterin

A study of the molecular, electronic, and vibrational characteristics of the molybdenum-containing enzyme complex xanthine oxidase with violapterin has been carried out using density functional theory calculations and resonance Raman spectroscopy. The electronic structure calculations were carried out on a model consisting of the enzyme molybdopterin cofactor [in the four-valent, reduced state; Mo(IV)O(SH)] covalently linked to violapterin (1H,3H,8H-pteridine-2,4,7-trione in the neutral form) via an oxygen bridge, Mo-O-C7. Full geometry optimizations were performed for all models using the SDD basis set and the three-parameter exchange functional of Becke combined with the Lee, Yang, and Parr correlational functional. Harmonic vibrational frequencies were determined for a variety of isotopes in an attempt to correlate experimentally observed shifts upon 18O-labeling of the Mo-OR bridge to bound product as well as shifts seen upon substitution of solvent-exchangeable protons in samples prepared in D2O. The theoretical vibrational frequencies compared favorably with experimentally observed vibrational modes in the resonance Raman spectra of the reduced xanthine oxidase-violapterin complex prepared in H2O and D2O and with 18O-labeled product. Correlating the isotopic shifts from the calculations with those from the resonance Raman experiments resulted in complete normal mode assignments for all modes observed in the 350-1750 cm(-1) range. The present work demonstrates that a model in which the violapterin is coordinated to the molybdenum of the active site in a simple end-on manner via the hydroxyl group introduced by an enzyme accurately predicts the observed resonance Raman spectrum of the complex. Given the numerous modes involving the bridging oxygen, a side-on binding mode can be eliminated.     

High resolution printing of DNA feature on poly(methyl methacrylate) substrates using supramolecular nano-stamping

In recent years, a large number of devices based on organic and biological materials have been developed. To scale-up the production of these systems to industrially acceptable standards, there is a need to develop soft-material stamping approaches with the needed resolution, complexity, and versatility. We have recently developed a DNA-based stamping method (supramolecular nano-stamping, SuNS) that has superior resolution and can print multiple molecules at the same time. A similar technique was independently developed by Crooks and co-workers. Here we show that SuNS can be used to efficiently print DNA features on a polymeric substrate (poly(methyl methacrylate), PMMA) with a 40 nm point resolution and a coverage that exceeds 100 mum2. The stamped PMMA substrate was also used as a master to print on a gold substrate. With PMMA being optically clear and electrically insulating, future applications of SuNS to print DNA micro- and nanoarrays are envisioned.     

Complex magnetic ordering in Eu3InP3: a new rare earth metal zintl compound

Eu3InP3 has been prepared as large single crystals with an indium flux reaction. The structure of the new compound is isotypic to Sr3InP3 and crystallizes in the orthorhombic space group Pnma with unit cell dimensions of a = 12.6517(15) A, b = 4.2683(5) A, and c = 13.5643(14) A (Z = 4, T = 140 K, R1 = 0.0404, wR2 = 0.0971 for all data). The structure consists of one-dimensional chains of corner-shared distorted [InP2P2/2]6- tetrahedra separated by rows of Eu2+ ions. Two of the three crystallographically distinct europium sites have a short Eu(1)-Eu(2) distance of 3.5954(7) A, which yields Eu-Eu dimers. The Eu-P bond distances range from 2.974(2) to 3.166(2) A. The temperature dependence of the conductivity indicates that Eu3InP3 is a small band gap semiconductor. Both magnetization and Eu-151 Mossbauer spectral measurements indicate that the europium in Eu3InP3 is divalent and that at least two magnetic transitions occur. Magnetization studies reveal magnetic transitions at 14, 10.4, and approximately 5 K. These transitions are also observed in heat capacity studies of Eu3InP3. The Mossbauer spectra indicate that the two europium sites are ordered at 12 K and that all three europium sites are ordered at 8 K.