Intramolecular cycloaddition/cycloreversion of (E)-3β,17β-diacetoxy-5,10-secoandrost-1(10)-en-5-one

Treatment of 3β,17β-diacetoxy-5,10-secoandrost-1(10)-en-5-one with BF₃·Et₂O was shown to proceed with cleavage of the macrocycle and formation of a new compound containing a cyclopentenone ring. Based on DFT calculations, an intramolecular Lewis acid promoted [2+2]cycloaddition, followed by a cycloreversion of the intermediate oxetane, is proposed as a possible reaction mechanism.     

Perfluoro anion-exchange polymeric films on glassy carbon electrodes

Solutions of the perfluoro anion-exchange membrane TosHex® in a solvent mixture composed of methanol + isopropanol + water (1:1:1) were prepared and applied in coating glassy carbon electrodes. The evaporated films were used to accumulate the Fe(CN) ₆ redox couple on the electrode surface. The magnitude of the electrochemical response of the loaded films is comparable with that for Nafion® incorporated cationic redox species. The multicharged Fe(CN) ₆ couple accumulated in Tosflex® film causes an ion cross-linking of the polymeric backbone, thus decreasing ion transport in the film substantially.     

Bioanalysis of (E)-5-(2-bromovinyl)-2'-deoxyuridine

(E)-5-(2-Bromovinyl)-2'-deoxyuridine is an antiviral drug that is experimentally used for modulation of the antitumour effect of fluoropyrimidines, such as ftorafur and 5-fluorouracil. The isolation of the analyte, in the presence of 5-fluorouracil, from the matrix is performed either by means of a simple protein precipitation (plasma) or by means of a liquid-liquid extraction with ethyl acetate (urine). Following pretreatment, the analyte is analysed by reversed-phase chromatography and quantified by absorbance detection at 307 nm. The minimum detectable concentration in plasma and urine samples is ca. 6 ng/ml. The recovery after deproteination of plasma samples is 75%, while after liquid-liquid extraction of urine the recovery amounts 92%. The degree of protein binding of the analyte, measured by ultrafiltration, is found to be 97%. These data allow the bioanalysis of (E)-5-(2-bromovinyl)-2'-deoxyuridine for pharmacokinetic studies.     

Apparent molar heat capacities and volumes of silver nitrate and silver perchlorate in aqueous solution at 298 K

We have made calorimetric and density measurements leading to apparent molar heat capacities and volumes of dilute aqueous solutions of silver nitrate and silver perchlorate at 298 K. Resulting apparent molar properties at infinite dilution are the following: φ^o_c(AgNO₃) = ? 36.8 J K^?1 mol^?1, φ^o_v(AgNO₃) = 29.1 cm³ mol^?1, φ^o_c(AgClO₄) = 11.0 J K^?1 mol^?1, and φ^o_v(AgClO₄) = 43.5 cm³ mol^?1.     

The Spectral Displacement Technique for Determining the Binding Constants of β-Cyclodextrin – Alkyltrimethylammonium Inclusion Complexes

A survey of the literature reveals that the spectroscopic displacement method, in which phenolphthalein is used as a competitive chromophore to determine binding constants of β-cyclodextrin with surfactants was successful in the case of homologous hydrocarbon and fluorocarbon anionic surfactants. We show here that this method can be applied also for homologous alkyltrimethylammonium bromides (with alkyl varying from octyl- to hexadecyl) under the condition that the concentration of the cationic surfactant is distinctly lower than the value of itscritical micelle concentration.     

Formation of M2+(O2)(C3H8) species in alkaline-earth-exchanged Y zeolite during propane selective oxidation

The adsorption of oxygen and d2-propane (CH3CD2CH3) on a series of alkaline-earth-exchanged Y zeolite at room temperature was studied with in situ infrared spectroscopy. Surprisingly at room temperature, oxygen adsorption led to the formation of supercage M2+(O2) species. Further, at low propane coverage, propane was found to adsorb linearly on Mg2+ cations, but a ring-adsorption structure was observed for propane adsorbing on Ca2+, Sr2+, and Ba2+ cations. It is demonstrated that O2 and propane can simultaneously attach to one active center (M2+) to form a M2+(O2)(C3H8) species, which is proposed to be the precursor in thermal propane selective oxidation. Selectivity to acetone in the propane oxidation reaction decreases with increasing temperature and cation size due to the formation of 2-propanol and carboxylate ions. An extended reaction scheme for the selective oxidation of propane over alkaline earth exchanged Y zeolites is proposed.     

H-aggregation of azobenzene-substituted amphiphiles in vesicular membranes

Photochemical switching has been studied of double-tailed phosphate amphiphiles containing azobenzene units in both tails in aqueous vesicular dispersions and in mixed vesicular systems with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Since the ease of switching depends on the strength of the bilayer packing, particular emphasis has been placed on the occurrence of H-aggregation in the hydrophobic core of the vesicles. UV-vis spectrometry was employed to monitor H-aggregation and showed how this process depends on the ionic strength and on the mode of preparation of the vesicles. Two types of H-aggregates were observed in mixed DOPC vesicles with 5 mol % of azobenzene phosphate: one with lambda(max) at around 300 nm and one with lambda(max) at 305-320 nm. Those with lambda(max) at 300 nm could not be trans-cis photoisomerized, whereas those with lambda(max) at 305-320 nm are more loosely packed and can be photochemically switched. The permeability of the vesicular bilayers, as probed with leakage experiments using calcein as a fluorescent probe, was examined as another measure for the strength of bilayer packing. Leakage occurred only for DOPC vesicles containing more than 20 mol % of azobenzenephosphate, irradiated with UV light to induce trans-cis photoisomerization. We contend that detailed information on bilayer packing will be of crucial importance for fine-tuning the lateral pressure in vesicular membranes with the ultimate aim to steer the opening and closing of mechanosensitive protein channels of large conductance.     

Methane-to-methanol oxidation by the hydrated iron(IV) oxo species in aqueous solution: a combined DFT and car-parrinello molecular dynamics study

Previously, we have shown that the ferryl ion ([FeIVO]2+) is easily produced from Fenton's reagent (i.e., a mixture of Fe2+ ions and H2O2 in aqueous solution), using DFT and Car-Parrinello MD calculations. To verify that the ferryl ion can indeed act as the active species in oxidation reactions with Fenton's reagent, we study in the present paper the reactivity of the ferryl ion toward an organic substrate, in particular the oxidation of methane to methanol. In the first part of this paper, we perform static DFT calculations on the reaction of CH4 with the [(H2O)5FeIVO]2+ complex in vacuo that show a strong prevalence of the oxygen-rebound mechanism over the methane coordination mechanism. This is in agreement with the static DFT results for methane oxidation by biocatalysts MMO and P450, but not with those for methane oxidation by bare metal-oxo ions, where the methane coordination mechanism prevails. The highest energy barrier in the oxygen-rebound mechanism is only 3 kcal/mol in vacuo, whereas in the methane coordination mechanism the highest barrier is 23 kcal/mol. Overall the oxidation reaction energy is downhill by 47 kcal/mol. We conclude that the ferryl ion can indeed act as the oxidative intermediate in the Fenton oxidation of organic species. In the second part of this paper, we perform a preliminary assessment of solvent effects on the oxidation by the ferryl ion in aqueous solution using the method of constrained (first principles) molecular dynamics. The free energy barrier of the H-abstraction reaction from methane by the ferryl ion (i.e., the first step in the rebound mechanism) in aqueous solution is, with 22 kcal/mol in solution, significantly higher than in vacuo. Given the fact that methane has a relatively strong C-H bond (ca. 10 kcal/mol stronger than the C-H bonds in the more typical Fenton's reagent substrates), we infer that for many organic substrates oxidation with the ferryl ion as an active intermediate may be a perfectly viable route.     

Analysis of anthracycline antitumour drugs in tissues and body fluids using liquid chromatography

Anthracyclines, with doxorubicin as the major representative, are amongst the most important chemotherapeutic agents used in cancer therapy. In order to reduce the severe side effects associated with their use, and to increase therapeutic efficacy, analogue development still continues, and analytical requirements change concomitantly. The available methods for bioanalysis of anthracyclines are summarized, with emphasis on high-performance liquid chromatography. Attention is paid to sample pretreatment, the possibilities of liquid—liquid and solid-phase extraction, and the chromatographic behaviour of the anthracyclines.     

Dynamics and lithium binding energies of polyelectrolytes based on functionalized poly(para-phenylene terephthalamide)

Polyelectrolyte materials are an interesting class of electrolytes for use in fuel cell and battery applications. Poly(para-phenylene terephthalamide) (PPTA, Kevlar) is a liquid crystalline polymer that, when sulfonated, is a polyelectrolyte that exhibits moderate ion conductivity at elevated temperatures. In this work, quasi-elastic neutron scattering (QENS) experiments were performed to gain insight into the effect of the presence of lithium counterions on the chain dynamics in the material. It was found that the addition of lithium ions decreases the dynamics of the chains. Additionally, the binding of lithium ions to the sulfonic acids groups was investigated by density functional theory (DFT) calculations. It was found that the local surroundings of the sulfonic acid group have very little effect on the lithium-ion binding energy. Binding energies for a variety of different systems were all calculated to be around 150 kcal/mol. The DFT calculations also show the existence of a structure in which a single lithium ion interacts with two sulfonic acid moieties on different chains. The formation of such "electrostatic cross-links" is believed to be the source of the increased tendency to aggregate and the reduced dynamics in the presence of lithium ions.