Nanoparticle-reinforced associative network hydrogels

ABA triblock copolymers in solvents selective for the midblock are known to form associative micellar gels. We have modified the structure and rheology of ABA triblock copolymer gels comprising poly(lactide)-poly(ethylene oxide)-poly(lactide) (PLA-PEO-PLA) through addition of a clay nanoparticle, laponite. Addition of laponite particles resulted in additional junction points in the gel via adsorption of the PEO corona chains onto the clay surfaces. Rheological measurements showed that this strategy led to a significant enhancement of the gel elastic modulus with small amounts of nanoparticles. Further characterization using small-angle X-ray scattering and dynamic light scattering confirmed that nanoparticles increase the intermicellar attraction and result in aggregation of PLA-PEO-PLA micelles.     

Enolate Stabilization by Anion–π Interactions: Deuterium Exchange in Malonate Dilactones on π‐Acidic Surfaces

Of central importance in chemistry and biology, enolate chemistry is an attractive topic to elaborate on possible contributions of anion–π interactions to catalysis. To demonstrate the existence of such contributions, experimental evidence for the stabilization of not only anions but also anionic intermediates and transition states on π‐acidic aromatic surfaces is decisive. To tackle this challenge for enolate chemistry with maximal precision and minimal uncertainty, malonate dilactones are covalently positioned on the π‐acidic surface of naphthalenediimides (NDIs). Their presence is directly visible in the upfield shifts of the α‐protons in the ¹H NMR spectra. The reactivity of these protons on π‐acidic surfaces is measured by hydrogen–deuterium (H–D) exchange for 11 different examples, excluding controls. The velocity of H–D exchange increases with π acidity (NDI core substituents: SO₂R>SOR>H>OR>OR/NR₂>SR>NR₂). The H–D exchange kinetics vary with the structure of the enolate (malonates>methylmalonates, dilactones>dithiolactones). Moreover, they depend on the distance to the π surface (bridge length: 11–13 atoms). Most importantly, H–D exchange depends strongly on the chirality of the π surface (chiral sulfoxides as core substituents; the crystal structure of the enantiopure (R,R,P)‐macrocycle is reported). For maximal π acidity, transition‐state stabilizations up to ?18.8 kJ mol^?1 are obtained for H–D exchange. The Brønsted acidity of the enols increases strongly with π acidity of the aromatic surface, the lowest measured pK_a=10.9 calculates to a ΔpK_a=?5.5. Corresponding to the deprotonation of arginine residues in neutral water, considered as “impossible” in biology, the found enolate–π interactions are very important. The strong dependence of enolate stabilization on the unprecedented seven‐component π‐acidity gradient over almost 1 eV demonstrates quantitatively that such important anion–π activities can be expected only from strong enough π acids.     

Synthetic pores with sticky pi-clamps

In this report, we describe design, synthesis, evaluation and molecular dynamics simulations of synthetic multifunctional pores with pi-acidic naphthalenediimide clamps. Experimental evidence is provided for the formation of unstable but inert, heterogeneous and acid-insensitive dynamic tetrameric pores that are sensitive to base and ionic strength. Blockage experiments reveal that the introduction of aromatic electron donor-acceptor interactions provides access to the selective recognition of pi-basic intercalators within the pore. This breakthrough is important for the application of synthetic pores as multianalyte sensors.     

Zipper assembly of photoactive rigid-rod naphthalenediimide pi-stack architectures on gold nanoparticles and gold electrodes

We introduce zipper assembly as a simple and general concept to create complex functional architectures on conducting surfaces. Rigid-rod pi-stack architecture composed of p-oligophenyl rods and blue naphthalenediimide (NDI) stacks is selected as an example. First, short p-quaterphenyl initiators with four anionic NDIs are deposited on gold. Then, long p-octiphenyl propagators with eight cationic NDIs are added. The lower half of the propagator pi-stacks with the initiator, whereas the upper half of the molecule remains free. These cationic sticky-ends zip up with anionic propagators to produce anionic sticky-ends, and so on. Zipper assembly on gold nanoparticles is demonstrated by the appearance of the absorption of face-to-face NDI pi-stacks and the shift of the surface plasmon resonance band with increasing layer thickness. Complete inhibition by zipper capping demonstrates that zipper assembly affords complex architectures that are more ordered than those obtained by conventional layer-by-layer (LBL) approaches. Zipper assembly on gold electrodes produces increasing photocurrents with increasing number of zipped layers. The photocurrents obtained by this method are much higher than those obtained by conventional LBL controls; zipper termination by capping cleanly stops any increase in photocurrent.     

Temperature dependence of oxygen isotope acid fractionation for modern and fossil tooth enamels

The oxygen isotope ratio of CO(2) liberated from structural carbonate in tooth enamel apatite was measured at phosphoric acid reaction temperatures of 25 degrees C, 60 degrees C and 90 degrees C, and it was found that apparent acid fractionation factors for pristine enamel, fossilized enamel, and calcite follow different temperature relationships. Using sealed vessel reactions normalized to alpha(25) = 1.01025 (the fractionation factor for calcite at 25 degrees C), the apparent fractionation factor at 90 degrees C (alpha*(90)) for pristine enamel ranged between 1.00771 and 1.00820, and between 1.00695 and 1.00772 for fossilized enamel. Apparent fractionation factors for common acid bath reactions are similar to those for sealed vessel reactions. A significant correlation exists between alpha*(90) and F(-) content, suggesting that change in the acid fractionation factor may be related to the replacement of OH(-) with F(-) during fossilization of bioapatite. These results have important implications for making accurate comparisons between modern and fossil tooth enamel delta(18)O values, and for the uniformity of isotope data produced in different laboratories using different acid reaction temperatures.     

Modeling the Selectivity of Potassium Channels with Synthetic,Ligand‐Assembled π Slides

A supramolecular ion channel model mediates transmembrane ion transport (shown schematically) with a selectivity topology similar to that of K⁺ channels. This supports the biological significance of flexible arene arrays as selective cation binding sites.     

Visualization of Magnetization Transfer Effect in Polyethylene Glycol Impregnated Waterlogged Wood

To visualize the condition of impregnation of polyethylene glycol (PEG) in waterlogged wood, we demonstrated magnetic transfer (MT) magnetic resonance imaging (MRI) through a series of process of PEG impregnation. Three different samples were examined; reference wood, 10 cm cut wood, and 5 cm cut wood. During this study, the upper section sample was kept immersed in water, for the middle and lower sections the concentration of PEG solution was changed at 20 wt% intervals from 20 to 100 wt%. The impregnated periods of each PEG solution concentration were 14 days. Then, MR imaging were performed with/without MT pulse. The MTR value for both 10 cm- and 5 cm-samples were shown to decrease at 20 wt% PEG at peak concentration. When the sample volume was large, e.g., 10 cm-sample, the MTR value decreased to 100 wt% PEG concentration. In contrast, when a sample volume was small, e.g., 5 cm-sample, MTR value decreased to 60 wt% PEG concentration. In conclusion, MTR analysis makes it possible to nondestructively visualize and evaluate the inner condition concerning the PEG impregnation method for waterlogged wood.     

Origin of the X‐ray magnetic circular dichroism at the L‐edges of the rare‐earths in RxR1−x′Al2 systems

An X‐ray magnetic circular dichroism (XMCD) study performed at the rare‐earth L_2,3‐edges in the R_xR_1?x′Al₂ compounds is presented. It is shown that both R and R′ atoms contribute to the XMCD recorded at the L‐edges of the selected rare‐earth, either R or R′. The amplitude of the XMCD signal is not directly correlated to the magnetization or to the value of the individual (R, R′) magnetic moments, but it is related to the molecular field acting on the rare‐earth tuned in the photoabsorption process. This result closes a longstanding study of the origin of the XMCD at the L‐edge of the rare‐earths in multi‐component systems, allowing a full understanding of the exact nature of these signals.     

Hydrogen-induced magnetic and structural transformations of GdCu

The magnetization of GdCu induced by hydrogen uptake was measured within the temperature range of 4.2 to 300 K, occurring phase changes were followed by X-ray diffraction measurements at ambient temperature. The prepared GdCu powder of CsCl-type structure readily absorbed hydrogen at ambient temperature, where hydrogen pressure was below 100 kPa. Hydrogenation changed the magnetism of GdCu in a complex manner from an antiferromagnetic-like type to a paramagnetic-like one. The changes in magnetic properties of GdCu by hydrogenation are governed by hydrogen-induced disproportionation. Within the composition range 0<[H]/[GdCu]<1, GdCu disproportionated according to 2GdCu+H₂→GdH₂+GdCu₂ . The magnetization was evaluated by the expression χ_total=(1-x)χ_GdCu+(x/2)(χ_GdH2+χ_GdCu2). GdCu hydride was not observed. Hydrogenation beyond [H]/[GdCu]>1 gave rise to the disproportionation of GdCu₂ causing the change in magnetization.