Nature of interaction energy anisotropy in the Li(<Superscript>2</Superscript>S)–HF (˜X<Superscript>1</Superscript>Σ<Superscript>+</Superscript>) van der Waals complex. A theoretical study

The potential-energy surface for the Li(²S)–HF (? X¹Σ⁺ interaction, where HF is kept rigid, is calculated using the supermolecular unrestricted fourth-order Møller–Plesset perturbation theory. The basis set superposition error corrected potential indicates two minima. The global minimum occurs for the bent Li...FH structure at R=1.95 Å and θ=70° with a relatively deep well of D_e=1,706 cm^?1 and the secondary minimum is found for the linear Li...HF configuration at R=4.11 Å with a well depth ofD_e=288 cm^?1. A barrier of 177 cm^?1 (with respect to the secondary linear minimum) separates these two minima. In this study 27 bound states of the bent Li...FH minimum and eight bound states of the linear Li...HF minimum up to the Li+HF dissociation threshold are calculated. The energy partitioning using the intermolecular perturbation theory scheme shows that the origins of the stability of the structures studied are entirely different. The global minimum is stabilised using the attractive Coulombic interaction and unrestricted Hartree–Fock deformation energy. The latter term originates from the mutual electric polarisation effects. The secondary linear minimum is mostly determined by the anisotropy of the repulsive Heitler–London exchange-penetration and attractive dispersion energies.     

Structure of 1,4,10,19,25,41-C70(CF3)6, isomer with unique arrangement of addends

A novel isomer of C₇₀(CF₃)₆ has been isolated by HPLC from a mixture prepared by trifluoromethylation of C₇₀ with CF₃COOAg. The X-ray structure revealed an unprecedented arrangement of CF₃ groups forming a p³mp ribbon. This result provides additional evidence of the preferable formation of trifluoromethylated fullerene molecules comprising a single continuous ribbon of edge-sharing para- and meta-C₆(CF₃)₂ hexagons.     

Partial donor-donor energy migration (PDDEM) as a fluorescence spectroscopic tool for measuring distances in biomacromolecules

A theoretical model is presented, tested and applied for determining the rates of energy migration and distances within pairs of chemically identical fluorophores, so-called donors (D), which are exposed to different physical properties. The model is a general extension of the recently developed donor-donor energy migration (DDEM) model [J. Chem. Soc., Faraday Trans. 92 (1996)1563; J. Chem. Phys. 105 (1996) 10896] that applies to examining structure-function of biomacromolecules, such as proteins. Most fluorescent groups of the same kind incorporated at different positions (alpha and beta) in a macromolecule exhibit shifts of the absorption and/or emission spectra, as well as different relaxation rates of the photophysics. As a consequence, the energy migration between the D(alpha) and D(beta) groups will be partially reversible. We refer to this case, as the partial donor-donor energy migration (PDDEM). The models of PPDEM presented can be used for analysing time-resolved fluorescence relaxation, as well as fluorescence depolarisation experiments. To explore the limitations of the PDDEM model, we have generated and re-analysed synthetic data that mimic time-correlated single photon counting (TCSPC) experiments. It was found that slow and fast rates of energy migration are most accurately recovered from the fluorescence relaxation and the depolarisation experiments, respectively. At comparable transfer and fluorescence rates, both kinds of experiments are equally useful. Real experiments on PDDEM were performed on an asymmetrically quenched bichromophoric molecule (1,32-dihydroxy-dotriacontane-bis-(Rhodamine 101) ester), that spans across the lipid bilayer of a vesicle. The depolarisation data were analysed by the PDDEM model and provide a distance between Rhodamine 101 groups, which agrees with independent studies.     

Capillary zone electrophoresis in wide bore capillary tubes with fiber-coupled diode array detection

This feasibility study deals with the use of a wide bore (320 μm I.D.) capillary tube for the detection and identification of capillary zone electrophoresis (CZE) analytes by optical fiber-coupled diode array detection. A 250-μm mean effective pathlength of the detection cell with an inherently enhanced photon flux through the cell were significant contributors in reaching 0.2–1 μmol/l concentration detectabilities of the CZE analytes by this combination. Experiments with model analytes (p-sulfanilic, sorbic and naphthalene-2-sulfonic acids, tryptophan and asulam) revealed that spectral confirmations of their identities were still possible when their concentrations in the loaded samples (200 nl) were 1–5 μmol/l. Here, chemometry procedures (target transformation factor analysis, fixed size moving window-target transformation factor analysis, fixed size moving window-evolving factor analysis and orthogonal projection approach) employed in the data processing effectively contributed to reliable confirmation of the identities of the analytes also in critical situations (e.g. peak overlaps). The CZE separations were carried out in tandem-coupled columns of identical I.D. This made it possible to use, in the first column of the tandem, carrier electrolyte solutions that provide the desired separative effects, while in the second (detection) column the compositions of the carrier electrolyte solutions employed could reflect favorable conditions for obtaining spectral data. Mixtures containing model constituents at significantly differing concentrations and Maillard’s reaction products spiked with tryptophan enantiomers were employed in experiments aimed at assessing practical applicabilities and limits of the present approach to the analysis of samples characterized by complex ionic matrices.     

Determination of orotic acid in urine by capillary zone electrophoresis in tandem-coupled columns with diode array detection

Analytical potentialities of capillary zone electrophoresis in the separation system with tandem-coupled columns to the spectral identification and determination of orotic acid (OA) in urine by diode array detection (DAD), coupled to the separation system via optical fibers, were investigated. A very significant “in-column” clean-up of OA from urine matrix was reached in the separation stage of the tandem by combining a low pH (2.8) with complexing effects of electroneutral agents [- and β-cyclodextrins, poly(vinylpyrrolidone) and 3-(N,N-dimethyldodecylammonio)propanesulfonate]. Due to this, its DAD spectral data could be acquired in the detection stage of the tandem with almost no disturbances by matrix co-migrants. The concentration limits of detection obtained under such working conditions for a 200-nl sample load of OA and 320 μm I.D. capillary tubes were 3.5 μmol/l (218 nm) and 0.4 μmol/l (280 nm). Using chemometry procedures (target transformation factor analysis, fixed size moving window-evolving factor analysis, orthogonal projection approach and fixed size moving window–target transformation factor analysis) in processing of the acquired spectral data, the presence of OA in the loaded urine matrix could be confirmed with confidence when its concentration was 10 μmol/l or slightly less.     

The detection of nitrated tyrosine in neuropeptides: a MALDI matrix-dependent response

Neuropeptides are a diverse class of signaling molecules that typically have one or more posttranslational modifications. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is an effective tool for identification and characterization of neuropeptides from samples as small as individual neurons. However, the detection of one particular posttranslational modification—nitrotyrosine—has been problematic because of the lability of the nitro group of nitrotyrosine under MALDI-MS conditions. The detection of nitrated tyrosine in peptide standards was dependent on the MALDI matrix used for the analysis. Specifically, sinapinic acid was the optimum matrix tested to observe this modification while it was not consistently detected with matrices such as 2,5-dihydroxybenzoic acid. Using the optimized procedures, several identified nitric-oxide-synthase positive neurons from Lymnaea stagnalis were tested to determine if the neuropeptides present were nitrated. In all cases, the nitrated form of the neuropeptide was not observed. The dependence on the sample-preparation procedures of observing this particular chemical modification demonstrates the need for careful selection of sample-preparation methods with MALDI or the use of other ionization methods.     

Ginkgolides and glycine receptors: a structure-activity relationship study

Ginkgolides from the Ginkgo biloba tree are diterpenes with a cage structure consisting of six five-membered rings and a unique tBu group. They exert a variety of biological properties. In addition to being antagonists of the platelet activating factor receptor (PAFR), it has recently been shown that native ginkgolides are potent and selective antagonists of the inhibitory glycine receptor. Forty new ginkgolide derivatives have been prepared in good to high yields on milligram scales and investigated for their antagonistic properties at homomeric alpha 1 glycine receptors, thus providing the first structure-activity relationship study of ginkgolides at glycine receptors. A high-throughput screening assay showed that native ginkgolide C was the most potent ligand, and that manipulation of any of the hydroxyl groups led to loss of activity at alpha 1 glycine receptors.     

Aperiodic capillary electrophoresis method using an alternating current electric field for separation of macromolecules

Switching from direct current (DC) to alternating current (AC) electric fields has provided substantial improvements in various instrument techniques that use electric fields for manipulating with various liquid-based systems. For example, AC fields are now used in both light scattering and electroacoustic instruments for measuring xi-potential, largely replacing more traditional microelectrophoresis techniques that use DC fields. In this paper, we suggest a novel way to make a similar transition in the area of separation techniques, capillary electrophoresis (CE) in particular. Dielectrophoresis is one well-known separation effect in which a drifting motion of particles is produced in a "spatially nonhomogeneous" AC electric field. However, there is another field effect that also causes a similar drift of particles. Instead of a "spatially nonhomogeneous" field, this method relies on a "temporally nonhomogeneous" field, normally referred to as "aperiodic electrophoresis". Despite a number of recently published experimental and theoretical papers describing this effect, it is less well-known than dielectrophoresis. We present a short overview of some of the relevant papers. We point out for the first time the idea that "aperiodic electrophoresis" might be useful for separation of macromolecules. We suggest several new mechanisms that could induce this effect in a sufficiently strong AC electric field. This effect can be used as a basis for a new separation method having several important advantages over traditional CE. We present a simple scheme as an example illustrating this new method.     

Binuclear complexes of 4-methoxyphenyltellurium trichloride with variably linked diphosphine disulfide ligands

Four neutral ligands of the diphosphine disulfide type with different central linking groups [(CH₂)₄, (CH₂)₆, (C₆H₄)₂, and HN-(C₆H₄)-NH] were reacted with 4-methoxyphenyltellurium trichloride in dichloromethane. The crystallization of products afforded crystals consisting of binuclear addition complexes of 4-MeOC₆H₄TeCl₃ with corresponding ligands. The ligands are coordinated to Te(IV) centres through weak Te–S bonds. The coordination of both tellurium atoms in each complex is pseudo-octahedral, built of the TeCl₃S tetragonal basis and the aryl group and the lone pair of electrons in axial positions. The weak donating properties of sulfur atoms are reflected in shortening of the trans-positioned Te–Cl bonds.     

The stable diiron(2.5) complex ion [(NC)(5)Fe(mu-tz)Fe(CN)5](5-), tz = 1,2,4,5-tetrazine, and its neighboring oxidation states

The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and M?ssbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.