Isolation, characterization, and theoretical study of La2@C78

A new metallofullerene, La2@C78, has been synthesized by DC arc discharge method, isolated by high-performance liquid chromatography, and characterized by laser desorption time-of-flight mass spectrometry, UV-vis-NIR absorption, differential pulse voltammetry, 13C NMR spectroscopy, and theoretical calculations. The La2@C78/CS2 solution is dark violet and presents several characteristic absorption features at 647, 561, 533, and 386 nm, with an onset around 1000 nm. With respect to empty D3-C78, the capability of La2@C78 as an electron acceptor or donor is stronger. Addition of 1,1,2,2-tetrakis(2,4,6-trimethylphenyl)-1,2-disirane to La2@C78 photochemically, as well as thermally, affords bis- and mono-adducts. Theoretical studies and 13C NMR spectroscopic analysis of La2@C78 indicate that it possesses a D3h-C78 cage (78:5).     

A calorimetric study of the enthalpy of enzymatic hydrolysis of components of a product from vegetable raw materials for enteral feeding

The enthalpy of hydrolysis of potato starch and some starch-containing components of Lider-1 nutrition product for enteral (tube) feeding in the presence of α-amylase enzyme was studied on a DAK-1-1 differential microcalorimeter.     

Calorimetric determination of solubility of water in pectin and physical state diagram of a pectin-water system

For the mixtures of pectin of different esterification degrees and water, heat capacities in the range of 80–320 K have been measured, and DTA measurements have been carried out in the range 80–450 K. The effect of water on the physical transitions of pectin has been studied, and the solubility of water in pectins of different esterification degrees at the temperature of melting of a liquid component has been determined. On the basis on the experimental data, the physical state diagram of the pectin-water system has been constructed and analyzed for the whole range of component concentrations within a wide temperature interval.     

Changes produced in the electrical resistivity of ErH2 thin films when converted to ErH3 due to hydrogen treatment

The resistivity of thin films (80–200 Å) of ErH₂ increases sharply when heated for 2 h at 300 °C in vacuum in the presence of hydrogen gas at 10^–2 Torr. This confirms that the films, originally metallic conductor, have become converted to semiconducting ErH₃ which is in conformity with the structural studies. The negative values of TCR also indicate the semiconducting nature of the hydrogen treated films. Activation energies of the films have been evaluated.     

Total synthesis of (+/-)-stemodinone via an efficient ring-exchange strategy

A total synthesis of (+/-)-stemodinone, a tetracyclic stemodane diterpene, from the known tricyclic methyl olefin 11 is described. The key steps involve an efficient ring-exchange reaction and palladium(0)-catalyzed lactone migration. The ring-exchange strategy for controlling the stereochemistry was based on an initial Diels-Alder reaction to form a new ring followed by cleavage of the original ring. Cleavage of the original ring of the Diels-Alder adduct 9 was achieved by an initial regio- and chemoselective Baeyer-Villiger oxidation followed by the Pd(0)-catalyzed lactone-migration reaction reported by us.     

Xanthine Sensors Based on Anodic and Cathodic Detection of Enzymatically Generated Hydrogen Peroxide

A xanthine biosensor was fabricated by the covalent immobilization of xanthine oxidase (XO) onto a functionalized conducting polymer (Poly‐5, 2′: 5′, 2″‐terthiophine‐3‐carboxylic acid), poly‐TTCA through the formation of amide bond between carboxylic acid groups of poly‐TTCA and amine groups of enzyme. The immobilization of XO onto the conducting polymer (XO/poly‐TTCA) was characterized using cyclic voltammetry, quartz crystal microbalance (QCM), and X‐ray photoelectron spectroscopy (XPS) techniques. The direct electron transfer of the immobilized XO at poly‐TTCA was found to be quasireversible and the electron transfer rate constant was determined to be 0.73 s^?1. The biosensor efficiently detected xanthine through oxidation at +0.35 V and reduction at ?0.25 V (versus Ag/AgCl) of enzymatically generated hydrogen peroxide. Various experimental parameters, such as pH, temperature, and applied potential were optimized. The linear dynamic ranges of anodic and cathodic detections of xanthine were between 5.0×10^?6?1.0×10^?4 M and 5.0×10^?7 to 1.0×10^?4 M, respectively. The detection limits were determined to be of 1.0×10^?6 M and 9.0×10^?8 M with anodic and cathodic processes, respectively. The applicability of the biosensor was tested by detecting xanthine in blood serum and urine real samples.     

Modification of carbon ceramic electrode prepared with sol-gel technique by a thin film of chlorogenic acid: application to amperometric detection of NADH

The carbon ceramic electrode prepared with sol-gel technique is modified by a thin film of chlorogenic acid (CGA). By immersing the carbon ceramic electrode in aqueous solution of chlorogenic acid at less than 2 s a thin film of chlorogenic acid adsorbed strongly and irreversibly on the surface of electrode. The cyclic voltammetry of the resulting modified CCE prepared at optimum conditions shows a well-defined stable reversible redox couple due to hydroquinone/quinone system in both acidic and basic solutions. The modified electrode showed excellent electrocatalytic activity toward NADH oxidation and it also showed a high analytical performance for amperometric detection of NADH. The catalytic rate constant of the modified carbon ceramic electrode for the oxidation of NADH is determined by cyclic voltammetry measurement. Under the optimised conditions the calibration curve is linear in the concentration range 1-120 μm. The detection limit (S/N = 3) and sensitivity are 0.2 μM and 25 nA μM⁻¹.The results of six successive measurement-regeneration cycles show relative standard deviations of 2.5% for electrolyte solution containing 1 mM NADH, indicating that the electrode renewal gives a good reproducible and antifouling surface. The advantages of this amperometric detector are: high sensitivity, excellent catalytic activity, short response time t < 2 s, remarkable long-term stability, simplicity of preparation at short time and good reproducibility.     

Chlorobiphenyls in sewage sludge; comparison of extraction methods

Six extraction methods for the analysis of PCBs (CB-28, CB-52, CB-101, CB-118, CB-138, CB-153 and CB-180) in sewage sludge were tested. A certified reference material (CRM 392) was used for the evaluation of the performance of the methods. Soxhlet-Dean-Starch with toluene as solvent, Soxhlet with hexane:acetone (2:3), cold digestion/saponification with 2 mol/L KOH in methanol followed by partition with hexane, and sonicated liquid-solid extraction with hexane:acetone (1:1) produced accurate results (97%, 93%, 104%, and 88%, respectively) with acceptable precisions (6.2%, 6.8%, 15% and 12%, respectively). Results in close agreement with the certified value for all congeners were obtained by treatment with BF₃-methanol prior to partition with dichloromethane. However, this is a tedious procedure and involves the use of hazardous compounds. Cyclic steam distillation produced results with an accuracy of around 80% and a good precision (5.2%). The very low consumption of solvents and other expensive chemicals by this technique and the possibility of analyzing the extract directly without clean-up make it an interesting alternative to the more sophisticated methods. Column elution with dichloromethane was found to be less efficient (61%), but it is a rapid, direct method with a low consumption of solvents and it may therefore serve as screening method. Received: 29 April 1997 / Revised: 30 July 1997 / Accepted: 6 August 1997     

Tandem on-line continuous separation and determination of arsenic by inductively coupled plasma atomic emission spectrometry

The principle of tandem on-line continuous separation techniques as an alternative means of introducing samples into plasmas was applied to the development of a sensitive, selective and convenient method for the determination of arsenic by inductively coupled plasma atomic emission spectrometry (ICP-AES). Arsenic is continuously extracted as AsI₃ into xylene from the sample dissolved in 0.1 M potassium iodide solution in 7.2 M hydrochloric acid. The xylene phase (containing the analyte) is continuously mixed on-line with NaBH₄ in dimethylformamide and acetic acid solutions. Arsine is thus continuously generated directly from the organic phase and is separated in a gas—liquid separation device which prevents most of the xylene phase vapour from reaching the ICP. The system was optimized for the continuous extraction of AsI₃, the direct generation of arsine from xylene and the final ICP determination of arsenic. Finally, the tandem on-line continuous separation ICP detection system was applied to the determination of arsenic in real samples (white metal, cast iron, cupro-nickel and orchard leaves standard materials). Very good agreement between the experimental results and the certified values was obtained.