Silicon (111) and (100) surfaces and their interactions with Cs,K, Na and Li; phase changes and kinetics of desorption studied by surface ionization

The rates of desorption of Cs, K, Na, and Li atoms from the (100) and (111) surfaces of silicon are followed by surface ionization. In the temperature range of the experiments, 800–1000 K, the surfaces are prepared in two forms. The first are metastable ones produced by cooling the crystals rapidly to temperatures below their surface phase transitions. These are a well-known one for the (111) face at 1120-40 K and one for the (100) face at 980 ± 20 K. The second forms are relatively stable surfaces to which the metastable ones convert spontaneously. On the metastable surfaces the desorption is first order and relatively rapid. Comparison with a model suggests that the atoms are highly mobile. On the more stable surfaces, where the desorption is mixed first and second order, a model fitting the results has the atoms also highly mobile but concentrated at steps or edges between terraces from which most of the desorption occurs. The sticking probability for the atoms is near unity on all the surfaces, and the smaller atoms, Na and Li, penetrate into the bulk to an appreciable extent.     

Mechanism for dehalogenation reactions in fast atom bombardment mass spectrometry

The mechanism of a dehalogenation reaction that occurs during fast atom bombardment (FAB) mass spectrometry was examined using halogenated nucleosides as model compounds. For aglycone-halogenated nucleosides, an inverse linear relationship exists between the extent of FAB dehalogenation and the calculated electron affinity of an individual nucleoside. The degree of dehalogenation for a given nucleoside also varies inversely with the calculated electron affinity of most FAB matrices. The observed dehalogenation reaction can be completely inhibited when matrices with positive electron affinities, such as 3-nitrobenzyl alcohol and 2-hydroxyethyl disulfide, are used. High-performance liquid chromatographic analysis of the bulk glycerol matrix following exposure to the FAB beam indicates measurable amounts of dehalogenated product, suggesting that this reaction occurs in the condensed phase prior to gas-phase ion formation. A dehalogenation mechanism involving thermal electron capture and subsequent negative charge stabilization is consistent with these observations.     

Synthesis of pyridyloxadiazoles 1. Characterization and thermal rearrangement of an unexpected 1,2,4-oxadiazol-5(4H)one

The synthesis of pyridyloxadiazoles by the reaction of acid chlorides with 5-(2-pyrklyl)-tetrazole or 2-pyridineamidoxime was studied. Reaction of 2-pyridineamidoxime with N,N-dimethylcarbamyl chloride produced 3-(2-pyridyl)-4-N,N-dimethylaminocarbony 1-1,2,4-oxadi-azol-5(4H)one instead of the expected oxadiazole. The oxadiazolone underwent thermal rearrangement with expulsion of carbon dioxide to yield the desired 3-(2-pyridyl)-5-N,N-dimethylamino-1,2,4-oxadiazole.     

Synthesis and anticonvulsant activity of 1-benzyl-4-alkylamino-1H-imidazo[4,5-c]pyridines

Four 3-deaza analogues of the potent anticonvulsant purine, BW A78U, were synthesized and tested for anticonvulsant activity. The imidazo[4,5-c]pyridines 9–12 were prepared in two steps from 4-chloro-1H-imidazo[4,5-c]pyridine ( 2 ). The compounds were potent anticonvulsant agents against maximal electroshock-induced seizures in rats with i.p. ED₅₀ ranging from 2 to 3.5 mg/kg. However, these 3-deazapurines were appreciably more toxic than BW A78U, which precluded their development as potential antiepileptic agents.     

Synthesis of 6-[[(hydroxyimino)phenyl]methyl]-1-[(1-methylethyl)sulfonyl]-1H-imidazo[4,5-b]pyridin-2-amine. An aza analogue of enviroxime

6-[[(Hydroxyimino)phenyl]methyl]-1-[(1-methylethyl)sulfonyl]-1H-imidazo[4,5-b]pyridin-2-amine ( 1 ), an aza analogue of enviroxime, was synthesized in eight steps from 6-hydroxynicotinic acid ( 2 ). Acid 2 was nitrated, chlorinated with phosphorus pentachloride, and subjected to Friedel-Crafts aroylation to give 6-chloro-5-nitro-3-pyridyl phenyl ketone ( 5 ). Amination of 5 was followed by reduction of the nitro group and condensation with ethoxycarbonylisothiocyanate to give 6-benzyl-2-ethoxycarbonylamino-1H-imidazo[4,5-d]pyridine ( 8 ). The ethoxycarbonyl moiety of 8 was cleaved, N-1 was isopropylsulfonylated, and the carbonyl moiety was condensed with hydroxylamine to give 1 . Compound 1 was inactive against rhinovirus 1B and poliovirus type 1 .     

Engineering an obligate domain-swapped dimer of cyanovirin-N with enhanced anti-HIV activity

The anti-HIV cyanobacterial protein cyanovirin-N can undergo domain swapping to form an intertwined dimer. The dimeric form is stable at low pH and millimolar concentrations. By deleting an amino acid from the hinge linker about which domain swapping occurs, we have constructed an obligate domain-swapped dimer of cyanovirin-N that represents a new tetravalent carbohydrate binding protein that is stable over a large range of pH values. This obligate dimer displays enhanced anti-HIV activity relative to the wild-type cyanovirin-N monomer with an observed 3.5-fold decrease in IC(50) (9nM for the dimer vs 32 nM for the monomer) for inhibition of HIV-1 envelope-mediated cell fusion and, when expressed in Escherichia coli, can be rapidly obtained in >98% purity in a single chromatographic step.     

Controlled hierarchical self-assembly of networked coordination nanocapsules via the use of molecular chaperones

Supramolecular chaperones play an important role in directing the assembly of multiple protein subunits and redox-active metal ions into precise, complex and functional quaternary structures. Here we report that hydroxyl tailed C-alkylpyrogallol[4]arene ligands and redox-active Mn^II ions, with the assistance of proline chaperone molecules, can assemble into two-dimensional (2D) and/or three-dimensional (3D) networked nanocapsules. Dimensionality is controlled by coordination between the exterior of nanocapsule subunits, and endohedral functionalization within the 2D system is achieved via chaperone guest encapsulation. The tailoring of surface properties of nanocapsules via coordination chemistry is also shown as an effective method for the fine-tuning magnetic properties, and electrochemical and spectroscopic studies support that the nanocapsule is an effective homogeneous water-oxidation electrocatalyst, operating at pH 6.07 with an exceptionally low overpotential of 368 mV.

Molecular chaperones play a critical role in directing the assembly of nanocapsules that assemble into 2D or 3D coordination networks.     

Effect of dielectric roughness on performance of pentacene TFTs and restoration of performance with a polymeric smoothing layer

The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness. Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed. The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths. Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness. The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also approximately 2 A rms).     

Synthesis,stereochemistry and anti-tetrabenazine activity of bicyclo analogues of 2-phenylmorpholines

A series of bicyclo analogues of several 2-phenylmorpholines were synthesized and tested for anti-tetrabenazine activity in mice. Most of the target compounds were prepared by reaction of 2-bromopropio-phenone ( 22 ) with the appropriate amino alcohol to form 2-phenylmorpholinols. Reduction of the 2-phenyl-morpholinols with sodium borohydride gave amino diols, which were cyclized to morpholines with acid. Alternatively, oxazines 17 and 18 were synthesized by alkylation of phenyl-(2-pyrrolo)carbinol (32a) and phenyl-(2-piperidyl)carbinol (32b) with 2-bromoethanol, followed by cyclization of the resulting amino diols with acid. Only the spirocyclic compounds 8 and 9 had i.p. anti-tetrabenazine activity comparable to the non-cyclic compounds 2a-3b , but 8 and 9 were less active by the oral route of administration.