Comparison of polydimethyl siloxane stationary phases with 5% and 50% phenyl substitution for the separation of organophosphorus and organonitrogen pesticides under optimized gas chromatographic conditions

The CGC analysis of 25 organophosphorus and organonitrogen pesticides was optimized on two different stationary phases, namely a 5% diphenyl dimethyl silicone (SE-54 type) and a 50% diphenyl dimethyl silicone (OV-17 type) by selecting the best temperature program conditions, using computer simulation.     

Evaluation of measurement uncertainty for analytical procedures using a linear calibration function

In the EURACHEM/CITAC draft ”Quantifying uncertainty in analytical measurement” estimations of measurement uncertainty in analytical results for linear calibration are given. In this work these estimations are compared, i.e. the uncertainty deduced from repeated observations of the sample vs. the uncertainty deduced from the standard residual deviation of the regression. As a result of this study it is shown that an uncertainty estimation based on repeated observations can give more realistic values if the condition of variance homogeneity is not correctly fulfilled in the calibration range. The complete calculation of measurement uncertainty including assessment of trueness is represented by an example concerning the determination of zinc in sediment samples using ICP-atomic emission spectrometry. Received: 9 February 2002 Accepted: 17 April 2002     

(+)- and (-)-2-aminocyclobutane-1-carboxylic acids and their incorporation into highly rigid beta-peptides: stereoselective synthesis and a structural study

[Chemical reaction: See text] Several derivatives of (+)- and (-)-2-aminocyclobutane-1-carboxylic acid, 1, have been prepared through enantiodivergent synthetic sequences. The stereoselective synthesis of free amino acid (+)-1 has been achieved, and this product has been fully characterized for the first time. Stereocontrolled alternative synthetic methodologies have been developed for the preparation of bis(cyclobutane) beta-dipeptides in high yields. Among them, enantio and diastereomers have been synthesized. beta,beta- and beta,delta-Dimers resulting from the coupling of a cyclobutane residue and a linear amino acid have also been prepared. The ability of the cyclobutane ring as a structure-promoting unit both in the monomers and in the dimers has been manifested. The NMR structural study and DFT theoretical calculations evidence the formation of strong intramolecular hydrogen bonds giving rise to cis-fused [4.2.0]octane structural units that confer high rigidity on these molecules both in solution and in the gas phase. The contribution of a cis-trans conformational equilibrium derived from the rotation around the carbamate N-C(O) bond has also been observed, the trans form being the major conformer. In the solid state, this equilibrium does not exist, and moreover, intermolecular hydrogen bonds are present.     

Chemosensors for the marine toxin saxitoxin

Eleven anthracylmethyl crown ethers have been synthesized and evaluated as fluorescence sensors for the marine toxin saxitoxin. Fluorescence enhancement data are consistent with a 1:1 binding complex for all crowns. The binding constants are in the range of 10(4) M(-)(1) in ammonium phosphate buffer (pH 7.1) in 80% ethanol solvent. Selectivity for sensing saxitoxin versus several organic analytes has been demonstrated for the first time. Possible modes of binding are presented, and relevance to saxitoxin monitoring programs are discussed.     

The fluoride ion-selective electrode in flow injection analysis: Part 1. General Methodology

A simple flow-injection system for determination of traces of fluoride by means of the fluoride-selective electrode is presented. A comparison of several flow-cell arrangements confirmed the advantages of a well-jet design. Systematic investigations of the parameters affecting response times (i.e., polishing procedure, flow rate, carrier composition) established the optimal experimental conditions for measurements down to 1 μg l^?1 fluoride. Calibration plots in the lower μg l^?1 range were neither Nernstein nor linear, but good precision (0.5–5%) was obtained even when the potential differences for concentration steps of one decade were as small as 3 mV.     

Applications of capillary electrochromatography

Applications performed by capillary electrochromatography (CEC) in all its modes, namely packed column CEC (packed-CEC), open tubular CEC (OT-CEC) and pressure-assisted CEC (pseudo-CEC), and published by June 1999 are reviewed. The review is divided into (i) separation of neutral, acidic and basic analytes with the main goal of evaluating column and system performance, (ii) separation according to field of application and/or chemical class, and (iii) separation of chiral analytes.     

Measuring intracellular calcium fluxes in high throughput mode

The measurement of intracellular calcium fluxes in real time is widely applied within the pharmaceutical industry to measure the activation of G-protein coupled receptors (GPCRhyp;s), either for pharmacological characterisation or to screen for new surrogate ligands. Initially restricted to G(q) coupled GPCRs, the introduction of promiscuous and chimeric G-proteins has further widened the application of these assays. The development of new calcium sensitive dyes and assays has provided sensitive, homogeneous assays which can be readily applied to high throughput screening (HTS). In this paper we describe the full automation of this assay type using a fluorometric imaging plate reader (FLIPR ) integrated into a Beckman/Sagian system to establish a simple robotic system that is well suited for the current medium throughput screening in this area of lead discovery. Using a recently completed HTS we discuss important determinants for FLIPR based screening, highlight some limitations of the current approach, and look at the requirements for future automated systems capable of keeping up with expanding compound files.     

Phage-display selection of a human single-chain fv antibody highly specific for melanoma and breast cancer cells using a chemoenzymatically synthesized G(M3)-carbohydrate antigen

Overexpression of the cell-surface glycosphingolipid G(M3) is associated with a number of different cancers, including those of the skin, colon, breast, and lung. Antibodies against the G(M3) epitope have potential application as therapeutic agents in the treatment of these cancers. We describe the chemoenzymatic synthesis of two G(M3)-derived reagents and their use in the panning of a phage-displayed human single-chain Fv (scFv) antibody library derived from the blood of cancer patients. Three scFv-phage clones, GM3A6, GM3A8, and GM3A15, were selected for recombinant expression and were characterized using BIAcore and flow cytometry. BIAcore measurements using the purified, soluble scFvs yielded dissociation constants (K(d)) ranging from 4.2 x 10(-7) to 2.1 x 10(-5) M. Flow cytometry was used to evaluate the ability of each scFv to discriminate between normal human cells (human dermal fibroblast, HDFa), melanoma cells (HMV-1, M21, and C-8161), and breast cancer cells (BCM-1, BCM-2, and BMS). GM3A6 displayed cross-reactivity with normal cells, as well as tumor cells, and GM3A15 possessed little or no binding activity toward any of the cell lines tested. However, GM3A8 bound to five of the six tumor cell lines and showed no measurable reactivity against the HDFa cells. Hence, we have demonstrated that a synthetic G(M3) panning reagent can be used to isolate a fully human scFv that is highly specific for native G(M3) on the surface of tumor cells. The result is a significant step toward effective immunotherapies for the treatment of cancer.     

Three‐Component Reactions with (S)‐Methyl Pyroglutamate: An Efficient Way to Diversely Substituted Asymmetric Amidocyclohexenes

Chiral N‐dienyl lactams are crucial building blocks for the synthesis of complex organic compounds. However, their generation is rather challenging. This paper reports the novel one‐pot reaction of (S)‐methyl pyroglutamate as the a mide component with different a ldehydes and d ienophiles (AAD reaction) to give novel chiral 1‐amido‐2‐cyclohexenes. The corresponding N‐dienyl lactams generated in situ undergo subsequent Diels–Alder reactions in good yield and diastereoselectivity. The scope and limitations of the three‐component protocol were investigated. X‐ray and NMR spectroscopic analysis of the products as well as DFT calculations of the intermediates were also performed to explain the observed stereoselectivity and structural features.     

Validation steps for traceability of linear calibrated chemical measurements

Based on the new draft of the EURACHEM/CITAC Guide Traceability in Chemical Measurement, this publication describes how traceability can be achieved for chemical measurements using a linear calibration function. Traceability can be accomplished without larger expenditure, if the measurement is calibrated on the basis of appropriate reference standards and the linear regression employed is selected and validated statistically in a suitable form. The determination of nickel in aqua regia eluates of sediment samples, employed for an ICP-OES measurement, is used as a practical illustration of this approach.