Computational study on the existence of organic peroxy radical-water complexes (RO2.H2O)

The existence of a series of organic peroxy radical-water complexes [CH3O2.H2O (methyl peroxy); CH3CH2O2.H2O (ethyl peroxy); CH3C(O)O2.H2O (acetyl peroxy); CH3C(O)CH2O2.H2O (acetonyl peroxy); CH2(OH)O2.H2O (hydroxyl methyl peroxy); CH2(OH)CH2O2.H2O (2-hydroxy ethyl peroxy); CH2(F)O2.H2O (fluoro methyl peroxy); CH2(F)CH2O2.H2O (2-fluoro ethyl peroxy)] is evaluated using high level ab initio calculations. A wide range of binding energies is predicted for these complexes, in which the difference in binding energies can be explained by examination of the composition of the R group attached to the peroxy moiety. The general trend in binding energies has been determined to be as follows: fluorine approximately alkyl < carbonyl < alcohol. The weakest bound complex, CH3O2.H2O, is calculated to be bound by 2.3 kcal mol-1, and the strongest, the CH2(OH)O2.H2O complex, is bound by 5.1 kcal mol-1. The binding energy of the peroxy radical-water complexes which contain carbonyl and alcohol groups indicates that these complexes may perturb the kinetics and product branching ratios of reactions involving these complexes.     

Extraordinary NO2 Removal by the Metal–Organic Framework UiO‐66‐NH2

Here we discuss the removal of nitrogen dioxide, an important toxic industrial chemical and pollutant, from air using the MOF UiO‐66‐NH₂. The amine group is found to substantially aid in the removal, resulting in unprecedented removal capacities upwards of 1.4 g of NO₂ /g of MOF. Furthermore, whereas NO₂ typically generates substantial quantities of NO on sorbents, the amount generated by UiO‐66‐NH₂ is significantly reduced. Of particular significance is the formation of a diazonium ion on the aromatic ring of the MOF, and the potential reduction of NO₂ to molecular nitrogen.     

Magnetic ionic liquids as non-conventional extraction solvents for the determination of polycyclic aromatic hydrocarbons

This work describes the applicability of magnetic ionic liquids (MILs) in the analytical determination of a group of heavy polycyclic aromatic hydrocarbons. Three different MILs, namely, benzyltrioctylammonium bromotrichloroferrate (III) (MIL A), methoxybenzyltrioctylammonium bromotrichloroferrate (III) (MIL B), and 1,12-di(3-benzylbenzimidazolium) dodecane bis[(trifluoromethyl)sulfonyl)]imide bromotrichloroferrate (III) (MIL C), were designed to exhibit hydrophobic properties, and their performance examined in a microextraction method for hydrophobic analytes. The magnet-assisted approach with these MILs was performed in combination with high performance liquid chromatography and fluorescence detection. The study of the extraction performance showed that MIL A was the most suitable solvent for the extraction of polycyclic aromatic hydrocarbons and under optimum conditions the fast extraction step required ∼20 μL of MIL A for 10 mL of aqueous sample, 24 mmol L⁻¹ NaOH, high ionic strength content of NaCl (25% (w/v)), 500 μL of acetone as dispersive solvent, and 5 min of vortex. The desorption step required the aid of an external magnetic field with a strong NdFeB magnet (the separation requires few seconds), two back-extraction steps for polycyclic aromatic hydrocarbons retained in the MIL droplet with n-hexane, evaporation and reconstitution with acetonitrile. The overall method presented limits of detection down to 5 ng L⁻¹, relative recoveries ranging from 91.5 to 119%, and inter-day reproducibility values (expressed as relative standard derivation) lower than 16.4% for a spiked level of 0.4 μg L⁻¹ (n = 9). The method was also applied for the analysis of real samples, including tap water, wastewater, and tea infusion.     

Measuring coexisting densities from a two-phase molecular dynamics simulation by voronoi tessellations

A new algorithm is presented that allows for the determination of bulk liquid and vapor densities from a two-phase Molecular Dynamics (2phiMD) simulation. This new method does not use any arbitrary cutoffs for phase definitions; rather it uses single-phase simulations as a self-consistency check. The method does not use any spatial bins for generating histograms of local properties, thereby avoiding the statistical issues associated with bins. Finally, it allows one to approach very close to the critical point. The new method utilizes Voronoi tessellations to determine the molecular volume of every point at every instance in a molecular dynamics simulation. Since the molecular volume is calculated throughout the simulation, statistical parameters such as the average molecular volume and average molecular variance are easy to obtain. To define the phases, the normalized variance of the molecular volume from 1phiMD and 2phiMD is used as a self-consistency check. The new method gives new insight into the nature of the near-subcritical fluid. The critical properties from this analysis are T(c) = 1.293 and rho(c) = 0.313. Direct simulation of the two-phase system was performed up to a temperature of 1.292. The results show excellent agreement to experimental results and Gibbs Ensemble Monte Carlo for coexisting densities. We see that well below the critical temperature, some particles are neither liquid nor vapor. These interfacial particles are primarily, but not exclusively, concentrated at the bulk interface. However, as we approach the critical point, some particles are considered both liquid and vapor. These interfacial particles are distributed through the system.     

Site-specific incorporation of a (19)F-amino acid into proteins as an NMR probe for characterizing protein structure and reactivity

19F NMR is a powerful tool for monitoring protein conformational changes and interactions; however, the inability to site-specifically introduce fluorine labels into proteins of biological interest severely limits its applicability. Using methods for genetically directing incorporation of unnatural amino acids, we have inserted trifluoromethyl-l-phenylalanine (tfm-Phe) into proteins in vivo at TAG nonsense codons with high translational efficiency and fidelity. The binding of substrates, inhibitors, and cofactors, as well as reactions in enzymes, were studied by selective introduction of tfm-Phe and subsequent monitoring of the 19F NMR chemical shifts. Subtle protein conformational changes were detected near the active site and at long distances (25 Angstrom). 19F signal sensitivity and resolution was also sufficient to differentiate protein environments in vivo. Since there has been interest in using 19F-labeled proteins in solid-state membrane protein studies, folding studies, and in vivo studies, this general method for genetically incorporating a 19F-label into proteins of any size in Escherichia coli should have broad application beyond that of monitoring protein conformational changes.     

The effects of dissolved halide anions on hydrogen bonding in liquid water

It is widely believed that the addition of salts to water engenders structural changes in the hydrogen-bond network well beyond the adjacent shell of solvating molecules. Classification of many ions as "structure makers" and "structure breakers" has been based in part on corresponding changes in the vibrational spectra (Raman and IR). Here we show that changes in O-H vibrational spectra induced by the alkali halides in liquid water result instead from the actions of ions' electric fields on adjacent water molecules. Computer simulations that accurately reproduce our experimental measurements suggest that the statistics of hydrogen-bond strengths are only weakly modified beyond this first solvation shell.     

Examination of analyte partitioning to monocationic and dicationic imidazolium-based ionic liquid aggregates using solid-phase microextraction-gas chromatography

Solid phase microextraction coupled to gas chromatography has been used to study the partitioning behaviour of several analytes to four monocationic and two dicationic imidazolium-based ionic liquid (IL) aggregates. The 14 different analytes studied consisted of aliphatic hydrocarbons, polycyclic aromatic hydrocarbons, phenols, and esters. The obtained partition coefficients for analytes that exhibited partitioning into the IL-aggregates ranged from 30 to 5200. Hydrophobic analytes (with octanol-water partition coefficients higher than 300) appear to be preferably extracted over more polar analytes revealing the possibility of carrying out selective extractions using these aggregate systems. Monocationic IL-aggregates generally exhibited higher partition coefficients compared to analogous dicationic ILs. The micellar shape of the IL-aggregates also influences the extent of analyte partitioning.     

Isovitexin-2'-O-beta-[6-O-E-p-coumaroylglucopyranoside] from UV-B irradiated leaves of rice, Oryza sativa L. inhibits fertility of Helicoverpa armigera

UV-B irradiated rice leaves (Oryza sativa L.) contained four closely related flavonoids, with either an isoorientin or isovitexin aglycone. These flavonoids have previously been purified and characterized, and were added to artificial diets of the African bollworm (Helicoverpa armigera Hübner) at 0.1x concentration found in irradiated rice leaves. Consumption of different diets had relatively small effects on laval, pupal and adult duration, weight and survival, indicating the insects lived near normal life cycles on all diets. However, one of the compounds, flavonoid IIa, isovitexin-2'-O-beta-[6-O-E-p-coumaroylglucopyranoside], dramatically reduced the number of fertile eggs laid to 7% of control insects (P<0.001) when added to insect diets at 18 nmol gFW(-1) (14 ppm). A similar antifertility effect was observed when only the male partner consumed diet containing flavonoid IIa, indicating that the reduced fertility may be male specific. In contrast, the fecundity and fertility of insects eating diets containing the closely related flavonoids, isoorientin-2'-O-beta-[6-O-E-p-coumaroylglucopyranoside] or isoorientin-2'-O-beta-[6-O-E-p-feruloylglucopyranoside], were not significantly different to control diets.     

The origin of the redshift in Brillouin spectra of silica films containing tin nanoparticles

The abrupt change of velocity in surface acoustic waves in thin films of amorphous SiO_x containing nanometre scale -Sn crystals is shown to be directly associated with the size-dependent melting of the nanoparticles, confirming preliminary experiments. High resolution thin film powder diffraction using synchrotron radiation shows that the abrupt redshift in the Brillouin spectra satellites occurs at the same temperature as the melting of the nanoparticles, evident for the loss of the Bragg peaks. Effective medium theory is used to explain the origin of the anomaly. A central peak in the Brillouin spectrum, the intensity of which shows a maximum at the melting temperature, can be interpreted in terms of overdamped fluctuations in the dielectric function. The melting temperature as a function of particle size is in agreement with theoretical predictions. No evidence for strain could be found on the X-ray diffraction profiles; the a- and c-axis thermal expansion coefficients are the same as those in bulk tin. Received 30 March 2000 and Received in final form 24 July 2000