The reaction of hydroxyethyl radicals with O2: A theoretical analysis and experimental product study

Reactions of α-hydroxyethyl (CH₃CHOH) and β-hydroxyethyl (CH₂CH₂OH) radicals with oxygen are of key importance in ethanol combustion. High-level ab initio calculations of the potential energy surfaces of these two reactions were coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 250-1000 K. The α-hydroxyethyl + O₂ reaction is controlled by the barrierless entrance channel and shows negligible pressure dependence; in contrast, the reaction of the β isomer displays pronounced pressure dependence. The high pressure limit rate coefficients of both reactions are about the same at the temperatures investigated. Products of the reactions were monitored experimentally at 4 Torr and 300-600 K using tunable synchrotron photoionization mass spectrometry. Hydroxyethyl radicals were produced from the reaction of ethanol with chlorine atoms and the β isomer was also selectively produced by the addition reaction C₂H₄ + OH → CH₂CH₂OH. Formaldehyde, acetaldehyde, vinyl alcohol and H₂O₂ products were detected, in qualitative agreement with the theoretical predictions.     

Determination of nitrogen content of methylamine and dimethylamine compounds by an automatic rapid nitrogen analyser

Summary It has been observed that when the Heraeus automatic nitrogen analyser is used for compounds containing anN-methyl group, values higher than theoretical are obtained, because the volume of methane arising from the methyl group is measured along with the nitrogen. This interference is found to be eliminated by increasing the temperature of the oven. When the traditional Pregl-Dumas method is used, the duration of combustion can be lengthened to eliminate this effect.

---

Zusammenfassung Bei Verwendung des automatischen Gerätes von Heraeus zur N-Bestimmung in N-Methylverbindungen werden überhöhte Werte erhalten, da sich Methan bildet, das mit Stickstoff gemeinsam gemessen wird. Diese Störung läßt sich durch Erhöhung der Ofentemperatur bescitigen. Wenn die herkömmliche Methode von Dumas-Pregl verwendet wird, kann man den gleichen Erfolg durch Verlängerung der Verbrennungszeit erzielen.

---

---

Work performed under the auspices of the U. S. Department of Energy under contract No. W-7405-Eng-48.     

A Situated and Sociocultural Perspective on Bilingual Mathematics Learners

My aim in this article is to explore 3 perspectives on bilingual mathematics learners and to consider how a situated and sociocultural perspective can inform work in this area. The 1st perspective focuses on acquisition of vocabulary, the 2nd focuses on the construction of multiple meanings across registers, and the 3rd focuses on participation in mathematical practices. The 3rd perspective is based on sociocultural and situated views of both language and mathematics learning. In 2 mathematical discussions, I illustrate how a situated and sociocultural perspective can complicate our understanding of bilingual mathematics learners and expand our view of what counts as competence in mathematical communication.     

Direct arylation of tetrazolo[1,5-a]pyridine and its benzenologues

Diphenyliodonium fluoroborate proved to be a suitable reagent for direct arylation of bridge head nitrogen containing fused heteroaromatic systems. By this one step method, 1-aryl-tetrazolo[1,5-a]pyridinium salt (3) can be obtained. Arylation of related benzenologues gave not only 1-aryl but also 2-aryltetrazolium salts.

---

Direkte Arylierung von Teterazolo[1,5-a]pyridin und dessen Benzologen (Kurze Mitteilung)

Zusammenfassung Kondensierte Heteroaromaten mit Brückenkopf-Stickstoff können direkt mit Diphenyl-Jodonium Fluoroborat aryliert werden. In dieser Weise wird 1-Aryltetrazolo[1,5-a]pyridinium Salz (3) in einer Reaktionsstufe synthetisiert. Arylierung der verwandten Benzologen führten nicht nur zu 1-Aryl, sondern auch zu 2-Aryltetrazolium Salzen.

     

Focal length variability and protein leakage as tools for measuring photooxidative damage to the lens

Hypericin is the ingredient used to standardize the popular over-the-counter antidepressant medication St. John's Wort. Because hypericin readily produces singlet oxygen and other excited state intermediates, it is a very efficient phototoxic agent in the eye that can potentially induce the development of the cataract photooxidative mechanism. Hypericin absorbs in the UV and visible ranges, binds to the lens crystallins (alpha, beta and gamma) and damages these proteins through a photooxidative mechanism. Effects were measured previously using fluorescence, UV and mass spectrometry. We report here two additional methods to monitor lens damage: (1) measuring focal length variability using a ScanTox instrument and (2) measuring protein leakage from the damaged lens. Because nonenzymic glycation results in free radical production, we chose to use elevated glucose concentrations as a convenient model for studying oxidative stress. To compare and contrast photooxidative damage against oxidative damage to the lens, we also measured the focal length variability and protein leakage induced by the presence of elevated glucose concentrations. We found that the total accumulated protein leakage was positively correlated (r = 0.9) with variability in focal length. Lenses treated with hypericin and irradiated with UVB had an increase in focal length variability as compared with the lenses that were only UVB-irradiated. Lenses without UVB irradiation had much lower focal length variability than irradiated lenses. For non-hypericin-treated lenses, UVB-irradiated lenses had a larger variability (4.58 mm) than the unirradiated lenses (1.78 mm). The lenses incubated in elevated glucose concentrations had a focal length variability (3.23 mm) equivalent to that of the unirradiated hypericin-treated lenses (3.54 mm). We conclude that photooxidative damage by hypericin results in changes in the optical properties of the lens, protein leakage and finally cataract formation. In contrast to this, high concentrations of glucose induced protein leakage but not changes in optical properties or the opacity associated with a cataract. This work provides further evidence that people should protect their eyes from intense sunlight when taking St. John's Wort.     

Preparation and Tungsten-183 NMR Characterization of [alpha-1-P(2)W(17)O(61)](10)(-), [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-), and [alpha-2-Zn(H(2)O)P(2)W(17)O(61)](8)(-)

The preparation of the alpha-1 and alpha-2 isomers of the Wells-Dawson 17 tungsto derivatives by standard methods is accompanied by a significant proportion of the other isomer present as an impurity. In this study, the alpha-1 and alpha-2 isomers of [Zn(H(2)O)P(2)W(17)O(61)](8)(-) have been prepared in >98% purity by reacting isomerically pure K(9)Li[alpha-1-P(2)W(17)O(61)] and K(10)[alpha-2-P(2)W(17)O(61)], respectively, with ZnCl(2), while rigorously controlling the pH at 4.7. The molecules were isolated as potassium salts. For (183)W NMR and (31)P NMR characterization, both molecules were ion exchanged by cation-exchange chromatography, maintaining the pH at 4.7, to obtain the lithium salts. Removal of water and isolation of a solid sample of [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) was achieved by lyophilization at -40 degrees C. The chemical shift data from (31)P and (183)W NMR spectroscopy of the isolated [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) and [alpha-2-Zn(H(2)O)P(2)W(17)O(61)](8)(-) isomers are consistent with a mixture of the alpha-1 and alpha-2 isomers reported previously;(1) the molecules have the expected C(1) and C(s)() symmetry, respectively. The [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) isomer is stable in the pH range of 4.6-6 at temperatures <35 degrees C. Using the same ion exchange and lyophilization techniques, the lacunary [alpha-1-P(2)W(17)O(61)](10)(-) isomer was isolated as the lithium salt; characterization by (183)W NMR spectroscopy confirms the C(1) symmetry.     

Homonuclear Selective One-Dimensional Gradient NMR Experiments in the Structure Determination of Natural Diterpene Derivatives

Summary.  The solution structure of two natural diterpene derivatives, the secondary metabolites esulatin-A and esulatin-B of Euphorbia esula, was investigated by homonuclear NMR experiments. Since the spectral dispersion of the ¹H NMR spectra at 500 MHz was sufficient to separate several skeletal protons of the title compounds, they were selectively excited with a double pulsed field gradient spin-echo (DPFGSE) sequence using 180°Gaussian pulses sandwiched between sine shaped gradients. With the use of selective excitation, scalar as well as dipolar interactions of the selected spins were monitored through one-dimensional (1D) COSY, TOCSY, and NOESY experiments. The chemical shifts of the coupling partners could be accurately extracted from the 1D COSY and TOCSY spectra recorded with high digital resolution. The selective TOCSY experiment provided an excellent opportunity to identify spins belonging to the same scalarly coupled spin system. The solution state conformation was investigated by selective gradient enhanced NOESY experiments. Proton–proton distances were evaluated from the cross-relaxation rates obtained from a quantitative analysis of the NOESY spectra recorded with different mixing times. The NMR derived distances were compared to the results of solid state X-ray diffraction measurements. Corresponding author. E-mail: pforgo@chem.u-szeged.hu Received November 21, 2001. Accepted (revised) January 9, 2002     

Principles of RNA base pairing: structures and energies of the trans Watson-Crick/sugar edge base pairs

Due to the presence of the 2'-OH hydroxyl group of ribose, RNA molecules utilize an astonishing variability of base pairing patterns to build up their structures and perform the biological functions. Many of the key RNA base pairing families have no counterparts in DNA. In this study, the trans Watson-Crick/sugar edge (trans WC/SE) RNA base pair family has been characterized using quantum chemical and molecular mechanics calculations. Gas-phase optimized geometries from density functional theory (DFT) calculations and RIMP2 interaction energies are reported for the 10 crystallographically identified trans WC/SE base pairing patterns. Further, stable structures are predicted for all of the remaining six possible members of this family not seen in RNAs so far. Among these novel six base pairs, the computations substantially refine two structures suggested earlier based on simple isosteric considerations. For two additional trans WC/SE base pairs predicted in this study, no arrangement was suggested before. Thus, our study brings a complete set of trans WC/SE base pairing patterns. The present results are also contrasted with calculations reported recently for the cis WC/SE base pair family. The computed base pair sizes are in sound correlation with the X-ray data for all WC/SE pairing patterns including both their cis and trans isomers. This confirms that the isostericity of RNA base pairs, which is one of the key factors determining the RNA sequence conservation patterns, originates in the properties of the isolated base pairs. In contrast to the cis structures, however, the isosteric subgroups of the trans WC/SE family differ not only in their H-bonding patterns and steric dimensions but also in the intrinsic strength of the intermolecular interactions. The distribution of the total interaction energy over the sugar-base and base-base contributions is controlled by the cis-trans isomerism.     

The Gauss-Bonnet theorem and Crofton-type formulas in complex space forms

We compute the measure with multiplicity of the set of complex planes intersecting a compact domain in any complex space form. The result is given in terms of the so-called hermitian intrinsic volumes. Moreover, we obtain two different versions for the Gauss-Bonnet-Chern formula in complex space forms. One of them gives the Gauss curvature integral in terms of the Euler characteristic, and some hermitian intrinsic volumes. The other one, which is shorter, involves the measure of complex hyperplanes meeting the domain. As a tool, we obtain variation formulas in integral geometry of complex space forms.