Conformational changes of a calix[8]arene derivative at the air-water interface

The particular behavior of a p-tert-butyl calix[8]arene derivative (C8A) has been studied at the air-water interface using surface pressure-area isotherms, surface potential-area isotherms, film relaxation measurements, Brewster angle microscopy (BAM), and infrared spectroscopy for Langmuir-Blodgett films. Thus, it is observed that the properties of the film, for example, isotherms, domain formation, and FTIR spectra, recorded during the first compression cycle differ appreciably from those during the second compression and following cycles. The results obtained are interpreted on the basis of the conformational changes of the C8A molecules by surface pressure, allowing us to inquire into the inter- and intramolecular interactions (hydrogen bonds) of those molecules. Thus, the compression induces changes in the kind of hydrogen bonds from intra- and intermolecular with other C8A molecules to hydrogen bonds with water molecules.     

Probing weak molecular orbital interactions in non-conjugated diene molecules by means of near-edge X-ray absorption spectroscopy

Carbon and oxygen near-edge X-ray absorption fine structure (NEXAFS) spectra of 1,4-cyclohexadiene, p-benzoquinone, norbornadiene, norbornadienone, and cis-cis-[4,4,2]propella-3,8-diene-11,12-dione were calculated by means of Hartree-Fock and hybrid density functional theory using the static-exchange (STEX) approximation. The NEXAFS spectra are used as a probe to identify weak molecular interactions between the two non-conjugated ethylenic pi* orbitals present in these molecules. We show that the X-ray absorption spectrum of 1,4-cyclohexadiene exhibits some particular spectral structures in the discrete energy region that evidence diene through-bond orbital interaction, whereas absorption peaks are identified in the norbornadiene and norbornadienone spectra that indicate effective through-space orbital interactions. The molecular structure of the cis-cis-[4,4,2]propella-3,8-diene-11,12-dione isomer is such that the indirect through-bond or through-space diene orbital interactions are too weak to be assigned by its C1s NEXAFS spectrum.     

Synthesis,structural and spectral properties of Au complexes: luminescence properties and their non‐covalent DNA binding studies

Gold complexes of 1,3‐ bis‐pyridylimidazolium chloride ( L1 ), 1,3‐bis‐[2,6‐diisopropylphenyl]imidazolium chloride ( L2 ) and 1,3‐bis‐[benzyl]benzimidazolium chloride ( L3 ) were synthesized and characterized by analytical methods. For the complexes, electronic spectral results show that there is a marked difference in the band feature observed in the spectra, ascribed to the greater relativistic effect of gold. In fluorescence studies, the complexes develop emission bands in the visible region (400–600 nm) after excitation at around 350 nm. Au complex–DNA binding was studied, and it was observed that genomic DNA isolated from the U373‐GB cell line was fragmented and in some cases degraded by the Au complexes. Furthermore, the intensity of the DNA band increased when concentration of the metal complex was augmented. This study shows that the DNA cleavage is mediated by the Au complex. Copyright © 2013 John Wiley & Sons, Ltd.     

Production, purification, and characterization of a low-molecular-mass xylanase from Aspergillus sp. and its application in baking

An extracellular xylanase produced by a Mexican Aspergillus strain was purified and characterized. Aspergillus sp. FP-470 was able to grow and produce extracellular xylanases on birchwood xylan, oat spelt xylan, wheat straw, and corncob, with higher production observed on corncob. The strain also produced enzymes with cellulase, amylase, and pectinase activities on this substrate. A 22-kDa endoxylanase was purified 30-fold. Optimum temperature and pH were 60°C and 5.5, respectively, and isoelectric point was 9.0. The enzyme has good stability from pH 5.0 to 10.0 retaining >80% of its original activity within this range. Half-lives of 150 min at 50°C and 6.5 min at 60°C were found. K _m and activation energy values were 3.8 mg/mL and 26 kJ/mol, respectively, using birch wood xylan as substrate. The enzyme showed a higher affinity for 4-O-methyl-d-glucuronoxylan with a K _m of 1.9 mg/mL. The enzyme displayed no activity toward other polysaccharides, including cellulose. Baking trials were conducted using the crude filtrate and purified enzyme. Addition of both preparations improved bread volume. However, addition of purified endoxylanase caused a 30% increase in volume over the crude extract.     

Methylene Blue Adsorption on a DMPA Lipid Langmuir Monolayer

Adsorption of methylene blue (MB) onto a dimyristoylphosphatidic acid (DMPA) Langmuir air/water monolayer is studied by molecular dynamics (MD) simulations, UV reflection spectroscopy and surface potential measurements. The free‐energy profile associated with MB transfer from water to the lipid monolayer shows two minima of ?66 and ?60 kJ mol^?1 for its solid and gas phase, respectively, corresponding to a spontaneous thermodynamic process. From the position of the free‐energy minima, it is possible to predict the precise location of MB in the interior of the DMPA monolayer. Thus, MB is accommodated in the phosphoryl or carbonyl region of the DMPA Langmuir air/water interface, depending on the isomorphic state (solid or gas phase, respectively). Reorientation of MB, measured from the bulk solution to the interior of the lipid monolayer, passes from a random orientation in bulk solution to an orientation parallel to the surface of the lipid monolayer when MB is absorbed.     

Conformational Dependence of Through‐Space Tellurium–Tellurium Spin–Spin Coupling in Peri‐Substituted Bis(Tellurides)

Three related series of peri‐substituted bis(tellurides) bearing naphthalene, acenaphthene and acenaphthylene backbones (Nap/Acenap/Aceyl(TeY)₂ (Nap=naphthalene‐1,8‐diyl N ; Acenap=acenaphthene‐5,6‐diyl A ; Aceyl=acenaphthylene‐5,6‐diyl Ay ; Y=Ph 1 ; Fp 2 ; Tol 3 ; An‐p­ 4 ; An‐o­ 5 ; Tp 6 ; Mes 7 ; Tip 8 ) have been synthesised and their solid‐state structures determined by X‐ray crystallography. Molecular conformations were classified as a function of the two C9‐C‐Te‐C(Y) dihedral angles (θ); in the solid all members adopt AB or CCt configurations, with larger Te(aryl) moieties exclusively imposing the CCt variant. Exceptionally large J(¹²⁵Te,¹²⁵Te) spin–spin coupling constants between 3289–3848 Hz were obtained for compounds substituted by bulky Te(aryl) groups, implying these species are locked in a CCt‐type conformation. In contrast, compounds incorporating smaller Te(aryl) moieties are predicted to be rather dynamic in solution and afford much smaller J values (2050–2676 Hz), characteristic of greater populations of AB conformers with lower couplings. This conformational dependence of through‐space coupling is supported by DFT calculations.     

Fully automated trace level determination of parent and alkylated PAHs in environmental waters by online SPE-LC-APPI-MS/MS

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous compounds that enter the environment from natural and anthropogenic sources, often used as markers to determine the extent, fate, and potential effects on natural resources after a crude oil accidental release. Gas chromatography-mass spectrometry (GC-MS) after liquid–liquid extraction (LLE+GC-MS) has been extensively used to isolate and quantify both parent and alkylated PAHs. However, it requires labor-intensive extraction and cleanup steps and generates large amounts of toxic solvent waste. Therefore, there is a clear need for greener, faster techniques with enough reproducibility and sensitivity to quantify many PAHs in large numbers of water samples in a short period of time. This study combines online solid-phase extraction followed by liquid chromatography (LC) separation with dopant-assisted atmospheric pressure photoionization (APPI) and tandem MS detection, to provide a one-step protocol that detects PAHs at low nanograms per liter with almost no sample preparation and with a significantly lower consumption of toxic halogenated solvents. Water samples were amended with methanol, fortified with isotopically labeled PAHs, and loaded onto an online SPE column, using a large-volume sample loop with an auxiliary LC pump for sample preconcentration and salt removal. The loaded SPE column was connected to an UPLC pump and analytes were backflushed to a Thermo Hypersil Green PAH analytical column where a 20-min gradient separation was performed at a variable flow rate. Detection was performed by a triple-quadrupole MS equipped with a gas-phase dopant delivery system, using 1.50 mL of chlorobenzene dopant per run. In contrast, LLE+GC-MS typically use 150 mL of organic solvents per sample, and methylene chloride is preferred because of its low boiling point. However, this solvent has a higher environmental persistence than chlorobenzene and is considered a carcinogen. The automated system is capable of performing injection, online SPE, inorganic species removal, LC separation, and MS/MS detection in 28 min. Selective reaction monitoring was used to detect 28 parent PAHs and 15 families of alkylated PAHs. The methodology is comparable to traditional GC-MS and was tested with surface seawater, rainwater runoff, and a wastewater treatment plant effluent. Positive detections above reporting limits are described. The virtual absence of sample preparation could be particularly advantageous for real-time monitoring of discharge events that introduce PAHs into environmental compartments, such as accidental releases of petroleum derivates and other human-related events. This work covers optimization of APPI detection and SPE extraction efficiency, a comparison with LLE+GC-MS in terms of sensitivity and chromatographic resolution, and examples of environmental applications.     

Fiber introduction mass spectrometry: determination of pesticides in herbal infusions using a novel sol-gel PDMS/PVA fiber for solid-phase microextraction

An application of the direct coupling of solid-phase microextraction (SPME) with mass spectrometry (MS), a technique known as fiber introduction mass spectrometry (FIMS), is described to determine organochlorine (OCP) and organophosphorus (OPP) pesticides in herbal infusions of Passiflora L. A new fiber coated with a composite of poly(dimethylsiloxane) and poly(vinyl alcohol) (PDMS/PVA) was used. Sensitive, selective, simple and simultaneous quantification of several OCP and OPP was achieved by monitoring diagnostic fragment ions of m/z 266 (chlorothalonil), m/z 195 (alpha-endosulfan), m/z 278 (fenthion), m/z 263 (methyl parathion) and m/z 173 (malathion). Simple headspace SPME extraction (25 min) and fast FIMS detection (less than 40 s) of OCP and OPP from a highly complex herbal matrix provided good linearity with correlation coefficients of 0.991-0.999 for concentrations ranging from 10 to 140 ng ml(-1) of each compound. Good accuracy (80 to 110%), precision (0.6-14.9%) and low limits of detection (0.3-3.9 ng ml(-1)) were also obtained. Even after 400 desorption cycles inside the ionization source of the mass spectrometer, no visible degradation of the novel PDMS/PVA fiber was detected, confirming its suitability for FIMS. Fast (ca 20 s) pesticide desorption occurs for the PDMS/PVA fiber owing to the small thickness of the film and its reduced water sorption.