A novel entry to xanthones by an intramolecular Diels-Alder reaction involving 2-(1,2-dichlorovinyloxy) aryl dienones

A wide array of synthetic methods are described in the literature for the preparation of xanthones—a prominent class of tricyclic molecules that occur widely in nature. Majority of these reported methods involve linking the two aromatic rings and forming the central pyrone ring using a variety of classical and non-classical cyclization strategies. In a conceptually different approach, we describe here a new xanthone synthesis wherein both the pyrone and the second aromatic rings were forged in a single step by an intramolecular cycloaddition reaction involving 2-(1,2-dichlorovinyloxy) aryldienones.     

Investigation of phenolic acids in yacon (Smallanthus sonchifolius) leaves and tubers

Thin-layer chromatographic (TLC) screening of crude extracts of dried leaves and tubers of yacon (Smallanthus sonchifolius, Asteraceae) and products of acid hydrolysis of tubers on the silica gel HPTLC plates using the developing solvents ethyl acetate-formic acid-water (85:10:15, v/v/v) and n-hexane-ethyl acetate-formic acid (20:19:1, v/v/v) proved the presence of chlorogenic, caffeic and ferulic acid. These phenolic acids were isolated from the crude extract of yacon leaves by preparative TLC, and identified after elution by HPLC/MS, as well as by direct injection of the crude extract into the HPLC/MS system. Acid hydrolysis of tubers released the increased amount of phenolic acids (e.g. caffeic acid and ferulic acid), flavonoid quercetin and an unidentified flavonoid, which was detected by TLC analysis. Ferulic acid, isomers of dicaffeoylquinic acid and still an unidentified derivative of chlorogenic acid (Mr = 562) as constituents of yacon leaves and ferulic acid as constituent of yacon tubers are reported here for the first time. These acids gave significant contribution to the radical scavenging activity detected directly on the TLC plate sprayed with 1,1-diphenyl-2-picrylhydrazyl (DPPH).     

N-Amination of pyrrole and indole heterocycles with monochloramine (NH2Cl)

A survey of several electrophilic ammonia reagents for the N-amination of indole- and pyrrole-containing heterocycles revealed that monochloramine (NH(2)Cl) is an excellent reagent for this transformation. Pyrroles and indoles containing a variety of substitution were aminated on nitrogen with isolated yields ranging from 45% to 97%.     

Capillary electrophoresis sequencing of small ssDNA molecules versus the Ogston regime: fitting data and interpreting parameters

We study the mobility of short ssDNA fragments (approximately 30-500 bases) separated by capillary electrophoresis in entangled polymer solutions. Although this corresponds to what is commonly called the Ogston regime, the corresponding sieving concept has never been defined properly nor tested quantitatively. We consider three formulas that have been suggested to fit data in this range of ssDNA sizes, and we discuss how their free parameters are related to actual physical parameters. We test these formulas with new data obtained in our laboratory using a commercial poly-N,N-dimethylacrylamide sieving matrix. Our results show that all three formulas provide decent fits. However, the traditional Ogston equation produces fitting parameters that appear to lack physical meaning. Surprisingly, all three approaches predict that the effective pore size and fiber radius are almost equal. This is the first step towards the development of a systematic approach to optimizing sequencing systems for this size range.     

Changes in the energy distribution between chlorophyll-protein complexes of thylakoid membranes from pea mutants with modified pigment content. I. Changes due to the modified pigment content

The low-temperature (77 K) emission and excitation chlorophyll fluorescence spectra in thylakoid membranes isolated from pea mutants were investigated. The mutants have modified pigment content, structural organization, different surface electric properties and functions [Dobrikova et al., Photosynth. Res. 65 (2000) 165]. The emission spectra of thylakoid membranes were decomposed into bands belonging to the main pigment protein complexes. By an integration of the areas under them, the changes in the energy distribution between the two photosystems as well as within each one of them were estimated. It was shown that the excitation energy flow to the light harvesting, core antenna and RC complexes of photosystem II increases with the total amount of pigments in the mutants, relative to the that to photosystem I complexes. A reduction of the fluorescence ratio between aggregated trimers of LHC II and its trimeric and monomeric forms with the increase of the pigment content (chlorophyll a, chlorophyll b, and lutein) was observed. This implies that the closer packing in the complexes with a higher extent of aggregation regulates the energy distribution to the PS II core antenna and reaction centers complexes. Based on the reduced energy flow to PS II, i.e., the relative increased energy flow to PS I, we hypothesize that aggregation of LHC II switches the energy flow toward LHC I. These results suggest an additive regulatory mechanism, which redistributes the excitation energy between the two photosystems and operates at non-excess light intensities but at reduced pigment content.     

Elimination of chemical shift artifacts of thoracic spine with contrast-enhanced FLAIR imaging with fat suppression at 3.0 T

The purpose of this study was to assess the effect of chemical shift artifacts and fat suppression between contrast-enhanced T1-weighted fast spin-echo (FSE) sequence with fat suppression and contrast-enhanced T1-weighted fluid attenuated inversion recovery (FLAIR) sequence with fat suppression in magnetic resonance imaging (MRI) of the thoracic spine at 3.0T. Forty patients, who underwent MRI examination, were recruited and analyzed both qualitatively and quantitatively. Due to chemical shift artifacts in the T1-weighted FSE, 14 of the patients were found to be of non-diagnostic value. On the contrary, in 11 of those 14 patients, no chemical shift artifacts were observed in the T1-weighted FLAIR sequence. Regarding the efficiency of fat suppression, both sequences achieved successful fat suppression. Consequently, the use of T1-weighted FLAIR fat suppression after contrast administration sequence seems to eliminate or significantly reduce image quality deterioration stemming from chemical shift artifacts in thoracic spine examinations.     

Novel Copolymers of Styrene. Alkyl and Alkoxy Ring-Substituted 2-Phenyl-1,1-dicyanoethylenes

Novel copolymers of trisubstituted ethylene monomers, alkyl and alkoxy ring-substituted 2-phenyl-1,1-dicyanoethylenes, RC₆H₄CH=C(CN)₂ (where R is 2-ethyl, 4-i-propyl, 4-butyl, 4-i-butyl, 4-t-butyl, 2-ethoxy, and 4-hexyloxy) and styrene were prepared at equimolar monomer feed composition by solution copolymerization in the presence of a radical initiator (ABCN) at 70°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by IR, ¹H and ¹³C-NMR. The order of relative reactivity (1/r ₁) for the monomers is 4-t-butyl (1.45) > 4-i-propyl (1.38) > 2-ethyl (1.37) > 4-hexyloxy (1.33) > 4-i-butyl (1.24) > 2-ethoxy (1.13) > 4-butyl (1.04). High T _g of the copolymers, in comparison with that of polystyrene) indicates a substantial decrease in chain mobility of the copolymer due to the high dipolar character of the trisubstituted ethylene monomer unit. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 200–500°C range with residue (1–10% wt.), which then decomposed in the 500–800°C range.     

Unpaired electrons,spin polarization,and bond orders in radicals from the 2‐RDM in orbital spaces: Basic notions and testing calculations

Adopting the second‐order reduced density matrix level, the conventional α‐ and β‐spin populations in radicals are split into paired and unpaired or electropon (referring to the simultaneous occurrence of an electron and a hole of opposite spins in an orbital) populations. This analysis gives the possibility to distinguish the (un)favorable for chemical bonding electronic interactions by means of positive or negative Coulomb and/or Fermi correlations of two electropons. To overcome the conceptual difficulties originated from the subtle superposition of unpaired electrons due to spin density and those responsible for chemical bonding, we use the notion of properly unpaired electrons. The quantity describing this notion provides a global picture for the ability of electrons of a given orbital to form covalent bonds with the electrons of all remaining orbitals. More detailed information, concerning the behavior of electrons in two distinct target orbitals, is obtained by means of the two‐electropon correlations. As shown, the boundary values of the used quantities are physically meaningful, and the whole theory is tested from various points of view concerning: localized and delocalized radical centers, orthogonal and nonorthogonal orbitals, uncorrelated and correlated levels, Coulomb and Fermi correlations. We also check the electropon based analysis by investigating the spin polarization effects and bond orders in radicals. The tests are achieved for well‐known radicals, and to preserve the stability of the numerical results and the invariance of the obtained conceptual pictures, we used natural basis sets introduced within the natural bond orbital methodology. © 2014 Wiley Periodicals, Inc.     

Analysis of Glycosaminoglycan Oligosaccharides by Combined HPTLC/MALDI-TOF MS: Reduced Silica Gel Thickness Leads to Improved Spectral Qualities and Reduced Side Reactions

Thin-layer chromatography (TLC) is a simple, fast and inexpensive separation method which can be applied to virtually all natural products including oligosaccharides. Unfortunately, however, the unequivocal identification of a TLC spot is normally difficult. Fortunately, this problem can be minimized when mass spectrometry (MS) such as matrix-assisted laser desorption and ionization time-of-flight is used to identify the TLC spots. This work is dedicated to the TLC/MS analysis of oligosaccharides derived from native chondroitin sulfate and hyaluronan. We will show that the thickness of the silica gel layer (200 versus 100 µm) has a tremendous influence on the quality of the mass spectra: a reduced silica gel thickness enhances the spectral quality and, in particular, improves the achievable signal-to-noise ratio. Additionally, unwanted formylation of the GAG oligosaccharides (which occurs due to the high moiety of formic acid in the mobile phase) can also be minimized if MS-grade HPTLC plates are used.

     

Evaluation of cyclodextrin- and cyclofructan-based chiral selectors for the enantioseparation of psychoactive substances in capillary electrophoresis

During this study, a simple and easy-to-prepare electrophoretic method was developed for the enantioseparation of amphetamine and cathinone derivatives. Different types of β-cyclodextrin and cyclofructan-based chiral selectors (CSs), both native and derivatized, were utilized, and the most effective ones, in terms of resolution and analysis time, were identified. In addition, several electrophoretic parameters, such as background electrolyte concentration and pH, and CS concentration, were examined to optimize the separation conditions. Under the optimal electrophoretic conditions, 10 psychoactive substances were enantiomerically separated using 1 mM sulfated cyclofructan-6 (SCF-6) for the amphetamine derivatives and 1 mM sulfated cyclofructan-7 (SCF-7) for the cathinone derivatives dissolved in an aqueous solution of 20-mM monobasic sodium phosphate at pH 2.5, a temperature of 25°C, and an applied voltage of 25 kV. In addition, the method was validated by estimating the intra- and interday precision.