Phase behavior of aqueous mixtures of 2-phenylbenzimidazole-5-sulfonic acid and cetyltrimethylammonium bromide: hydrogels, vesicles, tubules, and ribbons

We studied the phase behavior and aggregation in mixed aqueous solutions of the anionic UV-absorber 2-phenylbenzimidazole-5-sulfonic acid sodium salt, PhBSA (Na salt), and the cationic surfactant cetyltrimethylammonium bromide, CTAB. The mixtures of the two components behave similarly to catanionic surfactant mixtures. The samples on the PhBSA-rich side have low viscosity and are turbid. The turbidity, due to uni- and multilamellar vesicles (SUVs and MLVs), increases with the mole ratio of CTAB. The interbilayer distance inside the MLV changes with the mole ratio of the two components from a few 10 nm for the 7:3 (molar ratio of PhBSA, Na salt, to CTAB) system to practically zero for the 5:5 mixture. The latter mixture forms a precipitate within less than 1 h. With the exception of the 5:5 mixture, all samples on the PhBSA-rich side are stable for many days. After that period, within one more day, the turbid vesicle phases are transformed into more or less clear hydrogels. We found that the gelation is due to the formation of very long stiff tubules about 14 nm in diameter, which is independent of the mixing ratio of the samples. The hydrogels and the tubules melt around 45 degrees C. On the CTAB-rich side, the 4:6 sample behaves like the 6:4 sample, whereas at 3:7 a precipitate was found to form shortly after mixing. At still smaller PhBSA (Na salt) to CTAB ratios, only clear, viscoelastic solutions are found that do not change with time. We determined the micellar structures in the samples by cryo-TEM and by SAXS. The rheological properties of the hydrogels and of the viscoelastic samples were characterized by oscillating rheological measurements. DSC measurements indicated that the tubules are in a semicrystalline state and melt at around 45 degrees C. The semicrystalline bilayer of the tubules seems to have a 1:1 composition of PhBSA to CTAB. The excess PhBSA seems to be adsorbed on the tubules. It is assumed that the stiffness of the bilayer of the vesicles and the stiffness of the tubules are due to the stiffness of the PhBSA molecule.     

Synthesis,structure and characterization of [Mg7(μ3-OCH2CH2OMe)6(μ-OCH2CH2OMe)6][Al(n-Bu)4]2 – A cationic heptanuclear magnesium complex consisting of discrete cations and tetrabutylaluminate anions

Dibutylmagnesium (contaminated with Al(n-Bu)₃; n_Mg:n_Al ca. 1:0.2) was found to react with MeOCH₂CH₂OH followed by the addition of PhSCH(Me)Ph in the presence of 0.2 equiv n-butyllithium yielding [Mg₇(μ₃-OCH₂CH₂OMe)₆(μ-OCH₂CH₂OMe)₆][Al(n-Bu)₄]₂ (1) as the principal product (yield 40–45% referred to MeOCH₂CH₂OH). The single-crystal X-ray diffraction analysis revealed that the centrosymmetric cationic heptamagnesium complex is built up from seven edge-shared MgO₆ octahedra. The [Al(n-Bu)₄]⁻ anions adopt approximately a tetrahedral AlC₄ symmetry. ¹H, ¹³C and ²⁷Al NMR spectroscopic measurements showed that in THF solution the structures both of the heptamagnesium complex and the tetrabutylaluminate anion are preserved and that there are no cation–anion interactions reducing the symmetry. The ²⁷Al resonance (151.6 ppm) was found to be very sharp (w_1/2 = 5 Hz), the coupling constant ¹J(²⁷Al,¹³C) amounts to 72.3 Hz.     

On the Structure of the Solution Set of u″ = f(t,u), u(0) = u(1) = 0

The solution set of a Dirichlet problem x″ = f(t, x), x(0) = x(1) = 0, on a Banach space E and with f satisfying a Lipschitz condition, is homeomorphic to a closed subset of E. We prove that to an closed subset C of E there is a function f with Lipschitz constant arbitrarily close to π², such that the solution set of the corresponding Dirichlet problem is homeomorphic to C.     

N,N-Dimethylglycinatokomplexe von Platin(IV)

N,N -Dimethylglycinato Complexes of Platinum(IV) The aquapentachloroplatinic acid (H₃O)-[PtCl₅(H₂O)] · 2(18-cr-6) · 6 H₂O ( 1 ) reacts with N,N-dimethylglycine (Me₂glyH) to give cis-[PtCl₂(N,O-Me₂gly)₂] · (18-cr-6) ( 6 ) and (Me₂glyH₂)[PtCl₄(N,O-Me₂gly)] ( 7 ). Complexes 6 and 7 are characterized by microanalysis, ¹H-NMR and IR spectroscopy as well as by X-ray structure analysis. In both complexes the N,N-dimethylglycinato ligands are N,O-coordinated. In 6 , the amino groups are mutually trans and the carboxylato groups are cis (configuration index: OC-6–22). In the crystal, there are only weak C–H…O interactions between the N-methyl groups of the [PtCl₂(N,O-Me₂gly)₂] complex and the oxygen atoms of the crown ether (shortest C…O contacts: 3.10(2) Å and 3.21(2) Å). In the solid state, 7 exhibits strong cation-anion interactions: The carboxyl group of the cation (Me₂glyH₂)⁺ forms a strong O–H…O bridge to the exocyclic oxygen atom of the carboxylate group of the glycinato ligand (O…O 2.61(1) Å).     

Aromatic Polyhedral Hydroxyborates: Bridging Boron Oxides and Boron Hydrides

No explosion , but per-B-hydroxylation occurs if the icosahedral boron hydrides [closo-B₁₂H₁₂]²⁻ (see picture), [closo-CB₁₁H₁₂]⁻, or closo-1,12-(CH₂OH)₂-1,12-C₂B₁₀H₁₀ are refluxed in 30 % hydrogen peroxide. Thus, the three isoelectronic species [closo-B₁₂(OH)₁₂]²⁻, [closo-1-H-1-CB₁₁(OH)₁₁]⁻, and closo-1,12-H₂-1,12-C₂B₁₀(OH)₁₀ were obtained. ○=BH, ○=BOH.     

Precision and control in polymer synthesis why it's important and some recent examples of how to do it

This paper discusses some of the reasons why precision and control in polymer synthesis is of importance. By way of illustration it describes in outline recent results from the authors' laboratories in three areas. Namely; the controlled syntheses of poly(arylene vinylene)s and the influence of cis/trans vinylene content on luminescence in such polymers; the living polymerisation of highly functionalised polymers in water and the regulation of the crystallisation of calcium carbonate from water by the resultant well-defined water soluble polymers; and a simple route to hyperbranched polymers and the influence of the structure and topology of the products on solution properties. In each case the influence of control of architecture on properties will be discussed.     

Energy barriers of proton transfer reactions between amino acid side chain analogs and water from ab initio calculations

Proton transfer reactions were studied in all titratable pairs of amino acid side chains where, under physiologically reasonable conditions, one amino acid may function as a donor and the other one as an acceptor. Energy barriers for shifting the proton from donor to acceptor atom were calculated by electronic structure methods at the MP2/6-31++G(d,p) level, and the well-known double-well potentials were characterized. The energy difference between both minima can be expressed by a parabola using as argument the donor-acceptor distance R(DA). In this work, the fit parameters of the quadratic expression are determined for each donor-acceptor pair. Moreover, it was found previously that the energy barriers of the reactions can be expressed by an analytical expression depending on the distance between donor and acceptor and the energy difference between donor and acceptor bound states. The validity of this approach is supported by the extensive new data set. This new parameterization of proton transfer barriers between titratable amino acid side chains allows us to very efficiently estimate proton transfer probabilities in molecular modelling studies or during classical molecular dynamics simulation of biomolecular systems.     

Mechanism of UV-induced formation of Dewar lesions in DNA

The importance of a backbone: The mechanism of formation of Dewar lesions has been investigated by using femtosecond IR spectroscopy and ab?initio calculations of the exited state. The 4π?electrocyclization is rather slow, occurs with an unusual high quantum yield, and--surprisingly--is controlled by the phosphate backbone.     

Influence of nanoparticle surface functionalization on the thermal stability of colloidal polystyrene films

The installation of large scale colloidal nanoparticle thin films is of great interest in sensor technology or data storage. Often, such devices are operated at elevated temperatures. In the present study, we investigate the effect of heat treatment on the structure of colloidal thin films of polystyrene (PS) nanoparticles in situ by using the combination of grazing incidence small-angle X-ray scattering (GISAXS) and optical ellipsometry. In addition, the samples are investigated with optical microscopy, atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM). To install large scale coatings on silicon wafers, spin-coating of colloidal pure PS nanoparticles and carboxylated PS nanoparticles is used. Our results indicate that thermal annealing in the vicinity of the glass transition temperature T(g) of pure PS leads to a rapid loss in the ordering of the nanoparticles in spin-coated films. For carboxylated particles, this loss of order is shifted to a higher temperature, which can be useful for applications at elevated temperatures. Our model assumes a softening of the boundaries between the individual colloidal spheres, leading to strong changes in the nanostructure morphology. While the nanostructure changes drastically, the macroscopic morphology remains unaffected by annealing near T(g).