Oligoether-strapped calix[4]pyrrole: an ion-pair receptor displaying cation-dependent chloride anion transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.     

An analytical derivative procedure for the calculation of vibrational Raman optical activity spectra

We present an analytical time-dependent Hartree-Fock algorithm for the calculation of the derivatives of the electric dipole-magnetic dipole polarizability with respect to atomic Cartesian coordinates. Combined with analogous procedures to determine the derivatives of the electric dipole-electric dipole and electric dipole-electric quadrupole polarizabilities, it enables a fully analytical evaluation of the three frequency-dependent vibrational Raman optical activity (VROA) invariants within the harmonic approximation. The procedure employs traditional non-London atomic orbitals, and the gauge-origin dependence of the VROA intensities has, therefore, been assessed for the commonly used aug-cc-pVDZ and rDPS:3-21G basis sets.     

Coupled motion in proteins revealed by pressure perturbation

The cooperative nature of protein substructure and internal motion is a critical aspect of their functional competence about which little is known experimentally. NMR relaxation is used here to monitor the effects of high pressure on fast internal motion in the protein ubiquitin. In contrast to the main chain, the motions of the methyl-bearing side chains have a large and variable pressure dependence. Within the core, this pressure sensitivity correlates with the magnitude of motion at ambient pressure. Spatial clustering of the dynamic response to applied hydrostatic pressure is also seen, indicating localized cooperativity of motion on the sub-nanosecond time scale and suggesting regions of variable compressibility. These and other features indicate that the native ensemble contains a significant fraction of members with characteristics ascribed to the recently postulated "dry molten globule". The accompanying variable side-chain conformational entropy helps complete our view of the thermodynamic architecture underlying protein stability, folding, and function.     

Stereoselective synthesis of P-chirogenic dibenzophosphole-boranes via aryne intermediates

A new aryne-mediated tandem cross-coupling/P-cyclization sequence starting from tertiary phosphine-boranes and 1,2-dibromobenzenes is reported. P-chirogenic dibenzophospholes become accessible in a regio-, chemo-, and diastereoselective way.     

Air‐Stable {(C5H5)Co} Catalysts for [2+2+2] Cycloadditions

Cobalt cyclopentadienyl complexes incorporating a fumarate and a CO ligand (see picture) efficiently catalyze inter‐ and intramolecular [2+2+2] cycloadditions of alkynes, nitriles, and/or alkenes to give benzenes, pyridines, or 1,3‐cyclohexadienes. Unlike catalysts such as [CpCo(CO)₂] or [CpCo(C₂H₄)₂] (Cp=C₅H₅), they are air‐stable, easy to handle, compatible with microwave conditions, and do not necessarily require irradiation to be active.

     

Intramolecular Alkynylogous Mukaiyama Aldol Reaction Starting from Bicyclic Alkanones Tethered to Alkynyl Esters: Formal Total Synthesis of (±)‐Hamigeran B

Selecting the ring : tert‐Butyldimethylsilyltriflate (TBSOTf)/NEt₃ treatment of alkynyl esters tethered to bicycloalkanones leads to the formation of tricyclic allenoates with total diasteroselectivity at the ring junction. An intramolecular alkynylogous Mukaiyama aldol reaction promoted by a TBSOTf/NEt₃ dual activation is involved. This novel methodology was illustrated by a formal total synthesis of (±)‐hamigeran B.

     

Ba4KFe3O9: a novel ferrite containing discrete 6-membered rings of corner-sharing FeO4 tetrahedra

Single crystals of a new iron-containing oxide, Ba(4)KFe(3)O(9), were grown from a hydroxide melt, and the crystal structure was determined by single-crystal X-ray diffraction. This ferrite represents the first complex oxide containing isolated 6-membered rings of corner-sharing FeO(4) tetrahedra. M?ssbauer measurements are indicative of two tetrahedral high-spin Fe(3+) coordination environments. The observed magnetic moment (~3.9 μ(B)) at 400 K is significantly lower than the calculated spin-only (~5.2 μ(B)) value, indicating the presence of strong antiferromagnetic interactions in the oxide. Our density functional theory calculations confirm the strong antiferromagnetic coupling between adjacent Fe(3+) sites within each 6-membered ring and estimate the nearest-neighbor spin-exchange integral as ~200 K; next-nearest-neighbor interactions are shown to be negligible. The lower than expected effective magnetic moment for Ba(4)KFe(3)O(9) calculated from χT data is explained as resulting from the occupation of lower-lying magnetic states in which more spins are paired. X-band (9.5 GHz) electron paramagnetic resonance (EPR) spectra of a powder sample consist of a single line at g ~ 2.01 that is characteristic of Fe(3+) ions in a tetrahedral environment, thus confirming the M?ssbauer results. Further analysis of the EPR line shape reveals the presence of two types of Fe(6) magnetic species with an intensity ratio of ~1:9. Both species have Lorentzian line shapes and indistinguishable g factors but differ in their peak-to-peak line widths (δB(pp)). The line-width ratio δB(pp)(major)/δB(pp)(minor) ~ 3.6 correlates well with the ratio of the Weiss constants, θ(minor)/θ(major) ~ 4.     

Stability of Plasma Treated Non-vulcanized Polybutadiene Surfaces: Role of Plasma Parameters and Influence of Additives

Surface modification studies of non-vulcanized BR elastomers (butadiene rubber) by low-pressure air plasma treatment and the effect on ageing and adhesion performances are presented in this paper. In particular, the influence of discharge power and distance from the glow discharge, and impact of antioxidant molecules in the BR formulation were examined. To characterize the changes to the BR surface, XPS spectroscopy, contact angle measurements, AFM nanoindentation experiments and tack measurements were utilized. Oxidation and crosslinking were the main mechanisms observed on the polymer chains regardless of the plasma conditions used. Beyond a certain threshold of plasma energy (in our case, discharge power of ~60 W and exposure time of ~30 s), a steady state was reached irrespective of the distance from the glow discharge. The presence of antioxidant molecules considerably reduced crosslinking phenomena while maintaining oxidation processes on polymer chains and increasing the nitrogen content in the near surface region. The mechanisms responsible for these differences have been identified. Interestingly, the COOH/C=O ratio changed according to the balance between oxidation and crosslinking. The hydrophobic recovery rate was mainly driven by temperature-dependent dynamics and varied according to the degree of crosslinking in the surface region. It was found to be lower in air atmosphere in the presence of antioxidant molecules. Finally, the presence of antioxidant molecules in the BR formulation allowed the adhesion performances after plasma exposure to significantly increase.     

Rates of intra- and intermolecular electron transfers in hydrogenase deduced from steady-state activity measurements

Electrons are transferred over long distances along chains of FeS clusters in hydrogenases, mitochondrial complexes, and many other respiratory enzymes. It is usually presumed that electron transfer is fast in these systems, despite the fact that there has been no direct measurement of rates of FeS-to-FeS electron transfer in any respiratory enzyme. In this context, we propose and apply to NiFe hydrogenase an original strategy that consists of quantitatively interpreting the variations of steady-state activity that result from changing the nature of the FeS clusters which connect the active site to the redox partner, and/or the nature of the redox partner. Rates of intra- and intermolecular electron transfer are deduced from such large data sets. The mutation-induced variations of electron transfer rates cannot be explained by changes in intercenter distances and reduction potentials. This establishes that FeS-to-FeS rate constants are extremely sensitive to the nature and coordination of the centers.