Stereoselective total synthesis of protected sulfamisterin and its analogues

The stereoselective synthesis of sulfamisterin I and its unnatural analogues II and V in their protected form was achieved through a common strategy. The Wittig reaction of aldehydes VIII and IX with the C₁₄ hydrophobic side-chain X served as the key C-C connecting transformation. Subsequent functional group inter-conversions in the coupling products XI and XX completed the total synthesis.     

Determination of haloalkane dehalogenase activity by capillary zone electrophoresis

A new sensitive method has been developed for the determination of haloalkane dehalogenase activity. The enzymatic reactions were carried out directly in thermostatted autosampler vials and the formation of product - bromide or chloride ions - was monitored by sequential capillary zone electrophoresis runs. The determinations were performed in a 75 microm fused-silica capillary using 5 mM chromate, 0.5 mM tetradecyltrimethylammonium bromide (pH 8.4) as a background electrolyte, separation voltage 15 kV (negative polarity) and indirect detection at sample wavelength 315 nm, reference wavelength 375 nm for brominated and chlorinated substrates, respectively 0.1 M beta-alanine-HCl (pH 3.50) as a background electrolyte, separation voltage 18 kV (negative polarity) and direct detection at 200 nm for brominated substrates. The temperature of capillary was in both cases 25 degrees C. The method is rapid, can be automated, and requires only small amount of enzyme preparation and substrate.     

Kinetics of isotope exchange between [3-(iodophenyl)methyl]guanidine (mIBG) and [131I]-iodide

The reaction of isotope exchange between [3-(iodophenyl)methyl]guanidine, mIBG, and [¹³¹]-iodide in relatively concentrated solutions, in the presence of different ammonium salts, in a closed system, over the temperature range from 130 to 150°C, has been investigated. The reaction occurs either with (NH₄)₂SO₄ or CH₃COOH, which indicates that the reaction goes through some intermediate stages. Kinetic studies show the influence of the additives. The activation energies for the reaction with (NH₄)₂SO₄/H₂O, (NH₄)₂SO₄/CH₃COOH and CH₃COOH are 121.1, 115.1 and 84.5 kJ·mol^–1, respectively.     

Temperature dependent structure of spider silk by X-ray diffraction

We have investigated the structure of spider dragline silk by X-ray diffraction over a broad temperature range from room temperature up to thermal denaturation conditions. The dominating signal from the β-sheet crystallites is analyzed. Pronounced changes of scattering intensity starting at temperatures around 150 °C are observed. These changes are discussed in view of the respective lattice constants, crystal size, size distribution, crystallite number density and amino acid composition. PACS 87.68.+z; 87.15.-v; 87.64.Bx     

Two new XP(O)[NHC(CH3)3]2 phosphoramidates,with X = (CH3)2N and [(CH3)3CNH]2P(O)(O)

In N,N′‐di‐tert‐butyl‐N′′,N′′‐dimethylphosphoric triamide, C₁₀H₂₆N₃OP, (I), and N,N′,N′′,N′′′‐tetra‐tert‐butoxybis(phosphonic diamide), C₁₆H₄₀N₄O₃P₂, (II), the extended structures are mediated by P(O)...(H—N)₂ interactions. The asymmetric unit of (I) consists of six independent molecules which aggregate through P(O)...(H—N)₂ hydrogen bonds, giving R₂¹(6) loops and forming two independent chains parallel to the a axis. Of the 12 independent tert‐butyl groups, five are disordered over two different positions with occupancies ranging from to . In the structure of (II), the asymmetric unit contains one molecule. P(O)...(H—N)₂ hydrogen bonds give S(6) and R₂²(8) rings, and the molecules form extended chains parallel to the c axis. The structures of (I) and (II), along with similar structures having (N)P(O)(NH)₂ and (NH)₂P(O)(O)P(O)(NH)₂ skeletons extracted from the Cambridge Structural Database, are used to compare hydrogen‐bond patterns in these families of phosphoramidates. The strengths of P(O)[...H—N]_x (x = 1, 2 or 3) hydrogen bonds are also analysed, using these compounds and previously reported structures with (N)₂P(O)(NH) and P(O)(NH)₃ fragments.     

Forms of vibrations and structural changes in liquid state

Summary The forms of vibrations and displacements of particles in amorphous structures have been investigated. The particles, moving on highly non-linear amplitude, are responsible for the creation of disordered structures of amorphous bodies. The non-linear oscillators, even if 'few' in concentration, are characterized by unpredictable trajectories in phase space. The non-linear oscillators are fully developed in the liquid state above the crossover temperature T_cr and between T_cr and T_g their number decreases. Under T_g they completely disappear. The interconnection between the linear oscillators in blocks plays the most important role in the characteristic time spectra in liquid state. Using the additive properties of elements polarizibilities, the number of acoustical units in individual blocks at T_cr is estimated to be about 600 units. The diameter of blocks at T_cr was estimated to be about 1.8 nm. Even if the non-linear high amplitude motions disappear at solidification, the remnants of structural irregularity remain and the disordered structure of glass is formed.     

Sharp concentration of the chromatic number on random graphsG n,p

The distribution of the chromatic number on random graphsG _{n, p} is quite sharply concentrated. For fixedp it concentrates almost surely in √n ω(n) consecutive integers where ω(n) approaches infinity arbitrarily slowly. If the average degreepn is less thann ^1/6, it concentrates almost surely in five consecutive integers. Large deviation estimates for martingales are used in the proof.     

Study of the Reaction 2-(p-Nitrophenyl)Ethyl Bromide + OH(-) in Sulfobetaine Aqueous Micellar Solutions in the Presence and Absence of Added Salts

The reaction of dehydrobromination of 2-(p-nitrophenyl)ethyl bromide with hydroxide ions has been studied in aqueous micellar solutions of N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14. The kinetic effects of added salts (NaF, NaCl, NaBr, and NaNO(3)) on the reaction rate in SB3-14 aqueous micellar solutions have also been studied. They were rationalized by considering the binding of the anions, which come from the salt, to the sulfobetaine micelles and their competition with the reactive hydroxide ions for the micellar surface. The equilibrium binding constant of the 2-(p-nitrophenyl)ethyl bromide to the sulfobetaine micelles was estimated by recording the changes in the spectra of the organic substrate when the SB3-14 concentration in the micellar medium changed. This value was in agreement with that obtained from fitting of kinetic data. The second-order rate constant in the micellar pseudophase revealed that the reaction is faster in SB3-14 micelles than in water. This acceleration seems independent of the presence of added salts and can be explained by considering that SB3-14 micelles favor reactions in which charge is delocalized in the transition state. Copyright 2001 Academic Press.     

Speciation analysis of mercury in sediments, zoobenthos and river water samples by high-performance liquid chromatography hyphenated to atomic fluorescence spectrometry following preconcentration by solid phase extraction

A high-pressure microwave digestion was applied for microwave-assisted extraction (MAE) of mercury species from sediments and zoobenthos samples. A mixture containing 3 mol L⁻¹ HCl, 50% aqueous methanol and 0.2 mol L⁻¹ citric acid (for masking co-extracted Fe³⁺) was selected as the most suitable extraction agent. The efficiency of proposed extraction method was better than 95% with R.S.D. below 6%. A preconcentration method utilizing a “homemade” C18 solid phase extraction (SPE) microcolumns was developed to enhance sensitivity of the mercury species determination using on-column complex formation of mercury-2-mercaptophenol complexes. Methanol was chosen for counter-current elution of the retained mercury complexes achieving a preconcentration factor as much as 1000. The preconcentration method was applied for the speciation analysis of mercury in river water samples. The high-performance liquid chromatography-cold vapour atomic fluorescence spectrometric (HPLC/CV-AFS) method was used for the speciation analysis of mercury. The complete separation of four mercury species was achieved by an isocratic elution of aqueous methanol (65%/35%) on a Zorbax SB-C18 column (4.6 mm × 150 mm, 5 μm) using the same complexation reagent (2-mercaptophenol). The limits of detection were 4.3 μg L⁻¹ for methylmercury (MeHg⁺), 1.4 μg L⁻¹ for ethylmercury (EtHg⁺), 0.8 μg L⁻¹ for inorganic mercury (Hg²⁺), 0.8 μg L⁻¹ for phenylmercury (PhHg⁺).     

Optimization of the piezoelectric transducer response by means of electrical correcting circuits

A method of shortening the transient response of a piezoelectric transducer is described. It can be applied to thickness mode piezoelectric transducers of arbitrary electromechanical coupling. The system incorporates electrical correcting circuits and can produce a transient response with a duration much shorter than the transit time of an ultrasonic wave travelling through the piezoelectric plate.