Comparison of Different Sorbents for Solid-Phase Extraction of Phenoxyalkanoic Acid Herbicides

An HPLC procedure for determination of phenoxyalkanoic acid herbicides in water samples is proposed. The analytical column Phenomenex C18(2) Luna 5 µm and UV detection at 225 nm were applied. Baseline resolution was achieved in isocratic mode with a mobile phase consisting of acetonitrile/acetic acid (40/60, v/v), adjusted to pH 2.5. SPE sorbents – C₁₈ BondElut, phenyl-silica, LiChrolut SAX and polymeric sorbents – were compared for isolation and preconcentration of 6 phenoxyalkanoic acid herbicides. Higher (above 95%) and more reproducible recoveries were obtained with polymeric and phenyl-silica sorbents using pure methanol for elution. The method was tested for river water samples with the limit of detection in the range of 2–3 µg L⁻¹ (for 50 mL sample) and a reproducibility of 5% RSD.     

Multiresidue determination of fluoroquinolones in shrimp by liquid chromatography-fluorescence-mass spectrometry

An efficient multiresidue method for analysis of fluoroquinolones in shrimp has been developed in which quantitation by fluorescence and confirmation by Multiple Stage Mass Spectrometry (MS) is achieved simultaneously. In this method, shrimp tissue is extracted with ammoniacal acetonitrile and the extract is defatted and then evaporated. After dissolution in basic phosphate buffer, fluoroquinolones in the extract are separated by liquid chromatography and quantitated, taking advantage of their intense fluorescence. Eluate from the fluorescence detector enters the MS, which allows for confirmation by monitoring ratios of 2 prominent product ions in the MS3 or MS2 spectrum. Using this method, 8 fluoroquinolones have been analyzed in shrimp samples fortified at 10, 25, 50, or 100 ppb levels. Recoveries for desethyleneciprofloxacin, norfloxacin, ciprofloxacin, danofloxacin, enrofloxacin, orbifloxacin, sarafloxacin, and difloxacin ranged from 75 to 92%, with relative standard deviation values of <6%. The limits of quantitation ranged from 0.1 to 1 ng/g. Enrofloxacin and ciprofloxacin were also successfully determined in enrofloxacin-incurred shrimp using this method.     

The LOV2 domain of phototropin: a reversible photochromic switch

Light, oxygen, or voltage (LOV) domains constitute a new class of photoreceptor proteins that are sensitive to blue light through a noncovalently bound flavin chromophore. Blue-light absorption by the LOV2 domain initiates a photochemical reaction that results in formation of a long-lived covalent adduct between a cysteine and the flavin cofactor. We have applied ultrafast spectroscopy on the photoaccumulated covalent adduct state of LOV2 and find that, upon absorption of a near-UV photon by the adduct state, the covalent bond between the flavin and the cysteine is broken and the blue-light-sensitive ground state is regained on an ultrafast time scale of 100 ps. We thus demonstrate that the LOV2 domain is a reversible photochromic switch, which can be activated by blue light and deactivated by near-UV light.     

Self-assembly of [2]rotaxane exploiting reversible Pt(II)- pyridine coordinate bonds

A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine) coordinative bonds and having competitive triflate anions, as metal counterions, is used in the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the dynamics of the [2]rotaxane.     

Reactions of solute species at an electrode modified with titanocene functionalized polypyrrole film: ferrocene and titanocene dichloride

We have studied for the first time the ability of a conducting polymer film, p(Tc3Py), representing a polypyrrole matrix with covalently attached titanocene dichloride (TcCl₂) centers, to serve as an intermediator for the electron charge transport between the electrode and the reaction sites of solute reactants. The standard potential of the first of these electroactive species, ferrocene (Fc), is in the range where the polymer matrix is in its slightly oxidized state so that solute Fc species give a reversible response at the surface of this modified electrode. Another solute reactant, TcCl₂, was studied in solutions in which it demonstrates a (quasi)reversible behavior at bare electrode surfaces, THF+TBAPF₆ and AN+TEACl. The standard redox potential of this species belongs to the range of the electroactivity of immobilized TcCl₂ centers (where the matrix is in its non-conducting state) so that the electron charge has to be transported via stepwise redox reactions between neighboring centers inside the film. The combination, solute reactant+film, results in a greater CV current compared to the response of the film in background solution or of the solute species at the bare electrode surface. This current for THF solution even exceeds the sum of separate currents for the film and the reactant. This finding is attributed to a catalytic effect of solute species as redox intermediators for the transformation of immobilized electroactive centers leading to a greater degree of the film reduction. The presence of solute TcCl₂ species results in a much greater stability of immobilized centers (compared to the corresponding reactant-free solution), both in the course of CV with the passage of the range of their response and in experiments with the film holding at the potential within this range. This holding leads to an almost constant current related to the reaction of solute species at the film/solution interface. Our estimate shows that immobilized centers undergo above 10,000 reversible transformations (without an observed tendency to the degradation) to ensure the passage of this current. The conclusion has been drawn that immobilized TcCl₂ centers are able to serve as sufficiently stable redox intermediators for the electron charge transport across the film, a prerequisite for the catalytic applications of such films.Abbreviations AN acetonitrile - THF tetrahydrofuran - Cp cyclopentadienyl, C₅H₅ - Cp cyclopentadienyl radical, C₅H₄ - Fc ferrocene, Cp₂Fe - TcCl₂ titanocene=bis(cyclopentadienyl)titanium dichloride, Cp₂TiCl₂ or its radical CpCpTiCl₂ - PPy polypyrrole - Tc3Py titanocene-propyl-pyrrole, Cl₂TiCpCp(CH₂)₃NC₄H₄ - p(Tc3Py) polymer obtained from Tc3Py - TBAPF₆ tetrabuthylammonium hexafluorophosphate - TEACl tetraethylammonium chlorideDedicated to Zbigniew Galus on the occasion of his 70th birthday.     

Influence of chloride anions on the electrodeposition and electroactivity of the polymer matrix in polypyrrole,poly(<Emphasis Type="Italic">N</Emphasis>-methylpyrrole) and polypyrrole derivatives functionalized by titanocene centers,in dry non-aqueous solutions

We report electrochemical studies on the influence of a small concentration of chloride ions on the electroactivity of the polymer matrix of polypyrrole (PPy), poly(N-methylpyrrole) [p(N-MePy)] and a poly(titanocene-propyl-pyrrole) derivative, p(Tc3Py) [Tc(CH₂)₃NC₄H₄; Tc=CpCpTiCl₂; Cp=C₅H₅; Cp=C₅H₄] in acetonitrile (AN), tetrahydrofuran (THF) and N,N-dimethylformamide (DMF). The polymer films were obtained on Pt disc electrodes from AN solutions of the monomers containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte and then transferred to the corresponding monomer-free solution. Studies in Cl^–-containing solutions have shown that the p(Tc3Py) matrix is very sensitive to the presence of Cl^– ions in all the above solvents, namely that it was subjected to electrochemical degradation at potentials above 0.1 V vs. a Ag/0.01 M Ag⁺ in AN reference electrode. Degradation of the p(Tc3Py) matrix was also observed in chloride-free DMF+TBAPF₆ solutions. Addition of chloride ions to the AN solution containing pyrrole, N-methylpyrrole or Tc3Py inhibits the deposition of the polymer films. On the other hand, we have found that PPy and p(N-MePy) matrices after their deposition in chloride-free AN solutions show much more stable redox responses in contact with chloride and/or DMF solutions. Possible mechanisms of these effects are discussed.Abbreviations AN acetonitrile - Cp cyclopentadienyl - DMF N,N-dimethylformamide - N-MePy N-methylpyrrole - p(N-MePy) poly(N-methylpyrrole) - PPy polypyrrole - p(Tc3Py) poly[Tc(CH₂)₃NC₄H₄] - Py pyrrole - Tc titanocene=bis(cyclopentadienyl)titanium dichloride, Cp₂TiCl₂, or its radical CpCpTiCl₂ (Cp=C₅H₄) - Tc3Py titanocene-propyl-pyrrole, Tc(CH₂)₃NC₄H₄ - THF tetrahydrofuran Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

Analysis of First-Order Reactions with Distributed Parameters

Simple methods of analysis of first-order kinetic processes controlled by distributions of parameters (activation energies and/or frequency factors) are presented. Both the isothermal and nonisothermal regimes are considered. The procedures make use of numerically calculated time derivatives of the concentrations of decaying species (reactants in the case of chemical reactions). They do not require any adjustable initial or boundary values of parameters (like concentration at the infinite time) being thus more accurate and reliable than earlier methods in which kinetic parameters were determined from the concentration, and not from its time derivative. The methods were successfully employed in analysis of the thermally driven cis–trans isomerization of a photochromic crown ether containing the azobenzene moiety incorporated in the crown.     

The design and application of fluorophore-gold nanoparticle activatable probes

Fluorescence-based assays and detection techniques are among the most highly sensitive and popular biological tests for researchers. To match the needs of research and the clinic, detection limits and specificities need to improve, however. One mechanism is to decrease non-specific background signals, which is most efficiently done by increasing fluorescence quenching abilities. Reports in the literature of theoretical and experimental work have shown that metallic gold surfaces and nanoparticles are ultra-efficient fluorescence quenchers. Based on these findings, subsequent reports have described gold nanoparticle fluorescence-based activatable probes that were designed to increase fluorescence intensity based on a range of stimuli. In this way, these probes can detect and signify assorted biomarkers and changes in environmental conditions. In this review, we explore the various factors and theoretical models that affect gold nanoparticle fluorescence quenching, explore current uses of activatable probes, and propose an engineering approach for future development of fluorescence based gold nanoparticle activatable probes.     

Ring-expanded bicyclic β-lactams: a structure-chiroptical properties relationship investigation by experiment and calculations

In the present work, the validity of the helicity rule relating the absolute configuration of the bridgehead carbon atom in bicyclic β-lactams to the sign of the 220 nm band observed in their electronic circular dichroism (ECD) spectra is examined for ring-expanded cephalosporin analogues. To this end, a series of model compounds with a seven-membered ring condensed with the β-lactam unit was synthesized. A key step of their synthesis was either the ring-closing metathesis (RCM) or the free radical cyclization leading to the seven-membered ring with an S, O, or C atom at the 6 position in the bicyclic skeleton. To investigate the scope and limitations of the simple, empirically established helicity rule, a combination of ECD spectroscopy, variable-temperature ECD measurements, X-ray analysis, and time-dependent density functional theory (TD-DFT) calculations was used. A comparison of the experimental ECD spectra with the spectra simulated by TD-DFT calculations gives a reasonable interpretation of the Cotton effects observed in the 240-215 nm spectral range. The results suggest that the helicity rule does not apply to the investigated compounds because of the planarity of their amide chromophore. Thus, these compounds do not constitute an exception to the rule that was established for bi- and polycyclic β-lactams with the nonplanar amide chromophore only.     

Aminophosphinic Acids in a Pyridine Series: Cleavage of Pyridine-2- and Pyridine-4-methyl(amino)phosphinic Acids in Acidic Solutions

The synthesis of a series of new pyridine aminomethylphosphinic acids is described. These compounds were obtained in the reaction of the corresponding pyridine aldehydes with primary amines and with ethyl phenylphosphinate, or methylphosphinate, in the presence of bromotrimethylsilane. In aqueous, strong acid solutions, pyridine aminophosphinic acids were split, forming the phenyl-, or methylphosphonic, acid and the corresponding secondary pyridyl-alkylamines. The kinetics of some observed cleavages were measured, and a mechanism of the cleavage has been proposed.