A fundamental parameters approach including scattered radiation for mono-energetically excited samples in energy-dispersive x-ray fluorescence spectrometry

A fundamental parameters approach that incorporates data on scattered radiation is shown to be a useful tool for the determination of trace elements in light matrices. For thick, transparent samples, a computer program is developed that uses differetial intensity equations adjusted for decreasing geometric efficiency. Various organic and biological reference materials are analysed to assess the reliability of the procedure. A nested design for analysis of variance shows that major errors can originate from the deconvolution process and sample inhomogeneities in many cases. For loose powders, great care must be exercised to avoid errors caused by scatter from the sample cups. The program is also useful for alloys.     

Understanding binding affinity: a combined isothermal titration calorimetry/molecular dynamics study of the binding of a series of hydrophobically modified benzamidinium chloride inhibitors to trypsin

The binding of a series of p-alkylbenzamidinium chloride inhibitors to the serine proteinase trypsin over a range of temperatures has been studied using isothermal titration (micro)calorimetry and molecular dynamics simulation techniques. The inhibitors have small structural variations at the para position of the benzamidinium ion. They show small differences in relative binding affinity but large compensating differences in enthalpy and entropy. Binding affinity decreases with increased branching at the first carbon but increases with increasing the length of a linear alkyl substituent, suggesting that steric hindrance and hydrophobic interactions play dominant roles in binding. Structural analysis showed that the backbone of the enzyme was unaffected by the change of the para substituent. In addition, binding does not correlate strongly with octanol/water partition data. To further characterize this system, the change in the heat capacity on binding, the change in solvent-accessible surface area on binding, the effect of inhibitor binding on the hydration of the active site, the pK(a) of His57, and interactions within the catalytic triad have been investigated. Although the changes in inhibitor structure are small, it is demonstrated that simple concepts such as steric hindrance, hydrophobicity, and buried surface area are insufficient to explain the binding data. Other factors, such as access to the binding site and the cost of dehydration of the active site, are of equal or greater importance.     

Sampling and convergence in free energy calculations of protein-ligand interactions: The binding of triphenoxypyridine derivatives to factor Xa and trypsin

The binding of a set of 10 triphenoxypyridine derivatives to two serine proteases, factor Xa and trypsin, has been used to analyze factors related to sampling and convergence in free energy calculations based on molecular dynamics simulation techniques. The inhibitors investigated were initially proposed as part of the Critical Assessment of Techniques for Free Energy Evaluation (CATFEE) project for which no experimental results nor any assessment of the predictions submitted by various groups have ever been published. The inhibitors studied represent a severe challenge for explicit free energy calculations. The mutations from one compound to another involve up to 19 atoms, the creation and annihilation of net charge and several alternate binding modes. Nevertheless, we demonstrate that it is possible to obtain highly converged results (+/- 5-10 kJ/mol) even for such complex multi-atom mutations by simulating on a nanosecond time scale. This is achieved by using soft-core potentials to facilitate the creation and deletion of atoms and by a careful choice of mutation pathway. The results show that given modest computational resources, explicit free energy calculations can be successfully applied to realistic problems in drug design.     

Simultaneous Determination of Trichothecenes A, B, and D in Maize Food Products by LC–MS–MS

Two new methods of analysis, based on liquid chromatography–tandem mass spectrometry, for simultaneous determination of trichothecenes A, B, and D in maize flour and oil have been developed and validated in accordance with European Commission decision 2002/657/EC (recovery, CCα, CCβ, and precision). The trichothecenes were extracted from maize flour by matrix solid-phase dispersion, with recoveries ≥79%, and from maize oil by liquid–liquid extraction, with recoveries ≥78%. Limits of quantitation ranged between 0.03 and 50 μg kg^?1, depending on the electrospray response to each analyte and on the matrix. Monitoring of flour and oil samples with this HPLC–MS–MS method revealed the presence of deoxynivalenol, T-2 toxin, and diacetoxyscirpenol at very low concentrations.     

Voltammetric Detection of Free Sulfhydryl Compounds in Food Flours

Cyclic voltammetry has been applied to the rapid measurement of free SH‐compounds in food flours samples. The protocol is based on the electrochemical adaptation of the Ellman's test, where the DTNB reacts with SH‐compounds present in food flour extracts, resulting in the equimolar production of the electroactive compound TNBA which, in turns, reacts with phenylendiamine. The reaction is found to provide an analytical signal from which to quantify indirectly free sulfhydryl compounds in flours extracts.     

The role of different soil sample digestion methods on trace elements analysis: a comparison of ICP-MS and INAA measurement results

The measurement of trace-element concentration in soil, sediment and waste, is generally a combination of a digestion procedure for dissolution of elements and a subsequent measurement of the dissolved elements. “Partial” and “total” digestion methods can be used in environmental monitoring activities. To compare measurement results obtained by different methods, it is crucial to determine and to maintain control of the bias of the results obtained by these methods. In this paper, ICP-MS results obtained after matrix digestion with modified aqua regia (HCl+HNO₃+H₂O₂) method and two “total” digestion methods (microwave aqua regia+HF and HNO₃+HF) are compared with those obtained by instrumental neutron activation analysis, a non-destructive analytical method for the determination of the total mass concentrations of inorganic components in environmental matrices. The comparison was carried out on eight agricultural soil samples collected in one test area and measured by k₀-INAA and ICP-MS to determine As, Co, Cr, Sb and Zn mass concentration. The bias of results for As, Cd, Co, Cr, Cu, Ni, Pb, Sb and Zn of the three digestion methods were assessed using selected measurement standards. This paper highlights that the digestion procedure is an integral part of the measurement and can affect the measurement result in environmental analysis.     

Combined analysis by GC (RI), GC-MS and 13C NMR of the supercritical fluid extract of Abies alba twigs

Two samples (leaves and twigs) of Abies alba Miller from Corsica were extracted using supercritical CO2 and their chemical compositions were compared with those of the essential oils obtained from the same batch of plant material. In total 45 components were identified using combined analysis by GC (RI), GC-MS and 13C NMR. It was observed that the contents of monoterpenes (mainly represented by limonene, alpha-pinene and camphene) were significantly lower in the supercritical fluid extract (SFE) than in the essential oil (EO). Conversely, the proportions of sesquiterpenes were much higher in CO2 extracts than in essential oils (around 30% vs 4%). Cis-abienol, a diterpene alcohol, was identified only in SFE, and the proportions of this constituent (7.5% and 17.3%) were determined using quantitative 13C NMR since it was under estimated using the standard conditions of GC.     

Novel Copolymers of Styrene. 16. Halogen Ring-Disubstituted Methyl 2-Cyano-3-Phenyl-2-Propenoates

Electrophilic trisubstituted ethylenes, ring-disubstituted methyl 2-cyano-3-phenyl-2-propenoates, RPhCH?C(CN)CO₂CH₃, where R is 2,5-dichloro, 3,5-dichloro, 2,3-difluoro, 3-chloro-2-fluoro, 3-chloro-4-fluoro, 4-chloro-3-fluoro, 2-chloro-5-nitro, and 2-chloro-6-nitro were prepared and copolymerized with styrene. The monomers were synthesized by the piperidine catalyzed Knoevenagel condensation of ring-disubstituted benzaldehydes and methyl cyanoacetate, and characterized by CHN analysis, IR, ¹H and ¹³C-NMR. All the ethylenes were copolymerized with styrene (M₁) in solution with radical initiation (ABCN) at 70°C. The compositions of the copolymers were calculated from nitrogen analysis and the structures were analyzed by IR, ¹H and ¹³C-NMR. The order of relative reactivity (1/r₁) for the monomers is 4-Cl-3-F (4.87) > 2,3-F₂ (4.49) > 3-Cl-4-F (3.50) > 3-Cl-2-F (2.96) > 2-Cl-5-NO₂ (2.02) > 2,5-Cl₂ (1.54) > 2-Cl-6-NO₂ (1.00) > 3,5-Cl₂ (0.41). Relatively high T_g of the copolymers in comparison with that of polystyrene indicates a decrease in chain mobility of the copolymer due to the high dipolar character of the trisubstituted ethylene monomer unit. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 200–500ºC range with residue (1.5–34.5% wt), which then decomposed in the 500-800ºC range.