Mechanism of the Direct Hydrodenitrogenation of Naphthylamine on Sulfided NiMo/Al2O3

The hydrodenitrogenation of 1-naphthylamine was studied over a sulfided NiMo/Al2O3 catalyst between 300 and 350 °C. 1-Naphthylamine reacted to tetralin, naphthalene, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine. To elucidate the reaction mechanism, the reactions of the intermediates 1,2,3,4-tetrahydro-1-naphthylamine, 1,2-dihydronaphthalene and 5,6,7,8-tetrahydro-1-naphthylamine were studied as well. The results show that 1-naphthylamine reacts through hydrogenation to 1,2,3,4-tetrahydro-1-naphthylamine, which reacts by NH3 elimination to 1,2-dihydronaphthalene. The latter molecule subsequently reacts by hydrogenation to tetralin as well as by dehydrogenation to naphthalene. In addition, naphthalene is formed by direct denitrogenation from 1-naphthylamine. This direct denitrogenation may take place by hydrogenation of 1-naphthylamine to 1,2-dihydro-1-naphthylamine, followed by NH3 elimination or followed by a Bucherer-type NH2-SH exchange, dehydrogenation and C-S bond hydrogenolysi

Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/τ-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of τ-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of $\dot M$, and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in $\dot M$. These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses AAS LaTex v4.0 (5 pages plus 4 postscript figures

Mechanism of the hydrodenitrogenation of neopentylamine and adamantylamine on sulfided NiMo/Al2O3

Neopentanethiol, 1-adamantanethiol, and 2-adamantanethiol were primary products and neopentane and adamantane were secondary products in the hydrodenitrogenation of neopentylamine, 1-adamantylamine, and 2-adamantylamine, respectively, over sulfided NiMo/Al2O3. Dialkylamines and dialkylimines were formed as primary products in the reactions of 2-adamantylamine and neopentylamine as well. None of the three amines can react by ammonia elimination and a classic SN2 substitution of the NH2 group by H2S is not possible for the adamantylamines either. The formation of di(2-adamantyl)imine and di(neopentyl)imine indicates that dehydrogenation and hydrogenation reactions occur and that imine or iminium-cation intermediates play an important role. NH2-SH substitution takes place by dehydrogenation of the amine to an imine or iminium cation, which adds H2S and eliminates NH3. The secondary character of adamantane and neopentane demonstrates that hydrogenolysis of the aliphatic C-N bond does not take place over sulfided NiMo/Al2O3 below 340°C. Even though 1-adamantylamine can neither react by classic SN2, E1, and E2 reactions, nor via an imine or iminium cation, it formed 1-adamanethiol at 300°C. This reaction might take place by an SN1 reaction or by adsorption of the amine at a surface vacancy, followed by a shift of the adamantyl group to a neighboring sulfur ato

A conformational study of peptides with the general structure Ac-L-Xaa-Pro-D-Xaa-L-Xaa-NH_2: spectroscopic evidence for a peptide with significant β-turn character in water and in dimethyl sulfoxide

Several tetrapeptides, Ac-Val-Pro-D-Ser-His-NH_2, in particular, show significant type II β-turn character in water and in dimethyl sulfoxide. Evidence for this turn population is provided by 2D-rotating frame nuclear Overhauser effect (ROESY) spectroscopy, ^1H NMR amide temperature coefficients, and circular dichroism (CD) studies. To further investigate which residues specifically contribute to the integrity of the turn, studies on 10 tetrapeptides, having the general sequence AC-LXaa-Pro-D-Xaa-L-Xaa-NH_2, are described. The results show the effects of sequence variations on the type II β-turn forming propensity of these peptides in solution. Conclusions from these studies indicate that a cooperative effect between a sterically hindered, β-branched amino acid at the (i) position and a small, non-β-branched D-amino acid at the (i+2) position promotes turn formation. Implications for use of these sequences as structural nucleation elements in de novo protein design are discussed

Microglia, the missing link in maternal immune activation and fetal neurodevelopment; and a possible link in preeclampsia and disturbed neurodevelopment?

Disturbances in fetal neurodevelopment have extensively been related to neurodevelopmental disorders in early and later life. Fetal neurodevelopment is dependent on adequate functioning of the fetal immune system. During pregnancy, the maternal immune system is challenged to both tolerate the semi-allogenic fetus and to protect the mother and fetus from microbes. The fetal immune system is influenced by maternal immune disturbances; therefore, perturbations in maternal immunity likely do not only alter pregnancy outcome but also alter fetal neurodevelopment. A possible common pathway could be modulating the functioning of tissue macrophages in the placenta and brain. Maternal immune tolerance towards the fetus involves several complex adaptations. In this active maternal immune state, the fetus develops its own immunity. As cytokines and other players of the immune system-which can pass the placenta-are involved in neurodevelopment, disruptions in immune balance influence fetal neurodevelopment. Several studies reported an association between maternal immune activation, complications of pregnancy as preeclampsia, and altered neonatal neurodevelopment. A possible pathway involves dysfunctioning of microglia cells, the immune cells of the brain. Functionality of microglia cells during normal pregnancy is, however, poorly understood. The recent outbreak of ZIKA virus (ZKV), but also the literature on virus infections in general and its consequences on microglial cell function and fetal neurodevelopment show the devastating effects a virus infection during pregnancy can have