Structural Elucidation of Isomeric Pyranoflavones by 2D-Noesy

Abstract

Unequivocal assignments of the uncoupled protons at H-3 and H-6/H-8 of pyranoflavones by 2D-NOESY experiments have made it possible to distinguish linear and angular isomers. This study also suggests that the linear structure (carpachromene, 1) assigned to a pyranoflavone isolated from Atalantia ceylanica should be revised to angular form (atalantoflavone, 4). 2D-NOESY is also useful for differentiating 3′4′- and 2′4′- disubstituted B-ring flavones.     

Stereochemical Assignment of Tertiary Hydroxyl Group in Diterpene Furan Glycosides by Pyridine Induced Shifts, 13C- and 2D-NMR Spectroscopy

The stereochemistry of the tertiary hydroxyl group in diterpene furan glycosides viz. cordifolisides D (1) and cordifoliside E (2) has been assigned on the basis of pyridine solvent induced shifts (PIS) in the ¹H and ¹³C NMR spectra. The methyl and the methylene groups occupying positions vicinal, 1,3-diaxial and geminal to the tertiary hydroxyl group were deshielded to different extent depending on the dihedral angle. The stereochemical assignments are well supported by ¹³C-γ shifts and 2D Overhauser spectroscopy.     

Kinetics and mechanism of oxidation of phosphorus oxyacids by pyridinium hydrobromide perbromide

Oxidation of three lower oxyacids of phosphorus, viz. phosphinic, phenylphosphinic and phosphorous acids by pyridinium hydrobromide perbromide (PHPB), is first-order with respect to both oxyacid and PHPB. There is no effect on addition of acylonitrile and pyridinium bromide. On oxidation, deuterated phosphinic and phosphorous acids exhibit substantial kinetic isotope effects. The effect of solvent composition on reaction rate indicates that the transition state is more polar than the reactants. Reaction rates were determined at different temperatures and the activation parameters calculated. Alternative mechanisms, involving the two tautomeric forms of the oxyacid, have been formulated and it has been concluded that the reaction proceeds through the pentacoordinated tautomer. Transfer of a hydride ion from the oxyacid to PHPB, in the rate-determining step, has been proposed.     

Column isoelectric focusing in natural pH gradients generated by biological buffers

Using 2 or 3 simple Good zwitterionic buffers at a 16 or 18 mmol/L final column concentration of the mixture, natural pH gradients of 4 to 8 and 3 to 9.5, respectively, were generated in a liquid LKB column. The pH gradients, stabilized by an anticonvective sucrose gradient, were linear, reproducible and stable in the electric field up to 5h. The pH gradients were used for isoelectric focusing of a number of impure proteins such as human hemoglobin, bovine serum albumin and chicken egg white lysozyme. The protein components could be well separated in the gradient, were easily recovered and appeared to be quite pure when analyzed by sodium dodecyl sulfate-gel electrophoresis. Furthermore, the pH gradient 4-8 was effectively used to isolate one of the acidic isozyme (pI 5.6) components of mouse liver alcohol dehydrogenase (EC 1.1.1.1) in an enzymatically active state, suggesting that the procedure does not denature proteins. The low cost, the ease with which the pH gradients are formed, their linearity, stability for a sufficient period to allow proteins to reach equilibrium and their subsequent recovery from buffer eluates should make the procedure interesting for electrofocusing of proteins.     

Fiber-optic surface plasmon resonance for vapor phase analyses

Fiber-optic sensors based on surface plasmon resonance (SPR) for direct refractive index (RI) measurements of samples with the RI between 1.00 and 1.30 are described. Most applications of SPR sensors are designed to function near the refractive index of water (1.3330 RI). The RI changes of aqueous solution (RI, ca. 1.34) can easily be monitored by silica-fiber (RI, 1.4601 at 550 nm) based SPR sensor. With regard to gas species detection, the fiber-optic SPR sensor must be modified for sensitivity to changes in refractive index near 1.0008 (i.e., RI of air). However, the silica waveguide has a prohibitively high RI for unmodified monitoring of the RI changes of gas. The silica-fiber based SPR probe design presented here is based upon the modification of the probe geometry to the ability to tune the SPR coupling wavelength/angle pair. In this study, the tapered silica-based fiber SPR sensors are shown to directly determine the RI changes of gas species and the density change of dry air.     

Spectrophotometric determination of uranium(VI) with 7-chloro-8-hydroxyquinoline-5-sulphonic acid

Uranium(VI) and 7-chloro-8-hydroxyquinoline-5-sulphonic acid form a 1:2 complex with λ_max at 355 nm, in aqueous medium at pH 6.6. A procedure for accurate spectrophotometric determination of hexavalent uranium with 7-chloro-8-hydroxyquinoline-5-sulphonic acid has been described. Optimum conditions, including the effect of pH, time, temperature, order of addition of the reagents, and excess of the reagent have been studied. Range for adherance to Beer's law, effective photometric range, molar absorptivity, and Sandell sensitivity have also been reported as 1–38 ppm, 5.7–28.5 ppm, 6575 and 0.036 μg of uranium/cm² respectively. The method has been compared with some other methods for determination of uranium spectrophotometrically, with regard to their sensitivities. Interferences of various ions in the measurement of the color have been investigated, and the tentative structure of the colored species in solution has been proposed.     

Spectrophotometric Studies on the Chelate Formation between Zirconium(IV) and Chromotrope 2R

The composition and stability of the chelate formed between Zr(IV) and chromotrope 2R has been investigated using spectrophotometric methods. The chelate is violet in color and has λmax at 530 nm. The composition, determined by different methods is 1 : 1. The chelate is stable between pH 1.0-3.0. The value of log K as determined by two different methods are 4.48±0.2 and 4.21±0.1 and the values of free energy of formation are —6.21±0.2 and —5.83±0.1 respectively at 30°C. The system obeys Beer's Law over a concentration range of 2.50 to 20.0 ppm of zirconium. The effective range of photometric determination is 3.50 to 19.05 ppm of zirconium.     

Effect of the excitation wavelength on the ultrafast charge recombination dynamics of donor-acceptor complexes in polar solvents

The effect of the excitation wavelength on the charge recombination (CR) dynamics of several donor-acceptor complexes (DACs) composed of benzene derivatives as donors and of tetracyanoethylene or pyromellitic dianhydride as acceptors has been investigated in polar solvents using ultrafast time-resolved spectroscopy. Three different wavelength effects have been observed. (1) With complexes exhibiting two well-separated charge-transfer bands, the CR dynamics was found to be slower by a factor of about 1.5 upon excitation in the high-energy band. This effect was measured in both fast and slow relaxing solvents and was discussed in terms of different DAC geometries. (2) When the CR is faster than diffusive solvation, a slowing down of the CR with increasing excitation wavelength accompanied by an increase of the nonexponential character of the dynamics was measured. This effect appears only when exciting on the red edge of the charge-transfer absorption band. (3) When the driving force for CR is small, both nonequilibrium (hot) and thermally activated CR pathways can be operative. The results obtained with such a complex indicate that the relative contribution of these two paths depends on the excitation wavelength.     

The spectrophotometric determination of molybdenum and uranium with quinalizarin

Quinalizarin reacts very sensitively with molybdenum and uranium to form colored chelates having λ_max at 520 and 630 nm, respectively. The molar ratio for both the chelates is 1:1 (metal:reagent). Optimum conditions including the range for adherence to Beer's law, effect of pH on color intensity, effect of excess of the reagent, sensitvity, and interference of the foreign ions has been reported for the photometric determination of these metal ions using quinalizarin, in 50% ethanolic medium and at 30 °C.     

Complex Formation of Lead (II) with Bromopyrogallol Red

The complex formation of lead with Bromopyragallol Red has been studied spectrophotometrically in an attempt to establish composition, stability, thermodynamic parameters and optimum condition for determining small amounts of lead. The blue complex of lead had λ_max at 630 nm (against a reagent blank). The composition as determined by different methods is 1:1 at pH 5.5±0.1. The mean value of log K, free energy (ΔG), the heat content (ΔH) and entropy change (ΔS) of the complex are found to be 6.28, ?10.46 kcal/mole and 6.31 e. u. respectively at 25°C. The molecular extinction coefficient and Sendall's sensitivity is 5000 and 0.0140 μg of lead/cm². The effect of diverse ions was examined with eighteen cations and ten anions, in the determination of lead.