GC-NPD investigation of the recovery of organonitrogen and organophosphorus pesticides from apple samples: The effect of the extraction solvent

Summary A gas chromatographic method employing a capillary column and a selective nitrogen/phosphorus detector (NPD) has been developed for the determination of organophosphorus (OP) and organonitrogen (NP) pesticides in horticultural samples (apples). The separation of sixteen pesticides and the internal standard was performed in thirteen minutes. The analytical characteristics of the method, including linear response ranges, detection limits, and reproducibility, have been studied using a 11 mixture of ethyl acetate and xylene as extraction solvent. The possibility of mutual interference between pesticides has also been studied. A procedure for the quantitative extraction of the sixteen pesticides from apple samples has also been developed; for fifteen of the pesticides recoveries >85% were obtained after 90 minutes extraction. The effect of different solvents both on recovery and on the sensitivity of the subsequent chromatography were also investigated. It was found that the sensitivity required must be considered when the solvent for sample treatment is selected.     

Unusual behavior of pyrazolo[1,2-a]pyrazoles in fast atom bombardment (fab) and frit-fast atom bombardment (Frit-FAB) mass spectrometry. Influence of the matrix

The nature of the matrix used in Fast Atom Bombardment (FAB) mass spectrometry analyses of pyrazolo[1,2-a]pyrazoles was found to influence significantly their positive and negative ions mass spectra. Indeed the use of glycerol provided an abundant ion corresponding to the protonated molecule (M+H)⁺ whereas the meta-nitrobenzyl alcohol favored the formation of the radical ion M^+°. Such results which are in accordance with the oxidoreduction properties of the matrices studied were also established in Frit-FAB mass spectrometry analyses of pyrazolo[1,2-a]pyrazoles.     

Computer simulation study of the global phase behavior of linear rigid Lennard-Jones chain molecules: comparison with flexible models

The global phase behavior (i.e., vapor-liquid and fluid-solid equilibria) of rigid linear Lennard-Jones (LJ) chain molecules is studied. The phase diagrams for three-center and five-center rigid model molecules are obtained by computer simulation. The segment-segment bond lengths are L = sigma, so that models of tangent monomers are considered in this study. The vapor-liquid equilibrium conditions are obtained using the Gibbs ensemble Monte Carlo method and by performing isobaric-isothermal NPT calculations at zero pressure. The phase envelopes and critical conditions are compared with those of flexible LJ molecules of tangent segments. An increase in the critical temperature of linear rigid chains with respect to their flexible counterparts is observed. In the limit of infinitely long chains the critical temperature of linear rigid LJ chains of tangent segments seems to be higher than that of flexible LJ chains. The solid-fluid equilibrium is obtained by Gibbs-Duhem integration, and by performing NPT simulations at zero pressure. A stabilization of the solid phase, an increase in the triple-point temperature, and a widening of the transition region are observed for linear rigid chains when compared to flexible chains with the same number of segments. The triple-point temperature of linear rigid LJ chains increases dramatically with chain length. The results of this work suggest that the fluid-vapor transition could be metastable with respect to the fluid-solid transition for chains with more than six LJ monomer units.     

Organometallic gold(III) compounds as catalysts for the addition of water and methanol to terminal alkynes

Different inorganic and organometallic gold(III) and gold(I) complexes have been tested in the addition of water and methanol to terminal alkynes. Anionic and neutral organometallic gold(III) compounds can efficiently mediate these reactions in neutral media in refluxing methanol. The compounds are added in catalytic amounts (1.6-4.5 mol % with respect to the alkyne). Thus, compounds of the general formula Q[AuRCl(3)], Q[AuR(2)Cl(2)], [AuRCl(2)](2), and [AuR(2)Cl](2) (Q = BzPPh(3)(+), PPN: N(PPh(3))(2)(+) or N(Bu)(4)(+); R = C(6)F(5) or 2,4,6-(CH(3))(3)C(6)H(2)) seem to behave as Lewis acids in nucleophilic additions to triple bonds. Some intermediates could be detected in the stoichiometric reaction between [Au(C(6)F(5))(2)Cl](2) and phenylacetylene that was followed by variable temperature (1)H, (19)F[(1)H], COSY (19)F[(1)H]-(19)F[(1)H], and (2)H[(1)H] NMR experiments. Compound [Au(C(6)F(5))(2)Cl](2) is also able to catalyze the hydration of phenylacetylene at room temperature. A plausible mechanism for the hydration reaction has been proposed.     

Determination of zinc (II) in lubricant oils by stripping chronopotentiometry

A method for the determination of zinc (II) in lubricant oils by stripping chronopotentiometry is described. The only necessary sample pretreatment was the extraction of zinc (II) from the corresponding alkyl derivatives by hot concentrated hydrochloric acid in a suitable extractor. The metal ions were concentrated as the corresponding metals on a glassy carbon working electrode and then stripped by a suitable oxidant. Quantitative analysis was carried out by the method of standard additions; a good linearity was obtained in the range of concentrations examined. Recoveries of 94% were obtained from a lubricant oil spiked at different levels. The detection limit was 0.02 mg g(-1) and the coefficient of variation (mean of nine determinations) was 5.2%. Results obtained on commercial lubricant oils were not significantly different from those obtained by atomic absorption spectrometry.     

Axial and equatorial hydrogen-bond conformers and ring-puckering motion in the trimethylene sulfide.hydrogen fluoride complex

Axial and equatorial hydrogen-bond conformers of the trimethylene sulfide.hydrogen fluoride complex have been generated and characterized in the supersonic jet of a molecular beam Fourier transform microwave experiment. It is shown that the ring-puckering large amplitude motion of trimethylene sulfide is responsible for the observed conformers. The axial conformer has been found to be the most stable and has been proved by the existence of relaxation of the high-energy equatorial form to it. This conformational preference has been explained in the context of a delicate balance between primary and secondary hydrogen bonds. The interconversion between both conformers takes place through the ring-puckering motion of the heterocycle, provided that the barrier to the ring inversion remains low after complexation, as all experimental findings indicate. The structural parameters of the trimethylene sulfide and the hydrogen bond have been derived from the analysis of the rotational spectra of the C(3)H(6) (32)S.HF, C(3)H(6) (34)S.HF, (13)C(alpha) (12)C(2)H(6) (32)S.HF, and (13)C(beta) (12)C(2)H(6) (32)S.HF isotopomers. Both conformers have C(s) symmetry with the hydrogen fluoride located in the molecular symmetry plane of trimethylene sulfide, which is puckered at a similar angle to that found for the bare ring.     

NMR studies of interactions between periplasmic chaperones from uropathogenic E. coli and pilicides that interfere with chaperone function and pilus assembly

Adherence of uropathogenic Escherichia coli to host tissue is mediated by pili, which are hair-like protein structures extending from the outer cell membrane of the bacterium. The chaperones FimC and PapD are key components in pilus assembly since they catalyse folding of subunits that are incorporated in type 1 and P pili, respectively, and also transport the subunits across the periplasmic space. Recently, compounds that inhibit pilus biogenesis and interfere with chaperone-subunit interactions have been discovered and termed pilicides. In this paper NMR spectroscopy was used to study the interaction of different pilicides with PapD and FimC in order to gain structural knowledge that would explain the effect that some pilicides have on pilus assembly. First relaxation-edited NMR experiments revealed that the pilicides bound to the PapD chaperone with mM affinity. Then the pilicide-chaperone interaction surface was investigated through chemical shift mapping using 15N-labelled FimC. Principal component analysis performed on the chemical shift perturbation data revealed the presence of three binding sites on the surface of FimC, which interacted with three different classes of pilicides. Analysis of structure-activity relationships suggested that pilicides reduce pilus assembly in E. coli either by binding in the cleft of the chaperone, or by influencing the orientation of the flexible F1-G1 loop, both of which are part of the surface by which the chaperone forms complexes with pilus subunits. It is suggested that binding to either of these sites interferes with folding of the pilus subunits, which occurs during formation of the chaperone-subunit complexes. In addition, pilicides that influence the F1-G1 loop also appear to reduce pilus formation by their ability to dissociate chaperone-subunit complexes.     

Essential oil composition of the leaves and stems of Meum athamanticum Jacq., from Spain

The essential oil of the leaves and stems of Meum athamanticum Jacq., has been extracted by steam distillation and analysed by gas chromatography (GC) and gas chromatography coupled to mass spectrometry (GC-MS). The monoterpene fraction was predominant while the sesquiterpene one was practically absent. The principal constituents have been identified as (E)-beta-ocimene (29.6%), gamma-terpinene (17.9%), terpinolene (17.0%) and p-cymene (9.7%). Our results show that the chemical composition of the essential oil obtained of the leaves and stems of M. athamanticum from Spain is different to that obtained from plants of Germany, Italy and France.     

Surface thermodynamics study of monolayers formed with heteroacid phospholipids of biological interest

The interaction of 1-palmitoy-2-oleoyl-sn-glycero-3-phosphocoline (POPC) and 1-palmitoy-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), two of the major components in biological membranes, were investigated using the monolayer technique at the air–water interface. The pressure–area isotherms indicate that both phospholipids are miscible through all range of compositions. POPE–POPC form stable mixtures, with a minimum for the Gibbs energy of mixing at X_POPC = 0.4. A virial equation of state was fitted to the experimental values. Positive values found for the second virial coefficient indicate repulsion between POPC and POPE. The interaction parameter was evaluated which indicated that a corresponding decrease in the repulsion occurs when POPC molar fraction is low. This effect suggests the existence of hydrogen bonds between POPE and the water beneath the interface.     

Thermodynamics and kinetics of PNA-DNA quadruplex-forming chimeras

PNA-DNA chimeras present the interesting properties of PNA, such as the high binding affinity to complementary single-strand (DNA or RNA), and the resistance to nuclease and protease degradation. At the same time, the limitations of an oligomer containing all PNA residues, such as low water solubility, self-aggregation, and low cellular uptake, are effectively overcome. Further, PNA-DNA chimeras possess interesting biological properties as antisense agents. We have explored the ability of PNA-DNA chimeric strands to assemble in quadruplex structures. The rate constant for association of the quadruplexes and their thermodynamic properties have been determined by CD spectroscopy and differential scanning calorimetry (DSC). Thermal denaturation experiments indicated higher thermal and thermodynamic stabilities for chimeric quadruplexes in comparison with the corresponding unmodified DNA quadruplex. Singular value decomposition analysis (SVD) suggests the presence of kinetically stable intermediate species in the quadruplex formation process. The experimental results have been discussed on the basis of molecular dynamic simulations. The ability of PNA-DNA chimeras to form stable quadruplex structures expands their potential utility as therapeutic agents.