Mimicking the cell membrane with block copolymer membranes

Cell membranes are essential barriers in Nature. To understand their properties and functions and to develop desirable applications, a simple and elegant approach is to study membranes that mimic the cell membrane. Lipid bilayers represent simple models that are physiologically representative when in the form of mixtures of various lipids, but they are not adequately stable even when covered with amphipathic proteins or when combined with polymers, thus preventing technological applications. This makes necessary the design of completely synthetic membranes. In this respect, amphiphilic copolymers that self‐assemble under dilute aqueous conditions and generate supramolecular polymer vesicles or films are ideal candidates for synthetic membranes. Their versatility in terms of chemistry and properties (permeability, mechanical stability, thickness), if appropriately designed, enable the insertion of biological molecules, such as membrane proteins and biopores, or the attachment of biomolecules at their surfaces. Here, we present the domain of synthetic membranes based on amphiphilic copolymers beginning with their generation and up to their applications in medicine, the food industry, and technology. Even though significant progress has been made in combining them with membrane proteins, open questions remain with respect to desired properties that could accommodate biological molecules and support further development of the field, from both the point of view of fundamental understanding and of applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

HMBC‐1,1‐ADEQUATE via generalized indirect covariance: a high sensitivity alternative to n,1‐ADEQUATE

1,1‐ADEQUATE and the related long‐range 1,n‐ and n,1‐ADEQUATE variants were developed to provide an unequivocal means of establishing ²J_CH and the equivalent of ⁿJ_CH correlations where n = 3,4. Whereas the 1,1‐ and 1,n‐ADEQUATE experiments have two simultaneous evolution periods that refocus the chemical shift and afford net single quantum evolution for the carbon spins, the n,1‐variant has a single evolution period that leaves the carbon spin to be observed at the double quantum frequency. The n,1‐ADEQUATE experiment begins with an HMBC‐type ⁿJ_CH magnetization transfer, which leads to inherently lower sensitivity than the 1,1‐ and 1,n‐ADEQUATE experiments that begin with a ¹J_CH transfer. These attributes, in tandem, serve to render the n,1‐ADEQUATE experiment less generally applicable and more difficult to interpret than the 1,n‐ADEQUATE experiment, which can in principle afford the same structural information. Unsymmetrical and generalized indirect covariance processing methods can complement and enhance the structural information encoded in combinations of experiments e.g. HSQC‐1,1‐ or ?1,n‐ADEQUATE. Another benefit is that covariance processing methods offer the possibility of mathematically combining a higher sensitivity 2D NMR spectrum with for example 1,1‐ or 1,n‐ADEQUATE to improve access to the information content of lower sensitivity congeners. The covariance spectrum also provides a significant enhancement in the F₁ digital resolution. The combination of HMBC and 1,1‐ADEQUATE spectra is shown here using strychnine as a model compound to derive structural information inherent to an n,1‐ADEQUATE spectrum with higher sensitivity and in a more convenient to interpret single quantum presentation. Copyright © 2012 John Wiley & Sons, Ltd.     

Single crystal EPR studies of the reduced active site of [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F

In the catalytic cycle of [NiFe] hydrogenase the paramagnetic Ni-C intermediate is of key importance, since it is believed to carry the substrate hydrogen, albeit in a yet unknown geometry. Upon illumination at low temperatures, Ni-C is converted to the so-called Ni-L state with markedly different spectroscopic parameters. It is suspected that Ni-L has lost the "substrate hydrogen". In this work, both paramagnetic states have been generated in single crystals obtained from the [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F. Evaluation of the orientation dependent spectra yielded the magnitudes of the g tensors and their orientations in the crystal axes system for both Ni-C and Ni-L. The g tensors could further be related to the atomic structure by comparison with the X-ray crystallographic structure of the reduced enzyme. Although the g tensor magnitudes of Ni-C and Ni-L are quite different, the orientations of the resulting g tensors are very similar but differ from those obtained earlier for Ni-A and Ni-B (Trofanchuk et al. J. Biol. Inorg. Chem. 2000, 5, 36-44). The g tensors were also calculated by density functional theory (DFT) methods using various structural models of the active site. The calculated g tensor of Ni-C is, concerning magnitudes and orientation, in good agreement with the experimental one for a formal Ni(III) oxidation state with a hydride (H(-)) bridge between the Ni and the Fe atom. Satisfying agreement is obtained for the Ni-L state when a formal Ni(I) oxidation state is assumed for this species with a proton (H(+)) removed from the bridge between the nickel and the iron atom.     

Highly sensitive semi-quantitative field test for the determination of chromium (VI) in aqueous samples

A method for the semi-quantitative colorimetric determination of chromium(VI) at sub μg/L levels after sorptive preconcentration is presented. The method is based on the retention of the reaction product (preformed in liquid phase) between Cr(VI) and diphenylcarbazide on membrane embedded cation exchange material. The color intensity of the membrane can be correlated to Cr(VI) concentrations in the range 0.05–50 μg/L (i.e. almost three orders of magnitude lower than the conventional spectrophotometric procedure) with a detection limit of about 10 ng/L (using 50 mL of sample volume). Due to the visual inspection mode and comparative color detection the precision is only 30–80% rsd which, however, is regarded as sufficient for screening purposes. Analysis of real samples including different kinds of waters and extracts of soil and filter collected airborne particulate matter demonstrated the applicability of the method for fast and species selective screening. Recovery experiments generally gave reasonably good results, yet also revealed the risk of the conversion of chromium species during sample pretreatment procedures due to redox reactions. Received: 1 December 1997 / Revised: 23 February 1998 / Accepted: 10 March 1998     

Bismut(II)-chalkogenometallate(III) Bi2M4X8, Verbindungen mit Bi24+-Hanteln (M = Al,Ga; X = S,Se)

Bismuth(II) Chalcogenometallates(III) Bi₂M₄X₈, Compounds with Bi₂⁴⁺ Dumbbells (M = Al, Ga and X = S, Se) The ternary bismuth(II) chalcogenometallates(III) Bi₂M₄X₈ (with M = Al, Ga and X = S, Se) were synthesized from the binary chalcogenides M₂X₃ and Bi₂X₃ and elementary bismuth. All compounds are diamagnetic semiconductors with E_g (opt.) = 1.8–2.7 eV. The phases (except Bi₂Al₄Se₈) are thermodynamically stable and decompose peritectically above 965–1020 K. Bi₂Al₄Se₈ is metastable below 825 K and is obtained only by rapid quenching from T > 825 K. The isotypic compounds crystallize in a new tetragonal tP28 structure type (P4/nnc). The characteristic unit is the hitherto unknown clustercation Bi₂⁴⁺, with the mean bond length d(Bi–Bi) = 314.2 pm, the Raman frequency 102 cm^–1 ≤ ν_s ≤ 108 cm^–1, and the mean force constant of f = 0.68 N · cm^–1. The Electron Localization Function, ELF, shows the covalent Bi–Bi bond, the lone electron pairs of the ψ-octahedrally coordinated Bi(II) cations, and the polar character of the Bi–X bonds.     

Synthesis and Crystal Structure of KCuGd2S4: A Threedimensional Framework with Isolated Channels

The reaction of K₂S₅, Cu, Gd, and S in a 2 : 1 : 2 : 4 molar ratio at 450 °C yields yellow-orange needle-like cuboids of the new quaternary compound KCuGd₂S₄. The crystal structure represents a novel three-dimensional structure type of quaternary rare earth chalcogenides with alkali metal. The compound crystallizes in the orthorhombic space group Cmcm (No. 63) with a = 3.9921(1) Å, b = 13.523(3) Å, c = 13.802(3) Å, V = 745.1(3) ų, Z = 4. In the structure GdS₆ octahedra and CuS₄ tetrahedra are joined by common edges and corners forming corrugated layers parallel to (010). The GdS₆ octahedra are connected via common edges in the third dimension thus leading to the formation of a three-dimensional tunnel structure. The potassium cations are confined within the pentagonal shaped channels and are surrounded by eight sulfide anions each.     

Chloroselenate mit zwei- und vierwertigem Selen: 77Se-NMR-Spektren,Synthese und Kristallstrukturen von (PPh4)2SeCl6 · 2 CH2Cl2, (NMe3Ph)2SeCl6, (K-18-Krone-6)2SeCl6 · 2 CH3CN,PPh4Se2Cl9, (NEt4)2Se2Cl10, (PPh4)2Se3Cl8 · CH2Cl2 und (PPh4)2Se4Cl12 · CH2Cl2

Chloroselenates with Di- and Tetravalent Selenium: ⁷⁷Se-NMR-Spectra, Syntheses, and Crystal Structures of (PPh₄)₂SeCl₆ · 2 CH₂Cl₂, (NMe₃Ph)₂SeCl₆, (K-18-crown-6)₂SeCl₆ · 2 CH₃CN, PPh₄Se₂Cl₉, (NEt₄)₂Se₂Cl₁₀, (PPh₄)₂Se₃Cl₈ · CH₂Cl₂, and (PPh₄)₂Se₄Cl₁₂ · CH₂Cl₂ The title compounds were obtained from reactions of selenium and selenium tetrachloride with PPh₄Cl, NEt₄Cl, NMe₃PhCl, or (K-18-crown-6)Cl in dichloromethane or acetonitrile. (PPh₄)₂Se₃Cl₈ · CH₂Cl₂ was also formed from GeSe, PPh₄Cl and chlorine in acetonitrile. The ⁷⁷Se-NMR spectra of the solutions show the presence of dynamical equilibria which, depending on composition, mainly contain SeCl₂, SeCl₄, Se₂Cl₂, SeCl₆^2–, Se₂Cl₆^2–, and/or Se₂Cl₁₀^2–. Solutions of AsCl₃ and (PPh₄)₂Se₄ in acetonitrile upon chlorination with Cl₂ or PPh₄AsCl₆ yielded (PPh₄)₂Se₂Cl₆, while (PPh₄)₂As₂Se₄Cl₁₂ was the product after chlorination with SOCl₂. According to the X-ray crystal structure analyses the ions SeCl₆^2–, Se₂Cl₉^–, and Se₂Cl₁₀^2– have the known structures with octahedral coordination of the Se atoms. The structure of the Se₃Cl₈^2– ion corresponds to that of Se₃Br₈^2– consisting of three SeCl₂ molecules associated via two Cl^– ions. (PPh₄)₂Se₄Cl₁₂ · CH₂Cl₂ is isotypic with the corresponding bromoselenate and contains anions in which three SeCl₂ molecules are attached to a SeCl₆^2– ion; there is a peculiar Se–Se interaction.