Determination of a hydroxyl conformation in aqueous xylose using neutron scattering and molecular dynamics

Recently, it was shown using structural neutron diffraction with isotopic substitutions (NDIS) measurements, combined with molecular dynamics simulations, that in an aqueous solution of D-xylose the hydroxyl group on the C4 position does not significantly occupy the position trans to the H4 atom. Here, a similar combination of NDIS and MD studies is described which uses D-xylose deuterated at the C5 position to further characterize this hydroxyl conformation as being trans to the C5 atom, as predicted by constrained MD simulations, confirming the previous study.     

Processus de rotation empêchée autour de plusieurs liaisons dans les sels de (N,N-diméthylamino-1 propényl-1-ylidène-3) diméthylammonium

The structural identification of some N,N-dimethyl-aminopropenylylidene-dimethylammonium salts has been achieved by NMR and UV spectrographic studies. Several processes of hindered rotation around C? N and C? C bonds are demonstrated. Free activation enthalpies are determined and discussed in terms of effects of the substituent, anion and solvent.     

The structure of aqueous guanidinium chloride solutions

The combination of neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations to characterize the structuring in an aqueous solution of the denaturant guanidinium chloride is described. The simulations and experiments were carried out at a concentration of 3 m at room temperature, allowing for an examination of any propensity for ion association in a realistic solution environment. The simulations satisfactorily reproduced the principal features of the neutron scattering and indicate a bimodal hydration of the guanidinium ions, with the N-H groups making well-ordered hydrogen bonds in the molecular plane, but with the planar faces relatively deficient in interactions with water. The most striking feature of these solutions is the rich ion-ion ordering observed around the guanidinium ion in the simulations. The marked tendency of the guanidinium ions to stack parallel to their water-deficient surfaces indicates that the efficiency of this ion as a denaturant is due to its ability to simultaneously interact favorably with both water and hydrophobic side chains of proteins.     

DR-CALUX((R)) screening of food samples: evaluation of the quantitative approach to measure dioxin, furans and dioxin-like PCBs

European legislation laid down maximum tolerable levels of dioxin in feed and food as well as analytical method requirements. In order to face with large monitoring programs, it was foreseen in the EU strategy to integrate screening methods, using either a qualitative (screening) approach, or a quantitative approach. In this study, dioxin results obtained using the Dioxin Responsive Chemical-Activated LUciferase gene eXpression (DR-CALUX^®) cell-based assay (quantitative approach), were compared with gas chromatography–high resolution mass spectrometry (GC–HRMS) analyses data. Instead of using World Health Organization–toxic equivalent factor (WHO–TEF), the comparison was based on the assessment of relative effective potencies (REPs) for each congener of the 17 toxic 2,3,7,8-polychlorodibenzo-p-dioxins/furans (PCDD/Fs) and 12 dioxin-like polychlorobiphenyls (DL-PCBs). According to published data, DR-CALUX^®-REP evaluated here appear similar to WHO–TEF for PCDD/Fs while lower values were observed for DL-PCBs.

We analyzed two “home made” contaminated fat samples, displaying both the same WHO–toxic equivalent quantities (WHO–TEQ) concentration (12 pg WHO–TEQ g⁻¹). They were spiked with either a low or a high amount of DL-PCBs. In both cases, the DR-CALUX^® measured concentration (picogram 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) eq. g⁻¹) corresponded to the PCDD/Fs WHO–TEQ concentration only. A good agreement was nevertheless found between the DR-CALUX^® measurements and the recalculated DR-CALUX^®-TEQ contents (using DR-CALUX^®-REP instead of WHO–TEF), demonstrating that the observed response was due, in both cases, to the addition of the responses of the standards added to the fat. By contrast, in real contaminated samples (feed or cod liver samples), DR-CALUX^® measured concentrations were similar to WHO–TEQ GC–HRMS measured concentrations. But, depending on the PCDD/Fs and DL-PCBs congener content, the DR-CALUX^® measured concentrations were either lower or higher than calculated DR-CALUX^®-TEQ contents, demonstrating that possible co-extracted contaminants contributed to the CALUX response.

Owing to these divergences, the quantitative determination of dioxin-like content in food and feed using CALUX as screening method is questionable, except for samples displaying constant congener patterns, in which cases, correction factors could be applied.     

Solubilisation and binding characteristics of a recombinant beta2-adrenergic receptor expressed in the membrane of Escherichia coli for the multianalyte detection of beta-agonists and antagonists residues in food-producing animals

The on-line incorporation of cloud point extraction (CPE) to flow injection analysis (FIA) was previously based on the use of a cotton-packed column to entrap the analyte-containing surfactant aggregates after salt-induced CPE, and then the preconcentrated analyte was eluted into a separate detection cell for subsequent chemiluminescence (CL) detection (via the peroxyoxalate CL reaction). In the work, the on-line CPE/FIA technique was improved by the following: (1) sample preconcentration and CL detection were both carried out directly inside the collection column, thus avoiding the decrease in detection sensitivity due to sample dispersion and dilution, and (2) CL detection was performed through the reaction between nitrite and hydrogen peroxide, which is compatible with aqueous samples and should allow for chemical excitation to occur more efficiently inside the collection column. In addition to more effective sample preconcentration, the CL detection of the entrapped analytes directly inside the collection column, i.e., a unique heterogeneous microenvironment in which analyte-containing surfactant aggregates were embedded within the densely packed filtering material, may also contribute to the overall increase in CL intensity (e.g., a CL enhancement factor of ca. 1000). Under optimum experimental conditions, the calibration curve was found to be linear for the CL detection of bilirubin (5 to 120 μg L⁻¹), the limit of detection (S/N = 3) was 1.8 μg L⁻¹, and the R.S.D. was ca. 2.6% (n = 30) for 20 μg L⁻¹ bilirubin. Good agreements were obtained for the determination of total bilirubin in certified reference human serum samples between the present approach and an established clinical method.     

Pressurized-fluid extraction (PFE) of chlorinated paraffins from the biodegradable fraction of source-separated household waste

A method is presented in which pressurized-fluid extraction (PFE) is used for the extraction of chlorinated paraffins (CP) from the biodegradable fraction of source-separated household waste. The conditions that were optimized for high recovery in the extraction procedure were extraction time, temperature, and the use of different solvents and different sample particle sizes. Recoveries of CP from fortified household waste material were over 90%, with only few interferences when cyclohexane was used as solvent. Extraction yields from contaminated samples containing CP were further compared with recoveries obtained by use of Soxtec extraction. The results showed that PFE is a rapid, low-solvent-consuming technique, giving high yields. Received: 13 November 2000 / Revised: 2 March 2001 / Accepted: 7 March 2001     

Résonance Magnétique Dynamique du Carbone: IV—Application à l'Étude Conformationnelle des Thiourées

A study by ¹³C n.m.r. of a series of thioureas gives information about the conformational equilibrium. Hindered rotation of the tert-butyl groups is observed. Substituent effects are examined.