Chiral discrimination of phenoxypropionic acid herbicide enantiomers on teicoplanin phase: methanol dependence and eluent pH consideration

The chiral recognition mechanism for a series of d,l phenoxypropionic acid herbicides (PPAs) on a teicoplanin stationary phase was investigated in reversed phase liquid chromatography (RPLC) over a wide range of mobile phase pH and column temperature. The effect of methanol on the enantiomeric separation was studied by varying its fraction (v/v) in the mobile phase. The thermodynamic data indicated that the chiral recognition was controlled by the interaction between the anionic form of the solute and the teicoplanin phase while those with the neutral form played a minor role. In addition, it was demonstrated that the enhancement of the separation factor observed as the methanol fraction increased in the mobile phase was enthalpically controlled owing to stereoselective binding interactions. Such behavior was used to optimize the chromatographic conditions for separation of PPAs herbicides on teicoplanin.     

Microfluidic devices for culturing primary mammalian neurons at low densities

Microfluidic devices have been used to study high-density cultures of many cell types. Because cell-to-cell signaling is local, however, there exists a need to develop culture systems that sustain small numbers of neurons and enable analyses of the microenvironments. Such cultures are hard to maintain in stable form, and it is difficult to prevent cell death when using primary mammalian neurons. We demonstrate that postnatal primary hippocampal neurons from rat can be cultured at low densities within nanoliter-volume microdevices fabricated using polydimethylsiloxane (PDMS). Doing so requires an additional fabrication step, serial extractions/washes of PDMS with several solvents, which removes uncrosslinked oligomers, solvent and residues of the platinum catalyst used to cure the polymer. We found this step improves the biocompatibility of the PDMS devices significantly. Whereas neurons survive for > or = 7 days in open channel microdevices, the ability to culture neurons in closed-channel devices made of untreated, native PDMS is limited to < or = 2 days. When the closed-channel PDMS devices are extracted, biocompatibility improves allowing for reliable neuron cultures at low densities for > or = 7 days. Comparisons made to autoclaved PDMS and native, untreated PDMS reveal that the solvent-treated polymer is superior in sustaining low densities of primary neurons in culture. When neuronal affinity for local substrates is observed directly, we find that axons localize to channel corners and prefer PDMS surfaces to glass in hybrid devices. When perfusing the channels with media by gravity flow, cultured hippocampal neurons survive for > or = 11 days. Extracting PDMS improves biocompatibility of microfluidic devices and thus enables the study of differentiation of identifiable neurons and the characterization of local extracellular signals.     

Solid-phase microextraction and gas chromatography–mass spectrometry for analysis of phenols and nitrophenols in rainwater,as their <Emphasis Type="Italic">t</Emphasis>-butyldimethylsilyl derivatives

Solid-phase microextraction (SPME) coupled with gas chromatography–mass spectrometry has been used for analysis of four phenols and sixteen nitrophenols in rainwater samples. Analytes were extracted from the water in the immersion mode and derivatised for 5 min during direct desorption in the GC injector. Before desorption, 2 μL N-(t-butyldimethylsilyl)-N-methyltrifluoroacetamide (MDBSTFA) was introduced into the injector, which was maintained at 280 °C. Different conditions affecting extraction efficiency were studied, including temperature, type of microextraction fibre, and effect of pH and ionic strength. Five different fibre coatings were tested: 85-μm polyacrylate (PA), 100-μm polydimethylsiloxane (PDMS), 65-μm Carbowax–divinylbenzene (CW–DVB), 75-μm Carboxen–polydimethylsiloxane (CAR–PDMS), and 65-μm polydimethylsiloxane–divinylbenzene (PDMS–DVB). The best conditions were use of PA fibres for 40 min at ambient temperature (75 g NaCl per 100 mL, pH 3.0). MDBSTFA was used as derivatising agent because it enables analysis of phenols derivatives with high confidence in identification, because in electron-impact mode TBDMS–phenol derivatives produce the specific M–57 ion. Quantification was achieved by using 4-nitrophenol-d4, at 1 mg L⁻¹, as internal standard. Linearity was good, with correlation coefficients in the range 0.9888 (o-cresol) to 0.9987 (dinitro-o-cresol, DNOC). Detection limits varied between 0.208 and 99.3 μg L⁻¹ and quantification limits between 0.693 and 331 μg L⁻¹. Uncertainties varied between 8.7% (phenol) and 17.9% (4-methyl-2-nitrophenol). The method was successfully applied to the analysis of rainwater collected at urban and rural sites in Alsace (East of France). Because of derivatisation in the injector and the associated high temperature, the lifetime of the fibre is severely reduced.     

Rhenium fac tricarbonyl bisimine complexes: biologically useful fluorochromes for cell imaging applications

A series of lipophilic and hydrophilic fac tricarbonyl rhenium bisimine complexes have been prepared, their membrane-permeabilities explored in liposomes and their potential for application in fluorescence microscopy cell imaging demonstrated in the first application of MLCT-fluorescent rhenium complexes in cell imaging.     

A new scaffold for dipeptide β-turn mimetics: Expeditious synthesis of an unsaturated 6,5-fused bicyclic lactam

Reported here is an easy and short synthesis of 6-acetamido-5-oxo-1,2,3,5,6,7-hexahydro-3-indolizine-carboxylic acid, originating from β-enaminoesters derived from pyroglutamic acid. This key compound has been used as a scaffold in the synthesis of dipeptido mimetics.     

Measurement of Relaxation Rates of N and H Backbone Protons in Proteins with Tailored Initial Conditions

Several methods are presented for the selective determination of spin–lattice and spin–spin relaxation rates of backbone protons in labeled proteins. The relaxation rates of amide protons in ¹⁵N labeled proteins can be measured by using two-way selective cross-polarization (SCP). The measurement of H^α relaxation rates can be achieved by combining this method with homonuclear Hartmann–Hahn transfer using doubly selective irradiation. Various schemes for selective or nonselective inversion of the longitudinal proton magnetization lead to different initial recovery rates. The methods have been applied to lysine K6 in ¹⁵N-labeled human ubiquitin and to leucine L5 in ¹⁵N- and ¹³C-labeled octapeptide YG*G*F*LRRI (GFL) in which the marked residues are ¹⁵N- and ¹³C-labeled.