Molecular recognition in cyclodextrin complexes of amino acid derivatives: the effects of kinetic energy on the molecular recognition of a pseudopeptide in a nonconstraining host environment as revealed by a temperature-dependent crystallographic study

The crystal structure of a triclinic 2:2 inclusion complex of beta-cyclodextrin with N-acetyl-L-phenylalanine methyl ester has been determined at several temperatures between 298 and 20 K to further study molecular recognition using solid-state supramolecular beta-cyclodextrin complexes. The study reveals kinetic energy dependent changes in guest molecule conformations, orientations, and positions in the binding pocket presented by the crystal lattice. Accompanying these changes are observable differences in guest-guest interactions and hydrogen-bonding interactions in the binding pocket that involve guest molecules, water of hydration molecules, and beta-cyclodextrin molecules. On the basis of the differences observed in the crystal structures, we present a solid-state example of a system that displays the properties of both classical and quantum chemical models. At higher temperatures, the structure conforms to a classical mechanical model with dynamic disorder. At lower temperatures, the observations conform to examples in which there is static disorder representative of models in which quantum states differing in conformation, position, and orientation of components in the crystal structure are occupied. Ab initio theoretical calculations on the different guest molecule conformations have been carried out. Superpositions of theoretical electrostatic surface potential diagrams on the observed molecular positions in the complexes provide confidence that the deconvolution of the guest molecule disorder is acceptable. Temperature-dependent solid-state magic angle spinning deuteron NMR measurements provide evidence for large-amplitude, diffusive motion on a microsecond time scale in the complex.     

XPS analysis of aluminum nitride films deposited by plasma source molecular beam epitaxy

Ten samples of crystalline aluminum nitride (AlN) film were deposited on sapphire and silicon substrates by a plasma source molecular beam method. The samples were analyzed using X‐ray photoelectron spectroscopy (XPS) depth profiling and high‐resolution X‐ray diffraction. Oxygen levels were observed to decrease exponentially from the surface into the bulk film. Aluminum, nitrogen and oxygen peaks were fitted with subpeaks in a consistent manner and the subpeaks were assigned to chemical states. AlN subpeaks were observed at 73.5 eV for Al2p and 396.4 eV for N1s. An N1s subpeak at 395.0 eV was assigned to N? N defects. No direct N? O bonds are assigned; rather it is proposed that an N? Al? O bond sequence is the source of higher binding energy N1s subpeaks. The observations in this study support a model in which oxygen is bound only to aluminum in the form of Al? O octahedral complexes dispersed or clustered throughout the main AlN matrix or as Al? O bonds on the crystal grain boundaries. The data also suggest that the AlN lattice parameters are related to oxygen content, since the c‐axis is observed to increase with increasing oxygen content. Copyright © 2008 John Wiley & Sons, Ltd.     

Haloacetal radical cyclizations of alpha- and beta-hydroxyhydrazones

[reaction: see text] Haloacetal radical cyclizations of alpha- and beta-hydroxyhydrazones provide a direct access to aminosugarlike compounds. Stereocontrol of this process is influenced by stereogenic centers of both the hydroxyhydrazone and the acetal. The outcomes are consistent with chair and twist transition states with the anomeric alkoxy group in pseudoaxial orientations.     

Conformational preferences of jet-cooled melatonin: probing trans- and cis-amide regions of the potential energy surface

The hormone melatonin (N-acetyl-5-methoxytryptamine) is an indole derivative with a flexible peptide-like side chain attached at the C3 position. Using a combination of two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence excitation (LIF), resonant ion-dip infrared spectroscopy (RIDIRS), fluorescence-dip infrared spectroscopy (FDIRS), and UV-UV hole-burning spectroscopy, the conformational preferences of melatonin in a molecular beam have been determined. Three major trans-amide conformers and two minor cis-amide conformers have been identified in the R2PI spectrum and characterized with RIDIRS and FDIRS. Structural assignments are made using the infrared spectra in concert with density functional theory and localized MP2 calculations. Observation of cis-amide melatonin conformers in the molecular beam, despite the large energy gap (approximately 3 kcal/mol) between trans- and cis-amides, is striking because there are at least nine lower-energy trans-amide minima that are not detected. The implications of this observation for cooling and trapping conformational population in a supersonic expansion are discussed.     

Catalytic combustion of ethyl acetate over ceria-promoted platinum supported on Al2O3 and ZrO2 catalysts

Platinum-supported catalysts prepared by impregnation of mixed CeO₂/Al₂O₃ and CeO₂/ZrO₂ oxides using the sol-gel method were characterized and used in the combustion of ethyl acetate. In each series, the effect of CeO₂ loading (3 and 5 wt%) was studied. Characterization data from the studied catalysts (specific area measurements, hydrogen chemisorption, programmed temperature reduction (TPR), X-ray diffraction (XRD), photoelectron spectra (XPS) and transmission electron microscopy (TEM)) revealed significant changes in porosity and metal dispersion in each series. The catalytic activity of the solids, evaluated in the total combustion of ethyl acetate, exhibited a positive effect with the addition of Ce in the zirconia series whereas no significant changes was observed in the alumina series.     

The search for cosmic strangelets with the supersonic concorde and with JACEE's circumpolar balloon flight in Antarctica

The search for cosmic strangelet nuclei was carried out by two experiments with emulsion chambers. A balloon-borne JACEE emulsion chamber was flown at 3.5 g/cm² for 200 h in Antarctica (JACEE-10 experiment) and the Concorde flights were made by ECHOS at an atmospheric depth of 110 g/cm² between Paris and New York. No nuclei withZ⩾30 survived after traversing 60–120 g/cm² of the detector materials in the JACEE instruments. No evidence for a long mean free path were found in the zenith angle distribution forZ/β⩾26 nuclei. The exposure factor used by the JACEE was 72 m²hsr. The intensity upperbounds,I⩽(2.2–9.7)×10⁻²/m²h sr, were obtained for strangelets having an atmospheric attenuation length of 220−50 g/cm², which corresponds to the case for mass numberA=100–10000 andZ/β > 13. Concorde experiments (ECHOS) used both a thin and a thick emulsion chamber. The total exposure was 209 m² h sr and no candidates with chargeZ⩾30 were found. The largest track hadZ/β=28.6±1.29 withβ ∼ 1. Nuclei observed with charge 13⩽Z⩽30 were consistent with the survival intensity of ordinary nuclei. The flux bounds from the ECHOS experiments were I⩽(2.1–5.0) x 10⁻²/m²h for strangelets with mass number 100⩽A⩽1000.     

A frequency-locked, high power, X-band dielectric Cerenkovmaser

A high-power frequency-locked dielectric Cerenkov maser oscillator is presented. The device consists of an annular electron beam traveling down a dielectric-lined waveguide (wall radius 1.74 cm, liner thickness 2-3 mm, linear ∈=10). A 100 kW input signal is injected into the drift tube between the diode and liner, through a rectangular TE₁₀ to cylindrical TM₀₁ mode converter. When operated with a beam current of 500 A and a total output power of 13 MW, the device displays no RF quenching through the full width of the 100 ns beam pulse. Two higher current cases with power as high as 280 MW but severe RF quenching are presented. The results are compared with linear theory, and quenching mechanisms are discussed     

Efficient simulation of three-dimensional anisotropic cardiac tissue using an adaptive mesh refinement method

A recently developed space-time adaptive mesh refinement algorithm (AMRA) for simulating isotropic one- and two-dimensional excitable media is generalized to simulate three-dimensional anisotropic media. The accuracy and efficiency of the algorithm is investigated for anisotropic and inhomogeneous 2D and 3D domains using the Luo-Rudy 1 (LR1) and FitzHugh-Nagumo models. For a propagating wave in a 3D slab of tissue with LR1 membrane kinetics and rotational anisotropy comparable to that found in the human heart, factors of 50 and 30 are found, respectively, for the speedup and for the savings in memory compared to an algorithm using a uniform space-time mesh at the finest resolution of the AMRA method. For anisotropic 2D and 3D media, we find no reduction in accuracy compared to a uniform space-time mesh. These results suggest that the AMRA will be able to simulate the 3D electrical dynamics of canine ventricles quantitatively for 1 s using 32 1-GHz Alpha processors in approximately 9 h.     

Solar-neutrino results from SAGE

The solar-neutrino-capture rate measured by the Russian-American Gallium Experiment on metallic gallium during the period from January 1990 to December 1997 is (67.2 _?7.0?3.0 ^+7.2+3.5 ) SNU, where the uncertainties are statistical and systematic, respectively. This result represents a 7σ depression in the neutrino flux in relation to the predicted standard-solar-model rates. The experimental procedures used and data analysis are presented.     

Spins of resonances in the 12C + 16O system

The spins of resonances appearing at 22 MeV c.m. entrance channel energy in the ¹²C + ¹⁶O system are determined. Several inelastic transitions are used and a value of J^π = 15^? is deduced. This value disagrees with a previous J^π = 14⁺ assignment based on elastic scattering, but agrees with the J^π = 15^? value predicted at this energy by a recent microscopic calculation of ¹²C + ¹⁶O scattering.