Directly coupled CZE-NMR and CEC-NMR spectroscopy for metabolite analysis: paracetamol metabolites in human urine.

Direct coupling of NMR spectroscopic detection with both capillary zone electrophoresis (CZE) and capillary electrochromatography (CEC) was applied to the separation of metabolites of the drug paracetamol in an extract of human urine. Continuous-flow CZE-NMR and CEC-NMR allowed the detection of the major metabolites, the glucuronide and sulfate conjugates of the drug and the endogenous material hippurate. Identification of these substances was achieved by examination of individual rows of the NMR chromatogram and this also gave estimates of the detection limits. For CEC-NMR, spectra were also obtained in the stopped-flow mode including a two-dimensional TOCSY NMR experiment which afforded confirmatory evidence for paracetamol glucuronide. Characterisation of drug metabolites using NMR spectroscopy is therefore possible with nanolitre sample volumes.     

Application of on-line capillary high-performance liquid chromatography-nuclear magnetic resonance spectrometry coupling for the analysis of vitamin A derivatives

The direct on-line coupling between capillary high-performance liquid chromatography (capillary HPLC) and proton high-field nuclear magnetic resonance (NMR) spectrometry has been used to derive structural information about constituents of a mixture of vitamin A derivatives. ¹H NMR spectra were recorded in the stopped-flow and continuous-flow mode within a 180 μm I.D. capillary column mounted in a micro probe on a 600 MHz NMR spectrometer. The resolution of the ¹H NMR spectra obtained in capillary HPLC-NMR coupling experiments is sufficient to determine coupling constants in the order of 1.5 Hz. The detection limit is in the lower nanogram range. A stopped-flow 2D-TOCSY experiment of a 1% solution of vitamin A acetate acquired within 4 h reveals that the acquisition of 2D NMR spectra is possible in the nanoliter detection scale without any loss of structural information.     

Electrokinetic flow through an elliptical microchannel: effects of aspect ratio and electrical boundary conditions

The electrokinetic flow of an electrolyte solution through an elliptical microchannel is studied theoretically. The system under consideration simulates the flow of a fluid, for example, in vein. We show that, for a constant cross-sectional area, both the electroosmotic volumetric flow rate and the streaming potential increase monotonically with an increase in the aspect ratio, and both the total electric current and the electroviscous effect may exhibit a local minimum as the aspect ratio varies. Also, for a constant average potential on the channel wall, the difference between the results based on three kinds of boundary conditions, which include constant surface charge, constant surface potential, and charge-regulated surface, is inappreciable if the hydraulic diameter is larger than 1 mum.     

An operational supramolecular nanovalve

A functioning nanomachine in the form of a supramolecular nanovalve that opens and closes the orifices to molecular-sized pores and releases a small number of molecules on demand is reported. The nanovalve, which is used to open and close the nanocontainer, is a pseudorotaxane composed of two components-a long thread containing a 1,5-dioxnaphthalene donor unit, which is attached to the solid support, and the moving part, the tetracationic cyclophane acceptor/receptor, cyclobis(paraquat-p-phenylene), which controls access to the interior of the nanopore. The nanocontainer is made out of mesoporous silica by using a dip-coating method. Operating the nanovalve involves three steps: (i) filling the container, (ii) closing the valve, and (iii) opening the valve to release the contents of the container on demand. The tubular pores, which are approximately 2 nm wide, are filled with stable luminescent Ir(ppy)3 molecules by allowing them to diffuse into the open pores. The orifices are then closed by pseudorotaxane formation. An external reducing reagent (NaCNBH3) is used to effect dethreading of the pseudorotaxane so as to unlock the tubes and allow the guest molecules to be released. This nanovalve is a supramolecular machine consisting of a solid framework with moving parts capable of doing useful work.     

Design and performance of a low-flow capillary electrophoresis-electrospray-mass spectrometry interface using an emitter with dual beveled edge

A low-flow electrophoresis-mass spectrometry (CE-MS) interface has been developed for interfacing capillary zone electrophoresis (CZE) with electrospray- ionization-mass spectrometry (ESI-MS). The interface consists of two parallel capillary columns (a separation column and a makeup column), and an emitter with a dual beveled edge. While maintaining a relatively low optimum flow rate, the dual-beveled-edge ESI emitter allows the use of a tip with larger orifice. Therefore, this interface is less prone to column blocking in comparison with a flat tip. Primarily attributed to low sample dilution and smaller initial droplet, the interface showed better sensitivity than a conventional sheath liquid interface. Furthermore, the interface was found to be more resistant to the presence of nonvolatile salts. By using 40 mM borate and 20 mM alpha-cyclodextrin (alpha-CD) as the running buffer, four major forms of gangliosides were detected by CE-MS.     

Photoinduced Electron Transfer Reactions of 3,3‐Dialkylated 4,5‐Diphenyl‐3H‐Pyrazoles: A New Route to the Formation of the Solvent Adducts

3,3‐Dialkyl‐4,5‐diphenyl‐3H‐pyrazoles undergo readily photoinduced electron transfer (PET) reaction with 2,4,6‐triphenylpyrylium tetrafluoroborate (TPP⁺) in acetonitrile to produce cyclopropenes and 2H‐pyrroles. During prolonged irradiation, the new ring‐closure products derived from 2H‐pyrroles as the secondary photoproducts are also produced. However, the corresponding ester analog exhibits different behavior to obtain the cyclopropene as the primary photoproduct and a [2+2] dimer of the cyclopropene as the secondary photoproduct. A rationale for the different behavior is offered.     

Structural characterization and luminescence behavior of a silver(I) 1D polymeric chain constructed via a Bridge with unusual 4,5-diazospirobifluorene and perchlorate

A new polymeric silver complex, [Ag(2)(L(2))(ClO(4))(2)] (L = 4,5-diazospirobifluorene), has been synthesized and shown to exhibit interesting luminescence properties in a single crystal. Structural analysis reveals a one-dimensional chain, which contains a [Ag(2)(L(2))](2+) dimer bridged with ClO(4)(-). The Ag...Ag distances are 2.776(1) and 4.575(1) A incorporated by two L ligands and by a ClO(4)(-) bridge, respectively.     

An improved method of synthesis of neplanocin and related cyclopentenyl-containing nucleosides

A facile one-step condensation of the cyclopentenyl α,-tosylate $\text{2c}$ with the alkali metal salts of 6-chloropurine and uracil, afforded the protected carbocyclic nucleosides $\text{5a}$ and $\text{8}$, respectively, which were converted to neplanocin a ($\text{6}$) and cyclopentenyl uracil ($\text{9}$) by established methods.     

A Low Cost LED Based Spectrometer

A low cost LED based spectrometer is described. This LED based spectrometer could be operated as a standalone instrument or under PC control via serial link. A total of seven wavelength selections are available by the plug‐and‐measure LED light module. With the seven wavelength selections, the LED based spectrometer could provide qualitative visible absorption spectra that predict the absorption maximum. Based upon the qualitative visible spectra, quantitative photometric information could be obtained.     

Reaction mechanism and kinetics of the NCN+NO reaction: comparison of theory and experiment

The rate constants for the NCN + NO reaction have been measured by laser photolysis/laser-induced fluorescence technique in the temperature range of 254-353 K in the presence of He (40-600 Torr) and N2 (30-528 Torr) buffer gases. The NCN radical was produced from the photodissociation of NCN3 at 193 nm and monitored with a dye laser at 329.01 nm. The reaction was found to be strongly positive-pressure dependent with negative-temperature dependence, as was reported previously. The experimental data could be reasonably accounted for by dual-channel Rice-Ramsperger-Kassel-Marcus calculations based on the predicted potential-energy surface using the modified Gaussian-2 method. The reaction is predicted to occur via weak intermediates, cis- and trans-NCNNO, in the 2A" state which crosses with the 2A' state containing more stable cis- and trans-NCNNO isomers. The high barriers for the fragmentation of these isomers and their trapping in the 2A' state by collisional stabilization give rise to the observed positive-pressure dependence and negative-temperature effect. The predicted energy barrier for the fragmentation of the cis-NCNNO (2A') to CN + N2O also allows us to quantitatively account for the rate constant previously measured for the reverse process CN + N2O --> NCN + NO.