In situ assembly of octahedral Fe(II) complexes for the enantiomeric excess determination of chiral amines using circular dichroism spectroscopy

A method for discriminating between α-chiral primary amine enantiomers is reported. The method utilizes circular dichroism (CD) spectroscopy and a sensing ensemble composed of 2-formyl-3-hydroxypyridine (4) and Fe(II)(TfO)(2). Aldehyde 4 reacts rapidly with chiral amines to form chiral imines, which complex Fe(II) to form a series of diastereomeric octahedral complexes that are CD-active in both the UV and visible regions of the spectrum. NMR studies showed that for enantiomerically pure imine complexes, the Δ-fac isomer is preferred. A statistical analysis of the distribution of stereoisomers accurately modeled the calibration curves for enantiomeric excess (ee). CD signals appearing in the UV region were bisignate, and the nulls of the CD signals were coincident with maxima in the UV spectrum, consistent with exciton coupling. Time-dependent density functional theory and semiempirical calculations confirmed that the CD signals in the UV region arise from coupling of the π-π* transitions in the imine chromophores and that they can be used to describe the signs and magnitudes of the curves accurately. The CD signals in the visible region arise from metal-to-ligand charge-transfer bands, and these signals can be used to determine the ee values of chiral amines with an average absolute error of ±5%. Overall, the strategy presented herein represents a facile in situ assembly process that uses commercially available simple reagents to create large optical signals indicative of ee values.     

New anti-HIV aptamers based on tetra-end-linked DNA G-quadruplexes: effect of the base sequence on anti-HIV activity

This communication reports on the synthesis and biophysical, biological and SAR studies of a small library of new anti-HIV aptamers based on the tetra-end-linked G-quadruplex structure. The new aptamers showed EC(50) values against HIV-1 in the range of 0.04-0.15 μM as well as affinities for the HIV-1 gp120 envelope in the same order of magnitude.     

Spectroscopic Study of the Growth Mechanism of Silver Microclusters

Polymer-protected silver microclusters have been obtained by room-temperature alcoholic reduction of metal ions in presence of a polymeric stabilizer (i.e., poly(N-vinyl-2-pyrrolidone) (PVP)). The method allows one to prepare bulk quantities of the material with a control of size from a molecular level. Spectrophotometric monitoring technique has been employed to investigate the influence of the aging of the protective PVP layer on the microcluster growth rate, under different reaction temperatures and PVP/ethylene glycol weight ratios. The aging time of the polymeric stabilizer solution has been found to play a fundamental role in the reproducibility of the microcluster growth kinetic. In addition, the microcluster growth process has resulted governed by a diffusion-controlled mechanism.     

Radical anions containing the dioxidated 1,2,5‐thiadiazole heterocycle. Part II

Radical anions from several 3,4‐aryl‐disubstituted derivatives of 1,2,5‐thiadiazole 1,1‐dioxide were accumulated through chemical reduction of the substrates in aprotic solvents. The radical anions were characterized by electron spin resonance and cyclic voltammetry. DFT theoretical calculations were also performed for the 3,4‐diphenyl derivative. The course of the reductions was followed using cyclic voltammetry. Uncommon reductants, such as amides, were found to be effective under certain conditions. Copyright © 2010 John Wiley & Sons, Ltd.     

Passive Attitude Control of Flexible Spacecraft from Quaternion Measurements

In this work, we propose a dynamic controller for a spacecraft with flexible appendages and based on attitude measurements. This control ensures the asymptotic fulfillment of the objectives in the case of rest-to-rest maneuvers when a failure occurs on the accelerometer sensors, so that the angular velocity is not available for feedback. Also, it is assumed that the modal variables describing the flexible elements are not measured. This is a lower level controller and is to be selected at the higher level by a supervisor when an emergency situation is detected.     

Molecular structure refinement of a segmented thermotropic polyester containing an aromatic triad mesogen

The polymorphic behavior of a polyester consisting of an aromatic triad mesogen and a flexible spacer has been investigated. The effect of thermal and mechanical treatment on the appearance of two crystalline phases stable at room temperature is discussed. The molecular packing (triclinic cell, space group P1 ) and the morphological parameters of the crystalline phase stabilized by drawing process are evaluated and refined. The whole-pattern method, based on the analysis of the whole x-ray diffraction pattern from fiber samples of polymer, has been employed. The molecular packing resembles very much that of polyethylene terephthalate. ©1995 John Wiley & Sons, Inc.     

Separation of food-related biogenic amines by ion-interaction reversed-phase high performance liquid chromatography. Tyramine,histamine, 2-phenylethylamine,tryptamine and precursor aminoacids. Application to red wine

Summary Methods for the separation of food-related biogenic amines (histamine, tyramine, 2-phenylethylamine and tryptamine) have been developed based on ion-interaction reversed-phase liquid chromatography.Two different interaction reagents have been comparatively used, namely octylamine ortho-phosphate (at wave-lengths of 230, 254 and 280 nm) and octylamine salicylate (at a wavelength of 254 nm). The different elution sequence orders shown by the investigated amines for the two reagents are discussed and compared.The detection limits obtained were 20 ppb for tryptamine ( =280 nm), 500 ppb for 2-phenylethylamine (=254 nm), 400 ppb for tyramine (=230 or 280 nm) and 900 ppb for histamine (=230 nm).The method was applied to the analysis of a five years old Italian red wine, in which 2-phenylethylamine (at a concentration of 72±3 ppm) and tryptamine (at a concentration of 4.0±0.3 ppm) were found to be present.     

Synthesis and characterization of a bunchy oligonucleotide forming a monomolecular parallel quadruplex structure in solution

A cluster of four d(TG₄T) hexanucleotides with the 3′-ends linked together by a bunchy spacer has been synthesized and shown to form a monomolecular parallel quadruplex 5 in solution characterized by four G-quartets. ¹H NMR spectroscopy and CD thermal denaturation experiments indicate the monomolecular quadruplex (5) to be more stable than the tetramolecular counterpart [d(TG₄T)]₄.     

Titration of phytic acid

Titrimetric assays for phytic acid by titration with iron(III) or thorium are described. Stannous chloride interferes in both systems, causing negative errors directly proportional to the amount of stannous chloride. The thorium:phytate ratio in the complex formed is 2:1, which is interpreted as resulting from the possession of two chelating centres by phytic acid.     

Absolute configuration of 1β,10β-epoxydesacetoxymatricarin isolated from Carthamus oxycantha by means of TDDFT CD calculations

The absolute configuration of 1β,10β-epoxydesacetoxymatricarin 1, a sesquiterpenoid isolated from Carthamus oxycantha, was established by TDDFT CD calculations in combination with single crystal X-ray analysis. Since the solid-state CD spectrum shows contributions from intermolecular interactions in the crystal, 1 represents a test case for our solid-state CD/TDDFT approach.