The Role of Singlet Oxygen in Photoreactivity and Phototoxicity of Curcumin

Curcumin is a plant-derived yellow-orange compound widely used as a spice, dye and food additive. It is also believed to have therapeutic effects against different disorders. On the other hand, there are data showing its phototoxicity against bacteria, fungi and various mammalian cells. Since the mechanism of its phototoxic action is not fully understood, we investigated here the phototoxic potential of curcumin in liposomal model membranes and in HaCaT cells. First, detection of singlet oxygen (¹O₂) luminescence proved that curcumin generates ¹O₂ upon blue light irradiation in organic solvent and in liposomes. Then, HPLC-EC(Hg) measurements revealed that liposomal and cellular cholesterol is oxidized by ¹O₂ photogenerated by curcumin. Enrichment of liposome membranes with curcumin significantly increased the oxygen photo-consumption rate compared to the control liposomes as determined by EPR oximetry. Cytotoxicity measurements, mitochondrial membrane potential analyses and protein hydroperoxides detection confirmed strong phototoxic effects of curcumin in irradiated HaCaT cells. These data show that since curcumin is advertised as a valuable dietary supplement, or a component of cosmetics for topical use, caution should be recommended especially when skin is exposed to light.     

A selective optical chemical sensor for 2,6-dinitrophenol based on fluorescence quenching of a novel functional polymer

A bifurcated optical fiber based chemical sensor for continuous monitoring of 2,6-dinitrophenol (2,6-DNP) has been proposed based on the reversible chemical reaction between a novel functional poly(vinyl chloride) (PVC) as the sensing material and the analytes. The functional PVC (FPVC), containing a fluorescent curcumin moiety, was synthesized by the nucleophilic substitution of a fraction of the chlorine atoms bound to the PVC backbone by curcumin. When plasticized in a membrane of 5 μm thickness, FPVC extracts 2,6-DNP from aqueous solution into the bulk membrane phase and reacts with the analyte to form a complex with low fluorescence efficiency through hydrogen bonding. Formation of the complex gave a significant fluorescence quenching which is suitable for signalling the occurrence of the host-guest interaction. At pH 3.50, the sensor exhibits a dynamic detection range from 2.5 × 10⁻⁶ to 7.0 × 10⁻³ mol L⁻¹ with a limit of detection of 1.0 × 10⁻⁶ mol L⁻¹. As 2,6-DNP can provide an optimal space geometry matches to the formation of hydrogen bonds, the sensor shows excellent selectivity for 2,6-DNP over other nitrophenols. The forward and reverse response time (t₉₅) of the sensor both was within 1 min. The repeatability, reproducibility, and lifetime of the sensor were also satisfied. The sensor was applied to determine 2,6-DNP in water samples successfully.     

Bilayer lipid membrane biosensor with enhanced stability for amperometric determination of hydrogen peroxide

In this paper, a polydopamine (PDA) film is electropolymerized on the surface of bilayer lipid membrane (BLM) which is immobilized with horseradish peroxidase (HRP). The coverage of the PDA film on HRP/BLM electrode is monitored by electrochemical impedance spectroscopy (EIS). The electrocatalytic reduction of H₂O₂ at the PDA/HRP/BLM electrode is studied by means of cyclic voltammetry (CV). The biosensor has a fast response to H₂O₂ of less than 5 s and an excellent linear relationship is obtained in the concentration range from 2.5 × 10⁻⁷ to 3.1 × 10⁻³ mol L⁻¹, with a detection limit of 1.0 × 10⁻⁷ mol L⁻¹ (S/N = 3). The response current of BLM/HRP/PDA biosensor retains 84% of its original response after being stored in 0.1 mol L⁻¹ pH 7.0 PBS at 4 °C for 3 weeks. The selectivity, repeatability, and storage stability of PDA/HRP/BLM biosensor are greatly enhanced by the coverage of polydopamine film on BLM.     

Oxidation of substituted pyridines by dimethyldioxirane: kinetics and solvent effects

The oxidation of a series of substituted pyridines by dimethyldioxirane (1) produced the expected N-oxides in quantitative yields. The second order rate constants (k₂) for the oxidation of a series of substituted pyridines (2a-g) by dimethyldioxirane were determined in dried acetone at 23 °C. An excellent correlation with Hammett sigma values was found (ρ = −2.91, r = 0.995). Kinetic studies for the oxidation of 4-trifluoromethylpyridine by 1 were carried out in the following dried solvent systems: acetone (k₂ = 0.017 M⁻¹ s⁻¹), carbon tetrachloride/acetone (7:3; k₂ = 0.014 M⁻¹ s⁻¹), acetonitrile/acetone (7:3; k₂ = 0.047 M⁻¹ s⁻¹), and methanol/acetone (7:3; k₂ = 0.68 M⁻¹ s⁻¹). Kinetic studies of the oxidation of pyridine by 1 versus mole fraction of water in acetone [k₂ = 0.78 M⁻¹ s⁻¹ (χ = 0) to k₂ = 11.1 M⁻¹ s⁻¹ (χ = 0.52)] were carried out. The results showed the reaction to be very sensitive to protic, polar solvents.     

Toluidine blue adsorbed on alcohol dehydrogenase modified glassy carbon electrode for voltammetric determination of ethanol

A novel toluidine blue O (TBO) adsorbed alcohol dehydrogenase (ADH) biocomposite film have been prepared through simple adsorption technique with the help of electrostatic interaction between oppositely charged layers. Nafion (NF) coating was made on top of the biocomposite film modified glassy carbon electrode (GCE) to protect ADH from leaching. The fabricated ADH/TBO/NF biocomposite electrode remains highly stable in the pH range from 4 to 13. More facile electron transfer process occurs at ADH/TBO/NF biocomposite than at TBO/NF film, which is obvious from the six folds increase in k_s value. Maximum surface coverage concentration (Γ) of TBO is noticed at ADH/TBO/NF film, which is 82% higher than at TBO/NF and 15% higher than at ADH/TBO film modified GCEs. Electrochemical impedance spectroscopy studies reveal that ADH has been well immobilized in the biocomposite film. Scanning electron microscopy studies confirm the discriminate surface morphology of various components present in the biocomposite film. Cyclic voltammetry studies validate that ADH/TBO/NF biocomposite film exhibits excellent electrocatalytic activity for ethanol oxidation at low over potential (I_pa = −0.14 V). The same studies show biocomposite film possesses a good sensitivity of 7.91 μA M⁻¹ cm⁻² for ethanol determination. This above sensitivity value is 17.40% higher than the sensitivity obtained for TBO/NF film (6.74 μA M⁻¹ cm⁻²). Further, using differential pulse voltammetry, a sensitivity of 1.70 μA M⁻¹ cm⁻² has been achieved for ADH/TBO/NF biocomposite film.     

DNA interaction probed by evanescent wave cavity ring-down absorption spectroscopy via functionalized gold nanoparticles

Evanescent wave cavity ring-down absorption spectroscopy (EW-CRDS) is employed to study interaction and binding kinetics of DNA strands by using gold nanoparticles (Au NPs) as sensitive reporters. These Au NPs are connected to target DNA of study that hybridizes with the complementary DNA fixed on the silica surface. By the absorbance of Au NPs, the interaction between two DNA strands may be examined to yield an adsorption equilibrium constant of 2.2 × 10¹⁰ M⁻¹ using Langmuir fit. The binding efficiency that is affected by ion concentration, buffer pH and temperature is also examined. This approach is then applied to the label-free detection of the DNA mutation diseases using the sandwich hybridization assay. For monitoring a gene associated with sickle-cell anemia, the detection limit and the adsorption equilibrium constant is determined to be 1.2 pM and (3.7 ± 0.8) × 10¹⁰ M⁻¹, distinct difference from the perfectly matched DNA sequence that yields the corresponding 0.5 pM and (1.1 ± 0.2) × 10¹¹ M⁻¹. The EW-CRDS method appears to have great potential for the investigation of the kinetics of a wide range of biological reactions.     

Moisture barrier,wetting and mechanical properties of shellac/agar or shellac/cassava starch bilayer bio-membrane for food applications

Edible bilayer membrane composed of agar (AG) or cassava starch (CAS) as a cohesive structural layer and ethanol-cast shellac layer as a moisture barrier are investigated for their potential use in food preservation as bio-packaging film, membrane or coating. Bilayer membranes containing non-plasticized shellac exhibit low water vapor permeability (WVP), from 0.89 to 1.03 × 10⁻¹¹ g m⁻¹ s⁻¹ Pa⁻¹. A high value of contact angle (≈92°) and a low liquid water adsorption rate (26 × 10⁻³ μL s⁻¹) indicate that these barrier layers have a quite hydrophobic surface. However, the rigid and brittle characteristics of shellac induce a lack of integrity for this layer. It tends to be cracked and scaled off. The incorporation of PEG 200 (plasticizer) into shellac improves the flexibility that prevents the defects in structure and reinforces the adhesion between the shellac and the cohesive-structural layer. The use of plasticizer weakly affects the WVP of bilayer membranes; however, the surface hydrophobicity as well as the liquid water adsorption rate is comparable to that of non-plasticized shellac layer. Furthermore, PEG increases the stretchability of bilayer membranes. Either being plasticized or not, shellac layer could improve significantly the functional properties of bilayer barriers and give a promising use as biopackaging.     

Comparison of pyrimethanil-imprinted beads and bulk polymer as stationary phase by non-linear chromatography

A pyrimethanil-imprinted polymer (P1) was prepared by iniferter-mediated photografting a mixture of methacrylic acid and ethylene dimethacrylate onto homemade near-monodispersed chloromethylated polydivinylbenzene beads. The chromatographic behaviour of a column packed with these imprinted beads was compared with another column packed with irregular particles obtained by grinding a bulk pyrimethanil-imprinted polymer (P2). The comparison was made using the kinetic model of non-linear chromatography, studying the elution of the template and of two related substances, cyprodinil and mepanipyrim. Extension of the region of linearity, capacity factors for the template and the related substances, column selectivity, binding site heterogeneity, apparent affinity constant (K) and lumped kinetic association (k_a) and dissociation rate constant (k_d) were studied during a large interval of solute concentration, ranging between 1 and 2000 μg/ml. From the experimental results obtained, in the linearity region of solute concentration column selectivity and binding site heterogeneity remained essentially the same for the two columns, while column capacity (at 20 μg/ml, P1 = 23.1, P2 = 11.5), K (at 20 μg/ml, P1 = 8.3 × 10⁶ M⁻¹, P2 = 2.5 × 10⁶ M⁻¹) and k_a (at 20 μg/ml, P1 = 3.5 μM⁻¹ s⁻¹, P2 = 0.47 μM⁻¹ s⁻¹) significantly increased and k_d (at 20 μg/ml, P1 = 0.42 s⁻¹, P2 = 0.67 s⁻¹) decreased for the column packed with the imprinted beads. These results are consistent with an influence of the polymerisation method on the morphology of the resulting polymer and not on the molecular recognition properties due to the molecular imprinting process.     

Immobilization of [Cu(bpy)2]Br2 complex onto a glassy carbon electrode modified with α-SiMo12O40 and single walled carbon nanotubes: Application to nanomolar detection of hydrogen peroxide and bromate

A simple procedure has been used for preparation of modified glassy carbon electrode with carbon nanotubes and copper complex. Copper complex [Cu(bpy)₂]Br₂ was immobilized onto glassy carbon (GC) electrode modified with silicomolybdate, α-SiMo₁₂O₄₀⁴⁻ and single walled carbon nanotubes (SWCNTs)_. Copper complex and silicomolybdate irreversibly and strongly adsorbed onto GC electrode modified with CNTs. Electrostatic interactions between polyoxometalates (POMs) anions and Cu-complex, cations mentioned as an effective method for fabrication of three-dimensional structures. The modified electrode shows three reversible redox couples for polyoxometalate and one redox couple for Cu-complex at wide range of pH values. The electrochemical behavior, stability and electron transfer kinetics of the adsorbed redox couples were investigated using cyclic voltammetry. Due to electrostatic interaction, copper complex immobilized onto GC/CNTs/α-SiMo₁₂O₄₀⁴⁻ electrode shows more stable voltammetric response compared to GC/CNTs/Cu-complex modified electrode. In comparison to GC/CNTs/Cu-complex the GC/CNTs/α-SiMo₁₂O₄₀⁴⁻ modified electrodes shows excellent electrocatalytic activity toward reduction H₂O₂ and BrO₃⁻ at more reduced overpotential. The catalytic rate constants for catalytic reduction hydrogen peroxide and bromate were 4.5(±0.2) × 10³ M⁻¹ s⁻¹ and 3.0(±0.10) × 10³ M⁻¹ s⁻¹, respectively. The hydrodynamic amperommetry technique at 0.08 V was used for detection of nanomolar concentration of hydrogen peroxide and bromate. Detection limit, sensitivity and linear concentration range proposed sensor for bromate and hydrogen peroxide detection were 1.1 nM and 6.7 nA nM⁻¹, 10 nM-20 μM, 1 nM, 5.5 nA nM⁻¹ and 10 nM-18 μM, respectively.     

Direct electrochemistry of hemoglobin and myoglobin at didodecyldimethylammonium bromide-modified powder microelectrode and application for electrochemical detection of nitric oxide

Hemoglobin (Hb) and myoglobin (Mb) were immobilized at the didodecyldimethylammonium bromide (DDAB)-modified powder microelectrode (PME) to fabricate Hb-DDAB-PME and Mb-DDAB-PME. Direct electrochemistry of Hb and Mb were achieved on the DDAB-modified PME. The formal potential was −0.224 V for Hb and −0.212 V for Mb (vs. SCE). The apparent surface concentration of Hb and Mb at the electrode surface was 2.83 × 10⁻⁸ and 9.94 × 10⁻⁸ mol cm⁻². The Hb-DDAB-PME and Mb-DDAB-PME were successfully applied for measurement of NO in vitro. The anodic current peaks for NO oxidation at +0.7 V and the cathodic current peaks for NO reduction at −0.85 V on the CV curves were obtained on the modified electrodes. For detection of NO at +0.7 V, the sensitivity is 3.31 mA μM⁻¹ cm⁻² for Hb-DDAB-PME and 0.6 mA μM⁻¹ cm⁻² for Mb-DDAB-PME. The detection limit is 5 nM for Hb-DDAB-PME and 9 nM for Mb-DDAB-PME. The linear response range is 9-100 and 28-330 nM for Hb- and Mb-modified PME, respectively. For the electrochemical detection of NO at −0.85 V by using Hb-DDAB-PME, the detection sensitivity is 39.56 μA μM⁻¹ cm⁻²; the detection limit is as low as 0.2 μM; and the linear response range is 1.90-28.08 μM.     

Binding of the bioflavonoid robinetin with model membranes and hemoglobin: Inhibition of lipid peroxidation and protein glycosylation

Recent years have witnessed burgeoning interest in plant flavonoids as novel therapeutic drugs targeting cellular membranes and proteins. Motivated by this scenario, we explored the binding of robinetin (3,7,3′,4′,5′-pentahydroxyflavone, a bioflavonoid with remarkable ‘two color’ intrinsic fluorescence properties), with egg yolk phosphatidylcholine (EYPC) liposomes and normal human hemoglobin (HbA), using steady state and time resolved fluorescence spectroscopy. Distinctive fluorescence signatures obtained for robinetin indicate its partitioning (K_p = 8.65 × 10⁴) into the hydrophobic core of the membrane lipid bilayer. HbA–robinetin interaction was examined using both robinetin fluorescence and flavonoid-induced quenching of the protein tryptophan fluorescence. Specific interaction with HbA was confirmed from three lines of evidence: (a) bimolecular quenching constant K_q ? diffusion controlled limit; (b) closely matched values of Stern–Volmer quenching constant and binding constant; (c) τ₀/τ = 1 (where τ₀ and τ are the unquenched and quenched tryptophan fluorescence lifetimes, respectively). Absorption spectrophotometric assays reveal that robinetin inhibits EYPC membrane lipid peroxidation and HbA glycosylation with high efficiency.     

Layer by layer assembly of catalase and amine-terminated ionic liquid onto titanium nitride nanoparticles modified glassy carbon electrode: Study of direct voltammetry and bioelectrocatalytic activity

A novel, simple and facile layer by layer (LBL) approach is used for modification of glassy carbon (GC) electrode with multilayer of catalase and nanocomposite containing 1-(3-Aminopropyl)-3-methylimidazolium bromide (amine terminated ionic liquid (NH₂-IL)) and titanium nitride nanoparticles (TiNnp). First a thin layer of NH₂-IL is covalently attached to GC/TiNnp electrode using electro-oxidation method. Then, with alternative self assemble positively charged NH₂-IL and negatively charged catalase a sensitive H₂O₂ biosensor is constructed, whose response is directly correlated to the number of bilayers. The surface coverage of active catalase per bilayer, heterogeneous electron transfer rate constant (k_s) and Michaelis–Menten constant (K_M) of immobilized catalase were 3.32 × 10⁻¹² mol cm⁻², 5.28 s⁻¹ and 1.1 mM, respectively. The biosensor shows good stability, high reproducibility, long life-time, and fast amperometric response with the high sensitivity of 380 μA mM⁻¹ cm⁻² and low detection limit of 100 nM at concentration range up to 2.1 mM.     

An ion pair receptor showing remarkable enhancement of anion-binding strengths in the presence of alkali metal cations

An ion pair receptor was prepared by coupling of a diazacrown ether and a rigid biindole scaffold bearing hydrogen bond donors of two indole NHs. The former serves as the cation-binding site and the latter functions as the anion-binding site. The anion-binding affinities to the receptor, determined by ¹H NMR titration experiments in 10% (v/v) DMSO-d₆/CD₃CN at 24 ± 1 °C, have been greatly improved when an alkali metal cation binds to the adjacent diazacrown ether. For example, the association constant between chloride and receptor alone is 7 M⁻¹, but the magnitudes increase into 120 M⁻¹, 14,000 M⁻¹, and 6200 M⁻¹ in the presence of lithium, sodium, and potassium ions, respectively. The enhanced binding affinities must be attributed to electrostatic interactions by possibly forming contact ion pairs.     

Thiourea-isothiouronium conjugate for strong and selective binding of very hydrophilic H2PO4 anion at the 1,2-dichloroethane-water interface

Potential use of a surfactant-like receptor is demonstrated at the 1,2-dichloroethane-water interface for strong and selective binding of H₂PO₄⁻ over Br⁻ and Cl⁻. The analysis by interfacial tensiometry reveals that the interfacial adsorption of a thiourea-isothiouronium conjugate, BT-C₁, is significantly stabilized by the binding of H₂PO₄⁻ with the adsorption constant of 1.7 × 10⁵ M⁻¹ while the interfacial adsorptivity of this receptor is relatively moderate for Br⁻ (0.81 × 10⁵ M⁻¹) and Cl⁻ (0.63 × 10⁵ M⁻¹). Such complexation-induced interfacial adsorption behaviors of BT-C₁ are discussed as a basis for the development of receptor-based chemical sensors for phosphate anions.     

Simple and precise detection of lipid compounds present within liposomal formulations using a charged aerosol detector

In recent decades the use of liposomal preparations as drug delivery systems has become very attractive in pharmaceutical development. Therefore, thorough characterization and quantification of the lipids which form liposomes is wished from both investigators and regulatory authorities when the application in humans is being considered. In this study a new HPLC method for the detection of lipids in liposomal formulations was established using corona charged aerosol detection (CAD) which has the advantage to be independent of the chemical properties of the analytes. The superiority of this method over UV detection was demonstrated. Compared to UV detection no absorption effects of the organic solvent in the mobile phase interfering with the lipid signals were observed with CAD. CAD showed good linearity (R² > 0.990) for all liposomal compounds. The acceptance criteria for precision including repeatability were met. The average recovery for each of the excipients of the liposomal formulation was in the range of 90.0–110%.     

A capacitive immunosensor for detection of cholera toxin

Contamination of food with biological toxins as well as their potential use as weapons of mass destruction has created an urge for rapid and cost effective analytical techniques capable of detecting trace amounts of these toxins. This paper describes the development of a sensitive method for detection of cholera toxin (CT) using a flow-injection capacitive immunosensor based on self-assembled monolayers. The sensing surface consists of monoclonal antibodies against the B subunit of CT (anti-CT), immobilized on a gold transducer. Experimental results show that the immunosensor responded linearly to CT concentrations in the range from 1.0 × 10⁻¹³ to 1.0 × 10⁻¹⁰ M under optimized conditions. The limit of detection (LOD) was 1.0 × 10⁻¹⁴ M. Two more analytical methods were employed for detection of CT using the same antibody namely, sandwich ELISA and surface plasmon resonance (SPR)-based immunosensor. The former had an LOD of 1.2 × 10⁻¹² M and a working range from 3.7 × 10⁻¹¹ to 2.9 × 10⁻¹⁰ M whereas, the later had an LOD of 1.0 × 10⁻¹¹ M and a linearity ranging from 1.0 × 10⁻⁹ to 1.0 × 10⁻⁶ M. These results demonstrate that the developed capacitive immunosensor system has a higher sensitivity than the other two techniques. The binding affinity of CT to the immobilized anti-CT was determined using the SPR-based immunosensor and an association constant (K_A) of 1.4 × 10⁹ M⁻¹ was estimated.     

Reverse atom transfer radical polymerisation (RATRP) of methacrylates using copper(I)/pyridinimine catalysts in conjunction with AIBN

N-n-Propyl-2-pyridylmethanimine, 1, N-n-octyl-2-pyridylmethanimine, 2, N-n-lauryl-2-pyridylmethanimine, 3, and N-n-octadecyl-2-pyridylmethanimine, 4 have been used in conjunction with copper(II) bromide and azo initiators for the reverse atom transfer radical polymerisation of a range of methacrylates. AIBN to Cu^IIBr₂ ratios of 0.5:1, 0.75:1 and 1:1 give PMMA with M_n 11 500 g mol⁻¹ (PDi = 1.24) (at 22% conversion), 12 500 g mol⁻¹ (PDi = 1.06) (at 83% conversion) and 10 900 g mol⁻¹ (PDi = 1.11) (at 84% conversion), respectively. A Cu^IIBr₂ complex is demonstrated to be needed at the start of the reaction for good control over molecular weight and polydispersity as reactions using Cu(I)Br as catalyst yielded PMMA of M_n 31 000 g mol⁻¹ (PDi = 2.90), reactions with no copper yield PMMA of M_n 33 000 g mol⁻¹ (PDi = 2.95). The RATRP of styrene was carried out using Cu^IIBr₂ as catalyst. AIBN to Cu^IIBr₂ ratio of 0.5:1, 0.75:1 and 1:1 gave PS with M_n = 12 400 g mol⁻¹ (PDi = 1.27) at low conversion, M_n = 15 500 g mol⁻¹ (PDi = 1.11) and 12 400 g mol⁻¹ (PDi = 1.38), respectively at ∼85% conversion. A series of block copolymers of MMA with BMA, BzMA and DMEAMA (15 600 g mol⁻¹ (PDi = 1.18), 13 300 g mol⁻¹ (PDi = 1.14) 15 300 g mol⁻¹ (PDi) = 1.16), using a PMMA macroinitiator were prepared. Emulsion polymerisation of MMA using [initiator]:[Cu^(II)Br₂] ratio = 0.5:1 with Brij surfactant gave a linear increase of M_n with respect to conversion, final M_n = 112 800 g mol⁻¹ (PDi = 1.42). Further reactions were carried out with [initiator]:[Cu^(II)Br₂] ratio = 0.75:1 and 1:1. Both giving PMMA with M_n ∼ 32 000 g mol⁻¹ (PDi ∼ 2.4). These reactions exhibit no control, this is because the azo initiator is present in excess and all of the monomer is consumed by a free radical polymerisation as opposed to a controlled reaction. Particle size analysis (DLS) showed the particle size between 160 and170 nm in all cases.     

Photoinduced interactions between colloidal TiO2 nanoparticles and calf thymus-DNA

The interaction of colloidal TiO₂ nanoparticles with calf thymus-DNA was studied by using absorption, FT-IR, steady state and time resolved fluorescence spectroscopic techniques. The apparent association constant has been deduced (K_app = 2.85 × 10³ M⁻¹) from the absorption spectral changes of the DNA-colloidal TiO₂ nanoparticles using the Benesi–Hildebrand equation. Addition of colloidal TiO₂ nanoparticles quenched the fluorescence of EtBr–DNA. The number of binding sites (n = 0.97) and the apparent binding constant (K = 6.68 × 10³ M⁻¹) were calculated from relevant fluorescence quenching data. The quenching, through a static mechanism, was confirmed by time resolved fluorescence spectroscopy.     

2π + 2π cycloaddition kinetics of some fluoro olefins and fluoro vinyl ethers

The second order 2π + 2π homo- and co-dimerization between various classes of fluorinated olefins has been investigated. The fluorinated olefins examined in this study were: (1) R_fOCFCF₂ (perfluorinated vinyl ethers); (2) R_fCFCF₂ (perfluorinated terminal olefins); (3) RCH₂CFCF₂; (4) PhOCFCF₂ (aryl perfluorinated vinyl ethers).Homo-dimerizations between vinyl ethers have an E_a between 20 and 24 kcal mol⁻¹ while homo-dimerizations between terminal olefins have an average E_a between 35 and 40 kcal mol⁻¹; vinyl groups have a second order cyclodimerization rate constant of formation between 1 × 10⁻⁷ and 1 × 10⁻⁴ M⁻¹ S⁻¹ while vinyl ethers have a second order cyclodimerization rate constant of formation = 1 × 10⁻¹ M⁻¹ s⁻¹. If there is a CH₂ group α to the terminal olefin, the E_a of cyclodimerization is about 7 kcal mol⁻¹) lower with respect to those olefins with a CF₂ α to the instauration. At 270 °C co-dimerizations have an average ΔS^≠ = −45 cal K⁻¹ mol⁻¹ and a second order rate constant of cyclodimerization ranging between 0.1 × 10⁻⁴ M⁻¹ S⁻¹ and 16 × 10⁻⁴ M⁻¹ S⁻¹ while homo-dimerizations have an average ΔS^≠ = −17 cal K⁻¹ mol⁻¹ and a second order rate constant which can span from 7 × 10⁻⁷ M⁻¹ S⁻¹ to as much as 1 × 10⁻¹ M⁻¹ S⁻¹ depending on the electronic nature of the perfluorinated terminal olefin.A good correlation between the electronegativity χ and the activation energy E_a demonstrates that “polarizing” groups, O, PhO, α to the olefin play an important role in the formation and stabilization of the cyclodimerization biradical intermediate.     

Studies on electrochemical behaviors of acyclovir and its voltammetric determination with nano-structured film electrode

A multi-wall carbon nanotubes (MWNTs)-dihexadecyl hydrogen phosphate (DHP) film-coated glassy carbon electrode (GCE) was fabricated, and the electrochemical behaviors of acyclovir on the MWNTs-DHP film-coated GCE were investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and chronocoulometry (CC). The oxidation peak current of acyclovir increased significantly and the peak potential shifted negatively at the MWNTs-DHP film-modified GCE, compared with that at a bare GCE. The results showed that this nano-structured film electrode exhibited excellent enhancement effects on the electrochemical oxidation of acyclovir. Consequently, a simple and sensitive electroanalytical method was developed for the determination of acyclovir. The oxidation peak current was proportional to the concentration of acyclovir from 8.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol/L. The detection limit was about 3.0 × 10⁻⁸ mol/L for 60 s accumulation at 0.00 V. The proposed method was demonstrated by using acyclovir tablets and the result was satisfying.