Chiroptical spectroscopic determination of molecular structures of chiral sulfinamides: t-butanesulfinamide

The absolute configuration of t-butanesulfinamide has been determined as (-)-(S) using three different chiroptical spectroscopic methods, namely, vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotatory dispersion (ORD). Furthermore, the predominant conformation of this molecule is determined to have S=O and NH2 groups staggered with respect to the three methyl groups and to have amine hydrogens in gauche orientation with respect to S=O. The quality of predictions obtained for vibrational properties, namely, vibrational absorption and VCD, is found to be satisfactory with the B3LYP functional and 6-31G* basis set. However, this basis set is found to be inadequate for obtaining reliable predictions of electronic properties, namely, electronic absorption and ECD, but a larger aug-cc-pVDZ basis set is found to provide satisfactory prediction of electronic properties. t-Butanesulfinamide serves as an example which invalidates the recommendation of using the 6-31G* basis set for molecules that exhibit the same sign for the long-wavelength ECD band and ORD. This molecule also emphasizes the importance of simultaneous investigation of ECD and ORD, and the use of multiple chiroptical spectroscopic methods, for reliable determination of molecular stereochemistry.     

Presmectic wetting and supercritical-like phase behavior of octylcyanobiphenyl liquid crystal confined to controlled-pore glass matrices

The influence of controlled-pore glass (CPG) confinement on the phase behavior of octylcyanobiphenyl liquid crystal (LC) is studied by means of x-ray scattering and high precision calorimetry. For CPG samples with pore diameter 2R>24 nm, the smectic order parameter temperature dependence eta(T) reveals apparent presmectic ordering far above the bulk smectic A-nematic (SmA-N) phase transition for both nontreated and silane-treated CPG matrices. The behavior of eta(T) is qualitatively similar in all samples, well obeying the mean field approach (MFA) in which the surface wetting tendency plays the dominant role. In contrast, the critical fluctuations remain important in the specific heat data, which cannot be described within the MFA. We show experimentally that randomness and surface wetting become dominant over finite-size effects for 2R approximately<10 nm, in agreement with theoretical analysis. In nontreated samples, the noncritical character of the static disorder and the interfacial LC-CPG coupling almost completely suppress the quasi-SmA-N and nematic-isotropic phase transitions at 2R approximately 15.1 and approximately 7.5 nm, respectively.     

Synthesis and reactivity of copper(I) complexes containing a bis(imidazolin-2-imine) pincer ligand

The new pincer ligand 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine (TL(tBu)) has been prepared in high yield from 2,6-bis(hydroxymethyl)pyridine (1) and 1,3-di-tert-butylimidazolin-2-imine (3). Reaction of TL(tBu) with [Cu(MeCN)4]PF6 affords the highly reactive copper(I) complex [(TL(tBu))Cu]PF6, [5]PF6, which forms the stable copper(I) isocyanide complexes [6a]PF6 (nu(CN) = 2179 cm(-1)) and [6b]PF6 (nu(CN) = 2140 cm(-1)) upon addition of tert-butyl or 2,6-dimethylphenyl isocyanide, respectively. For the cations 6a and 6b, DFT calculations reveal ground-state electronic structures of the type [(TL(tBu)-kappaN(1):kappaN(2))Cu(CNR)] with tricoordinate geometries around the copper atoms. Exposure of [5]PF6 to the air readily leads to trapping of atmospheric CO2 to form the square-planar complex [(TL(tBu))Cu(HCO3-kappaO)]PF6, [7]PF6, with the bicarbonate ligand adopting a rarely observed monodentate coordination mode. In chlorinated solvents such as dichloromethane or chloroform, [5]PF(6) rapidly abstracts chloride by reductive dechlorination of the solvent to yield [(TL(tBu))CuCl]PF6, [8]PF6 quantitatively. Reaction of TL(tBu) with copper(I) bromide or chloride affords complexes 9a and 9b, respectively, for which X-ray diffraction analysis, low-temperature NMR experiments and DFT calculations reveal the presence of a kappa(2)-coordinated ligand of the type [(TL(tBu)-kappaN(1):kappaN(2))CuX]. In solution, complex 9b undergoes slow disproportionation forming the mixed-valence copper(II)/copper(I) system [(TL(tBu))CuCl][CuCl2], [8]CuCl2 with a linear dichlorocuprate(I) counterion.     

Total synthesis and structural elucidation of azaspiracid-1. Synthesis-based analysis of originally proposed structures and indication of their non-identity to the natural product

The key building blocks (6, 7, and 8) for the intended construction of the originally proposed structures of azaspiracid-1, a potent marine-derived neurotoxin, were coupled and the products elaborated to the targeted compounds (1a,b) and their C-20 epimers (2 and 3). The assembly of the three intermediates was accomplished by a dithiane-based coupling reaction that united the C(1)-C(20) (7) and C(21)-C(27) (8) fragments, followed by a Stille-type coupling which allowed the incorporation of the C(28)-C(40) fragment (6) into the growing substrate. Neither of the final products (1a,b) matched the natural substance by TLC or (1)H NMR spectroscopic analysis, suggesting one or more errors in the originally proposed structure for this notorious biotoxin.     

Comparison of different separation technologies for proteome analyses: isoform resolution as a prerequisite for the definition of protein biomarkers on the level of posttranslational modifications

In this article we evaluate methods used to reveal the molecular complexity, which is generated in biological samples by posttranslational modifications (PTM) of proteins. We show how distinct molecular differences on the level of phosphorylation sites in a single protein (ovalbumin) can be resolved with different success using 1D and 2D gel-electrophoresis and reversed-phase liquid chromatography (LC) with monolithic polystyrol-divinylbenzol (PS-DVB) columns for protein separation, and matrix-assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF MS) for protein identification. Phosphorylation site analysis was performed using enzymatic dephosphorylation in combination with differential peptide mass mapping. Liquid chromatography-MALDI-TOF MS coupling with subsequent on-target tryptic protein digestion turned out to be the fastest method tested but yielded low resolution for the analysis of PTM, whereas 2D gel-electrophoresis, due to its unique capability of resolving highly complex isoform pattern, turned out to be the most suitable method for this purpose. The evaluated methods complement one another and in connection with efficient technologies for differential and quantitative analysis, these approaches have the potential to reveal novel molecular details of protein biomarkers.     

Morphological characterization of ex situ prepared bismuth film electrodes and their application in electroanalytical determination of the biomolecules

A study on the preparation and characterization of the potentiostatically prepared bismuth films (BiFs), in order to obtain satisfactory electroanalytical tool, is presented. BiFs formed on glassy carbon were characterized by electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The dependence of the BiFs properties upon electrolytes composition and electrochemical parameters are discussed and diagnostic criteria that allow estimation of the BiF morphology are proposed. Analytical performance data of the formed BiF electrodes were obtained by their application to the determination of glutathione (GSH) and folic acid (FA) using square-wave cathodic stripping voltammetry. The dependence of the analytical performance of the formed BiFs on their specific surface areas, along with their different morphology, is discussed. Adequate method and parameters for the electrochemical formation of optimal BiF, in order to fulfill the analytical requirements, are proposed. The best analytical performance was obtained with films formed from acetic buffer solution spiked with EDTA, as a consequence of the improved surface coverage and most arranged homogenous structure of the film. This electrode displays a linear response range toward GSH with estimated detection limit of 0.005 μM and sensitivity of 3.28 μA μM^?1 for linear range of 0.01 to 0.1 μM. Also, the utilization of the BiF electrode for the determination of FA was demonstrated by direct electroreduction of FA.     

Thermal destruction of spin-polaron bands in the narrow-gap correlated semiconductors FeGa3 and FeSb2

We report muon spin rotation spectra in the narrow-gap semiconductors FeGa(3) and FeSb(2) consistent with a narrow band of small spin polarons (SPs). The characteristic sizes obtained for these SPs are R(FeGa(3)) ≈ 0.3-0.6 nm and R(FeSb (2)) ≈ 0.3 nm, respectively. Such SP states are expected to originate from the exchange correlations between localized and itinerant electrons. Our data suggest that SP bands are formed at low temperature, but are destroyed by thermal fluctuations above 10 K in FeGa(3) and above 7 K in FeSb(2). Formation of such SP band states can explain many of the low-temperature properties of these materials.