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.     

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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.     

Calibration of polyacrylamide gel columns for the separation of oligonucleotides by capillary electrophoresis

Polyacrylamide-filled gel columns are used to separate oligonucleotide samples. For homopolymeric standard samples, plots of migration time versus molecular size are presented over a range of 30-160 bases. With 2.5-4% T and 3.3% C gels, good resolution over the examined mass range, with peak width at half height of 3 to 6 s, is obtained by applying electrical fields of 200-400 V/cm. The separation of heteropolymeric nucleotides by slab gel electrophoresis under routine conditions was compared with capillary gel electrophoresis. Using the same column and the same separation conditions, the plot of migration time versus base number is linear with an identical slope for three oligonucleotide samples which were examined, allowing a calibration of a gel-filled capillary for molecular mass determination.     

Inter- and intramolecular carbene reactions of diazoketones tethered to tricarbonyliron coordinated acyclic dienes. New tricarbonyliron complexes of cyclohexa-2,4-dienone and cyclopent-2-enone

A series of diazoketones tethered to tricarbonyliron coordinated dienes at the terminal position have been synthesized, and their decomposition reactions as carbene precursors studied. Intermolecular reactions with nucleophilic olefins were observed with Cu(acac)₂ as catalyst, the tricarbonyliron playing the role of an efficient protecting group of the diene fragment. With rhodium^II acetate, intramolecular reactions predominate, with formation of five-membered rings, in the side chain, or less commonly, as a cyclopentenone joined side by side to a rearranged diene ligand. Unusually, due to cationic stabilization by the iron, six-membered rings in the side chain are also formed here. A still different reaction was observed during the thermal decomposition of a diazoketone tethered to a coordinated diene at position 2: the formation of a stable complex of a cyclohexa-2-4-dienone (iron assisted carbene reaction and multiple hydrogen migrations).     

Conjugated Biradical Intermediates: Spectroscopic,Kinetic, and Trapping Studies of 2,2-Dimethyl-1,3-perinaphthadiyl

The biradical 2,2-dimethyl-1,3-perinaphthadiyl ( a ) was generated from two different precursors, the naphthocyclopropane 1 and the azo compound 2 , and from each by three different pathways (pyrolysis, direct photolysis, and triplet sensitization, Scheme 1). The combined evidence from flash photolysis, low-temperature spectroscopy, and product analyses provides a detailed mechanistic picture of the formation and decay of this reactive intermediate which is persistent at 77 K in the triplet ground state (³ a ) and rather long-lived (400 μs) at room temperature. When formed in its lowest singlet state (¹ a ), the biradical is too short-lived to undergo intersystem crossing to ³ a or bimolecular reactions. Thus, ³ a is formed exclusively from the excited triplet state of the precursor compounds, ³ 1 * or ³ 3 *. The monomolecular decay of ³ a is retarded by the spin barrier; ³ a initiates the polymerization of acrylonitrile and is trapped by ³O₂.     

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.