In spite of containing three conformationally accessible β‐H atoms, palladacycle 1 a is an isolable intermediate in the asymmetric Heck cyclization of 2 a. Although 1 a is stable in the presence of the hydrotriflate salt of 1,2,2,6,6‐pentamethylpiperidine, it is converted into the oxindole Heck product when exposed to the more acidic hydrotriflate salt of 2,6‐di‐tert‐butylpyridine. Heck cyclization of 2 b is also believed to proceed by way of a palladacyclic intermediate 1 b, which in this case undergoes β‐methoxide elimination. Bn=benzyl.
lH Nuclear magnetic resonance measurements are reported for the methanol-synthesis heterogeneous catalyst system based on Cu/Zn/Aloxides. Spectra of the hydrogen-reduced catalyst at room temperature show two peaks, one of which is narrow and shifted to high frequency by 85 ppm. This is assigned to hydrogen dissociatively adsorbed onto the copper metal surface, the interaction of the protons with the metal conduction electrons giving a Knight shift. The shift is independent of hydrogen pressure up to 50 Torr,y and the isotherm obtained from the intensity of the Knight-shifted peak as a function of hydrogen pressure is consistent with the assignment. The amount of copper metal surface available for hydrogen chemisorption, as detected by lH n.m.r. spectroscopy, is very variable.The second peak in the 'H n.m.r. spectra of the reduced catalyst occurs close to the normal resonance position for protons in diamagnetic systems. Partially T,-relaxed lH spectra show this peak to be a composite, with up to three components distinguishable on the basis of their spin-lattice relaxation and linewidth characteristics. Measurements of 'H spin-lattice relaxation behaviour are presented for samples of zinc oxide, the spinel catalyst precursor and a copper-free catalyst sample to assist in assignment of lines in the spectra of reduced catalyst samples. The data are interpreted in terms of four proton populations, these being hydrogens associated with the oxide phase (probably ZnH species), hydroxyl hydrogens on the zinc oxide surface, hydrogens associated with the alumina-containing oxide phase and the hydrogen atoms chemisorbed on copper metal surfaces. Comparisons of the n.m.r. properties for the reduced catalyst samples with those for the noncopper-containing samples suggest that the copper component in the reduced catalyst has the effect of substantially increasing the population assigned as ZnH species. In addition it substantially lowers their spin-lattice relaxation time and that of the alumina-associated proton population. The values of these relaxation times are suggestive of an electronic mechanism, being too short for typical nuclear dipoledipole and similar mechanisms. The peaks assigned to ZnH and CuH species are shown to derive directly from adsorbed hydrogen. Measurements of the spin-lattice relaxation of the total proton magnetisation for the reduced catalyst in the presence of hydrogen gas demonstrate that the relaxation of the component populations are coupled uia exchange through surface or spin diffusion.Preliminary measurements are reported of 13C spectra for reduced catalyst samples reacted with 13C-enriched methanol, CO, CO, and CO-H, and C0,-H, mixtures. All samples show evidence of a carbon species which has no detectable dipolar coupling to protons. The sample exposed to 13C0
The impact of synthetic amyloid β (1–42) (Aβ1–42) oligomers on biophysical properties of voltage-gated potassium channels Kv 1.3 and lipid bilayer membranes (BLMs) was quantified for protocols using hexafluoroisopropanol (HFIP) or sodium hydroxide (NaOH) as solvents prior to initiating the oligomer formation. Regardless of the solvent used Aβ1–42 samples contained oligomers that reacted with the conformation-specific antibodies A11 and OC and had similar size distributions as determined by dynamic light scattering. Patch-clamp recordings of the potassium currents showed that synthetic Aβ1–42 oligomers accelerate the activation and inactivation kinetics of Kv 1.3 current with no significant effect on current amplitude. In contrast to oligomeric samples, freshly prepared, presumably monomeric, Aβ1–42 solutions had no effect on Kv 1.3 channel properties. Aβ1–42 oligomers had no effect on the steady-state current (at −80 mV) recorded from Kv 1.3-expressing cells but increased the conductance of artificial BLMs in a dose-dependent fashion. Formation of amyloid channels, however, was not observed due to conditions of the experiments. To exclude the effects of HFIP (used to dissolve lyophilized Aβ1–42 peptide), and trifluoroacetic acid (TFA) (used during Aβ1–42 synthesis), we determined concentrations of these fluorinated compounds in the stock Aβ1–42 solutions by 19F NMR. After extensive evaporation, the concentration of HFIP in the 100× stock Aβ1–42 solutions was ∼1.7 μM. The concentration of residual TFA in the 70× stock Aβ1–42 solutions was ∼20 μM. Even at the stock concentrations neither HFIP nor TFA alone had any effect on potassium currents or BLMs. The Aβ1–42 oligomers prepared with HFIP as solvent, however, were more potent in the electrophysiological tests, suggesting that fluorinated compounds, such as HFIP or structurally-related inhalational anesthetics, may affect Aβ1–42 aggregation and potentially enhance ability of oligomers to modulate voltage-gated ion channels and biological membrane properties.
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