Thermal degradation of a palladium coating on hydrogen-tight niobium membranes

The thermal stability of palladium-covered niobium, a composite used in hydrogen-tight membranes, is studied. Palladium is applied by the method of plasma sputtering. The thermal degradation of the coating is judged from a decrease in its ability to prevent niobium oxidation. It is found that the degree of oxidation of the samples heated in air at 693 K drops when the thickness of the coating increases in the range 2.5–1000.0 nm. The reason is heating-induced destructive modifications of the coating morphology, which are observed under a scanning electron microscope. The thicker the coating, the higher the temperature of onset of destruction. At temperatures above 873 K, the initially smooth continuous coating becomes highly porous and this porous layer does not change upon subsequent heating (up to 1273 K). Critical factors and possible mechanisms of thermal degradation of palladium coatings on hydrogen-tight composite membranes are discussed.     

Hydrogel nanoparticles covered by neuroligin-2-derived peptide-protected <Emphasis Type="Italic">β</Emphasis> cells under oxidative stress and increase their proliferation

Protein complexes that mediate secretion and adhesion are located on the plasma membrane of pancreatic β cells. Neuroligins and their binding partners, the neurexins, are among these complexes. β cell maturation and physiologically regulated insulin secretion, as a response to high levels of blood glucose, are dependent on their three-dimensional (3D) arrangement. Both insulin secretion and the proliferation rates of β cells dramatically increase when β cells are co-cultured with clusters of a member of the neuroligin family: NL-2. A membranal protein, such as NL-2, has very limited drugability owing to its low biostability and bioavailability. Thus, based on in silico modeling, a short NL-2 peptide (HSA-28), which was able to mimic NL-2-positive effects on β cells, was designed, as we described in previous publication. However, the peptide was active only as a cluster, created by the covering the maghemite (γ-Fe₂O₃)-based nanoparticles (NPs) with limited biocompatibility. In this brief communication, we will show that conjugation of HSA-28 to biocompatible hydrogel NPs exhibits an impressive protective effect on INS-1E β cells under oxidative stress and induces their proliferation rate via augmentation of PDX1 nuclear translocation. The diameter of coated by the peptide NPs was 206?±?63 nm (DLS) and 114?±?27 nm (cryo-TEM). This significant change in size can be explained by the very hydrophilic character of the proteinoid NPs, inducing adsorption of many water molecules on their surface, which are accounted only by the DLS. The ability of biocompatible hydrogel NPs to prevent apoptosis and increase β cell mass might be used for developing novel β cell protective therapies.

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Graphical abstract Effect of covered by bioactive peptide NPs on PDX1 nuclei translocation.

     

How do dimethyl oxalate ions CH3O-C(=O)-C(=O)-OCH 3 ·+ break in half? Loss of CH 3 · + CO2 versus CH3O-C-O·

The interesting unimolecular dissociation chemistry of dimethyl oxalate (DMO) ions, CH₃O-C(=O)-C(=O)-OCH ₃ ^·+ , has been studied by vacuum ultraviolet photoionization and tandem mass spectrometry based experiments. The measured appearance energy (AE) for the generation of CH₃O-C=O⁺ (10. 5 eV) is not compatible with a simple bond cleavage involving the cogeneration of the radical CH₃O-C=O^· whose calculated AE is 11 kcal/mol higher. However, because the CH₃O-C=O^· radical is thermodynamically less stable than its dissociation products CH₃ ^· and CO₂, by 19 kcal/mol, a two-step dissociation of ionized DMO into CH₃O-C=O⁺ + CH ₃ ^· + CO₂ is energetically feasible. Collision induced dissociative ionization experiments clearly show that low energy DMO ions dissociate into CH₃ ^· + CO₂ without the intermediacy of CH₃O-C=O^·. Experiments using a charged collision chamber further indicate that CO₂ is released first, followed by loss of CH₃ ^· and not vice versa and a mechanism is proposed. The measured AE, which we assign to the two-step process, is 8 kcal/mol higher than the calculated value. This could be due to a competitive shift caused by a prominent low energy decarbonylation reaction yielding the hydrogen bridged radical cation CH₂=O … H … O=C-OCH₃ ^·+. However, from metastable ion observations and AE measurements on deuterium labeled DMO ions, it follows that there is no competitive shift and that the elevated AE for the two-step process corresponds to the barrier for the first step, loss of CO₂. Finally, neutralization-reionization experiments on ionized DMO and CH₃O-C=O⁺ provide evidence for the existence of CH₃O-C=O^· as a kinetically stable radical.     

Rate of the retro-Diels-Alder dissociation of 1,2,3,6-tetrahydropyridine over a wide temperature range

Rate constants for the retro-Diels-Alder dissociation of 1,2,3,6-tetrahydropyridine, to 1,3-butadiene and methanimine, have been measured over 650–1450 K. To cover this range, three separate techniques were used at three laboratories: laser schlieren and single pulse shock tube methods, and a comparative rate flow system technique. The derived rate constants are extrapolated to the high-pressure limit with an RRKM model parameterized to fit the falloff observed in the laser-schlieren measurements. The resulting high-pressure rate constants show a reduction in activation energy of about 10 kcal/mol, comparing the isoelectronic cyclohexene, but little change in A-factor. There is an apparent increase in activation energy of 4 kcal/mol over the temperature range of these experiments, which is just outside probable error. Such a rise in activation energy is in marked contrast to the drop usually seen in simple bond fission, and may reflect a change from a concerted to a stepwise, biradical mechanism at high temperatures.     

Time-dependent mass spectra and breakdown graphs. The kinetic shift in aniline

Time-resolved appearance energies and metastable peak shapes were determined by trapped-ion mass spectrometry (TIMS) for the unimolecular dissociation of aniline cations. The long-time (milliseconds) appearance energy limit, AE(C₅H⁺₆) = 11.26 = 0.2 eV. suggests the formation at threshold energies of the cyclopentadienyl cation with neutral hydrogen isocyanide. HNC.     

Optical and Conductivity Properties of PbS Nanocrystals in Amorphous Zirconia Sol-Gel Films

Lead sulfide (PbS) nanocrystals (NCs), embedded in amorphous zirconia sol-gel film with different PbS mole concentration (5–30%), were grown at temperature, ranging from 200°C to 350°C. The size of PbS NCs was determined by TEM and by blue shift of the absorption edge. The size increased with an increase of the synthesis temperature and PbS mole concentration. The optical and electrical properties of various sizes of PbS NCs in zirconia film are investigated utilizing absorption, photoluminescence (PL) and current-voltage measurement. The PL spectra were Stokes shifted from the corresponding absorption edge by about 0.5 eV. The latter can be associated with recombination process from surface state. The electrical properties were investigated by the deposition of the PbS NCs-zirconia films on ITO/glass substrate, followed by their coverage with gold contact. The current-voltage characteristics depend on the PbS NCs size and exhibits nonlinear nearly symmetric curve, associated with the space-charge limited current or the tunneling of carriers through the nanocrystalline film.     

Elimination of water from the carboxyl group of GlyGlyH+

The elimination of water from the carboxyl group of protonated diglycine has been investigated by density functional theory calculations. The resulting structure is identical to the b(2) ion formed in the mass spectrometric fragmentation of protonated peptides (therefore named "b2" in this study). The most stable geometry of the fragment ion ("b2") is an O-protonated diketopiperazine. However, its formation is kinetically disfavored as it requires a free energy of 58.2 kcal/mol. The experimentally observed N-protonated oxazolone is 3.0 kcal/mol less stable. The lowest energy pathway for the formation of the "b2" ion requires a free energy of 37.5 kcal/mol and involves the proton transfer from the amide oxygen of protonated diglycine to the hydroxyl oxygen. Fragmentation initiated by proton transfer from the terminal nitrogen has also a comparable free energy of activation (39.4 kcal/mol). Proton transfer initiating the fragmentation, from the highly basic terminal nitrogen or amide oxygen to the less basic hydroxyl oxygen is feasible at energies reached in usual mass spectrometric experiments. Amide N-protonated diglycine structures are precursors of mainly y(1) ions rather than "b2" ions. In the lowest energy fragmentation channels, proton transfer to the hydroxylic oxygen, bond breaking and formation of an oxazolone ring occur concertedly but asynchronously. Proton transfer to hydroxyl oxygen and cleavage of the corresponding C-O bond take place at the early stages of the fragmentation step, while ring closure to form an oxazolone geometry occurs at the later stages of the transition. The experimentally observed low kinetic energy release is expected to be due to the existence of a strongly hydrogen bonded protonated oxazolone-water complex in the exit channel. Whereas the threshold energy for "b2" ion formation (37.1 kcal/mol) is lower than for the y(1) ion (38.4 kcal/mol), the former requires a tight transition state with an activation entropy, DeltaS++ = -1.2 cal/mol.K and the latter has a loose transition state with DeltaS++ = +8.8 cal/mol.K. This leads to y(1) being the major fragment ion over a wide energy range.