The Direct Partial Oxidation of Methane to Dimethyl Ether over Pt/Y2O3 Catalysts Using an NO/O2 Shuttle

Using a mixture of NO + O₂ as the oxidant enabled the direct selective oxidation of methane to dimethyl ether (DME) over Pt/Y₂O₃. The reaction was carried out in a fixed bed reactor at 0.1 MPa over a temperature range of 275–375 °C. During the activity tests, the only carbon‐containing products were DME and CO₂. The DME productivity (μmol g_cat^?1 h^?1) was comparable to oxygenate productivities reported in the literature for strong oxidants (N₂O, H₂O₂, O₃). The NO + O₂ mixture formed NO₂, which acted as the oxygen atom carrier for the ultimate oxidant O₂. During the methane partial oxidation reaction, NO and NO₂ were not reduced to N₂. In situ FTIR showed the formation of surface nitrate species, which are considered to be key intermediate species for the selective oxidation.     

Trace LC/MS/MS quantitation of 17β‐estradiol as a biomarker for selective estrogen receptor modulator activity in the rat brain

A sensitive LC/MS/MS method has been developed by derivatization of 17β‐estradiol (E2) with dansyl chloride to quantitate 17β‐E2 in female rat serum. The use of E2‐d₅ minimized interferences from endogenous 17β‐E2 in order to achieve a limit of quantitation (LOQ) of 2.5 pg/ml using 150 µl of female rat serum. The recovery of the dansyl derivative was 95% or greater in quality control samples. The intra and interday assay precision was better than 8.2 and 6.2%, respectively, with accuracies ranging from 97 to 101% in the quality control samples. The assay was used for the quantitation of serum E2 as a biomarker for the estrogen receptor (ER) antagonist activity of small molecule SERMs (selective estrogen receptor modulators) in the female rat brain. The study revealed that a statistically significant upregulation of serum 17β‐E2 occurred for rats dosed with SERMs that are known to penetrate the brain and disrupt the hypothalamic‐pituitary‐ovarian (HPO) axis. Variations in 17β‐E2 in ascending dose studies also correlated with the corresponding trends in CYP17a1 levels, an mRNA biomarker for ovarian hyperstimulation. This biomarker assay has provided a useful screen for medicinal chemistry optimization to produce SERMs that do not interfere with negative feedback of estrogens on the brain and for biological hypothesis testing. Copyright © 2009 John Wiley & Sons, Ltd.     

The link between sequence and conformation in protein structures appears to be stereochemically established

In search of the link between sequence and conformation in protein structures, we perform molecular dynamics analysis of the effect of stereochemical mutation in end-protected octa-alanine Ac-Ala8-NHMe from poly-L to an alternating-L,D structure. The mutation has a dramatic effect, transforming the peptide from a condition of extreme sensitivity to one of extreme insensitivity to solvent. Examining the molecular folds of poly-L and alternating-L,D structure in atomistic detail, we find them to differ in the relationship between peptide dipolar interactions at the local and nonlocal levels, either conflicting or harmonious depending upon the chain stereochemistry. The stereochemical transformation of interpeptide electrostatics from a condition of conflict to one of harmony explains the long-standing puzzle of why poly-L and alternating-L,D peptides strongly differ in properties such as "stiffness" and solvent sensitivity. Furthermore, it is possible that poly-L stereochemistry is also the fulcrum of protein sensitivity to the effects of amino acid side-chain structures via dielectric arbitrations in interpeptide electrostatics. Indeed the evidence is accumulating that the amino acid side chains differing in alpha-helix and beta-sheet propensities also differ in their desolvating effects in the adjacent and nearest-neighbor peptides and thus possibly in the solvent screening of peptide dipolar interactions.     

Spectral signatures and molecular origin of acid dissociation intermediates

The existence of a broad, mid-infrared absorption ranging from 1000 to 3000 cm(-1) is usually interpreted as a signature for the existence of protonated water networks. Herein, we use cryogenic mixtures of water and hydrogen fluoride (HF) and show experimental and computational evidence that similarly wide absorptions can be generated by a broad distribution of proton-shared and ion pair complexes. In the present case, we demonstrate that the broadening is mainly inhomogeneous, reflecting the fact that the topology of the first solvation shell determines the local degree of ionization and the shared-proton asymmetric stretching frequency within H2O x HF complexes. The extreme sensitivity of the proton transfer potential energy hypersurface to local hydrogen bonding topologies modulates its vibrational frequency from 2800 down to approximately 1300 cm(-1), the latter value being characteristic of solvation geometries that yield similar condensed-phase proton affinities for H2O and fluoride. By linking the local degree of ionization to the solvation pattern, we are able to propose a mechanism of ionization for HF in aqueous solutions and to explain some of their unusual properties at large concentrations. However, an important conclusion of broad scientific interest is our prediction that spectral signatures that are normally attributed to protonated water networks could also reveal the presence of strong hydrogen bonds between un-ionized acids and water molecules, with important consequences to spectroscopic investigations of biologically relevant proton channels and pumps.     

Elasticity of ion stuffing in chemically strengthened glass

The chemical strengthening of glass involves the stuffing of large ions into network sites previously occupied by smaller ions. Typically this involves an exchange of Li⁺ or Na⁺ ions in the glass for larger K⁺ ions from a molten salt bath. This swapping of ions creates compressive stress in the surface of the glass, significantly increasing the strength of the final glass product. The magnitude of this compressive stress is governed by the linear network dilation coefficient (LNDC), which defines the amount of linear strain per unit of ion substitution. However, the amount of strain attainable through ion exchange is much smaller compared to what is expected from as-melted versions of the same final glass composition. This effect, which we have termed the “network dilation anomaly,” is a result of the different local environment around the invading ion species in as-melted versus ion-exchanged glasses. A remaining question concerns the nature of the network strain due to ion stuffing. Using molecular dynamics simulations, we show that the strain induced by ion stuffing is entirely elastic. In other words, when a reverse ion exchange is performed to swap the original ions back into the glass, the initial volume of the as-melted glasses is entirely recovered. Moreover, we show that the local structural environment around the alkali ions is restored to the as-melted conditions. The elastic nature of ion stuffing demonstrates that the network dilation anomaly is not a result of plasticity, but rather a failure to achieve the full amount of expected elastic strain during ion exchange. The elasticity itself consists of both instantaneous and delayed contributions.     

Cellular uptake and subcellular localization of highly luminescent silica-coated CdSe quantum dots--in vitro and in vivo

With excellent optical properties, quantum dots (QDs) have been made as attractive molecular probes for labeling cells in biological research. The purpose of the present work is to explore the possible role of silica-coated cadmium selenide (CdSe) QDs in the in vitro and in vivo cellular uptake and their subcellular localization. The in vitro uptake characteristics of silica-coated CdSe QDs were performed in cultured New Zealand rabbit adipose tissue-derived mesenchymal stem cells (RADMSCs) and Human cervical cancer cells (HeLa) using fluorescence microscopy after staining with 4,6-diamidino-2-phenylindole (DAPI). The in vitro results showed that the silica-coated CdSe QDs were efficiently taken up by the cells and it was localized in the intracellular vesicles giving strong fluorescence from the cytoplasm and nearby nucleus. Subsequently, the in vivo localization and distribution of QDs were studied by the hematoxilin stained semithin cryosections of tissues (~15 μm thickness) under fluorescence microscopy and ultrathin sections of tissues (~100 nm thickness) under confocal laser scanning microscopy at the distribution maxima. Our in vivo results confirmed the effective cellular uptake and even distribution pattern of QDs in tissues. Overall, these in vitro and in vivo results are represented with focus on internalization, subcellular localization and distribution of the QDs, in view of their potential applications in biomedical field.     

Atomistic understanding of the network dilation anomaly in ion-exchanged glass

Chemically strengthened glasses are of increasing technological importance for personal electronic devices, tablet computers, and a variety of other applications. However, there are many unexplained phenomena associated with the physics of the ion exchange process used for strengthening. One of the most puzzling of these is the anomalous behavior of the network dilation coefficient, i.e., the parameter governing the amount of linear strain of the glass per unit of alkali ions exchanged, which is inevitably a factor of 2–4 higher for as-melted glasses as compared to chemically strengthened versions of the same glass compositions prepared via ion exchange. In this paper, we investigate the atomistic origin of this discrepancy between as-melted and ion-exchanged glasses based on molecular dynamics simulations of a series of alkali tetrasilicate glasses, viz., xNa₂O·(20 ? x)K₂O·80SiO₂ (mol%). The network dilation coefficient of the ion-exchanged glasses is dependent on composition and ranges from 30% to 54% of the ideal value obtained from the as-melted glasses. This anomalous behavior of the network dilation coefficient is explained in terms of different local environments between sodium and potassium sites in the glass network and a two-stage relaxation process of the local potassium environment after ion exchange.     

Defect-mediated self-diffusion in calcium aluminosilicate glasses: A molecular modeling study

The mechanism of self-diffusion in calcium aluminosilicate glasses is investigated at the atomistic level using molecular dynamics (MD) simulations. We study nine glass compositions having the fixed ratio R = [CaO]/[Al₂O₃] = 1 and the concentration of SiO₂ varied from 11.8 to 76.5 mol%. The diffusion coefficient is calculated for each composition at different temperatures from 300 to 6000 K in steps of 300 K. The self-diffusivities of the various elements are found to be close to each other in magnitude, signifying the cooperative nature of the atomic movement. Network “defects” such as miscoordinated cations, non-bridging oxygen, and oxygen triclusters are also studied as a function of temperature and composition. We find that the behavior of self-diffusion correlates well with the concentration of network defects. A model of self-diffusion in calcium aluminosilicate glasses is proposed where diffusion is considered as a defect-mediated process resulting from bond-switching reactions at high temperature.     

Cytotoxicity and fluorescence studies of silica-coated CdSe quantum dots for bioimaging applications

The toxicological effects of silica-coated CdSe quantum dots (QDs) were investigated systematically on human cervical cancer cell line. Trioctylphosphine oxide capped CdSe QDs were synthesized and rendered water soluble by overcoating with silica, using aminopropyl silane as silica precursor. The cytotoxicity studies were conducted by exposing cells to freshly synthesized QDs as a function of time (0–72 h) and concentration up to micromolar level by Lactate dehydrogenase assay, MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide] assay, Neutral red cell viability assay, Trypan blue dye exclusion method and morphological examination of cells using phase contrast microscope. The in vitro analysis results showed that the silica-coated CdSe QDs were nontoxic even at higher loadings. Subsequently the in vivo fluorescence was also demonstrated by intravenous administration of the QDs in Swiss albino mice. The fluorescence images in the cryosections of tissues depicted strong luminescence property of silica-coated QDs under biological conditions. These results confirmed the role of these luminescent materials in biological labeling and imaging applications.     

Existence of specific "folds" in polyproline II ensembles of an "unfolded"alanine peptide detected by molecular dynamics

Equilibrium ensembles of octaalanine (Ac-Ala8-NHMe) in water, prepared with MD, are analyzed for contributing microstates with an RMSD-based conformational clustering algorithm. The extracted ensemble-averaged properties are in excellent agreement with numerous spectroscopic measurements reported with small alanine model peptides in water. However, the dominantly polyproline II-like ensemble of the peptide is found to be populated with a handful of highly position-specific "folds", including beta-turns, beta-hairpins, and helix nuclei, which could be the "seeds" that initiate proteins along their folding pathways.