High-fidelity Recognition of Organotrifluoroborate Anions (R−BF3−) as Designer Guest Molecules

The recognition of boron compounds is well developed as boronic acids but untapped as organotrifluoroborate anions (R−BF₃⁻). We are exploring the development of these and other designer anions as anion-recognition motifs by considering them as substituted versions of the parent inorganic ion. To this end, we demonstrate strong and reliable binding of organic trifluoroborates, R−BF₃⁻, by cyanostar macrocycles that are size-complementary to the inorganic BF₄⁻ progenitors. We find that recognition is modulated by the substituent's sterics and that the affinities are retained using the common K⁺ salts of R−BF₃⁻ anions.     

High-Throughput Synthesis and Characterization of Aryl Silicones by Using the Piers–Rubinsztajn Reaction

Arylsilicones are widely exploited for their thermal and optical properties. The creation of phenylsilicone elastomers with specific physical properties is typically done by a “one-off” formulation and test process. Herein, it is demonstrated that high-throughput synthesis methods can be used to rapidly prepare a series of arylsilicone elastomers and then the relative impact of different aryl groups on their physical properties is assessed. Aromatic groups were incorporated into polydimethylsiloxane (PDMS) elastomers by exploiting the relative reactivity of different functional groups in the Piers–Rubinsztajn reaction. To analyze trends in the silicone mechanical properties as a function of increasing aryl concentration—structure/property relationships—libraries of elastomers were both quickly synthesized and characterized by using high-throughput suites starting from low viscosity silicone oils/monomers in 96-well plates. Liquid handling parameters were optimized to effectively work with the silicones. Incorporating aryl instead of alkyl crosslinkers into the PDMS backbone increased the silicone elastomer modulus by approximately 50 % (at a crosslink density of 6 %); elastomers prepared with an aromatic crosslinker with three contact points led to much higher moduli compared with those with one contact point at the same crosslink density. When located at precise rather than random points on the silicone chains, diphenylsilicones had lower moduli than analogous monophenylsilicones.     

Matrix effects on the multi-collector inductively coupled plasma mass spectrometric analysis of high-precision cadmium and zinc isotope ratios

Resin-derived contaminants added to samples during column chemistry are shown to cause matrix effects that lead to inaccuracy in multi-collector inductively coupled plasma mass spectrometry measurement of small natural variations in Cd and Zn isotopic compositions. These matrix effects were evaluated by comparing pure Cd and Zn standards and standards doped with bulk column blank from the anion exchange chromatography procedure. Doped standards exhibit signal enhancements (Cd, Ag, Zn and Cu), instrumental mass bias changes and inaccurate isotopic compositions relative to undoped standards, all of which are attributed to the combined presence of resin-derived organics and inorganics. The matrix effect associated with the inorganic component of the column blanks was evaluated separately by doping standards with metals at the trace levels detected in the column blanks. Mass bias effects introduced by the inorganic column blank matrix are smaller than for the bulk column blank matrix but can still lead to significant changes in ion signal intensity, instrumental mass bias and isotopic ratios. Chemical treatment with refluxed HNO₃ or HClO₄/HNO₃ removes resin-derived organic components resulting in matrix effects similar in magnitude to those associated with the inorganic component of the column blank.Mass bias correction using combined external normalization-SSB does not correct for these matrix effects because the instrumental mass biases experienced by Cd and Zn are decoupled from those of Ag and Cu, respectively. Our results demonstrate that ion exchange chromatography and associated resin-derived contaminants can be a source of error in MC-ICP-MS measurement of heavy stable element isotopic compositions.     

A B3LYP investigation of the conformational and environmental sensitivity of carbon�Cdeuterium frequencies of aryl-perdeuterated phenylalanine and tryptophan

Carbon?Cdeuterium labeled amino acids can serve as sensitive probes for biophysical characterization. Although multiple research groups have used infrared spectroscopy in conjunction with alkyl backbone or side-chain deuterated amino acids for the biophysical characterization of conformational and/or environmental changes, it was not entirely clear to the authors that perdeuterated aryl rings would demonstrate a similar sensitivity toward conformational or environmental changes. In an effort to evaluate the sensitivity of aryl carbon?Cdeuterium (C?CD) IR frequencies, a B3LYP investigation of the sensitivity of aryl C?CD frequencies toward conformational and environmental changes was conducted for phenylalanine (Phe) and tryptophan (Trp). To compensate for the low molar absorptivity of C?CD frequencies, perdeuterated aryl rings were investigated, which are commercially available and can be readily compared to experimental data. B3LYP results suggest that aryl-deuterated Phe and Trp will exhibit moderate sensitivities toward conformational and environmental changes with frequency shifts upward of 13 and 26?cm^?1 for Phe and Trp, respectively. B3LYP predicts that conformational sensitivity arises from dipole changes and not orbital alignment changes. In an effort to mimic what might be observed experimentally, simulated IR spectra were created and show absorption band changes with conformational and environmental changes, which indicate that IR characterization of perdeuterated aryl rings in amino acids could serve as a biophysical tool.     

Syntheses and Crystal Structures of a Series of Manganese-Lanthanide-Sodium 12-Metallacrown-4 Dimers

A series of heterotrimetallic manganese-lanthanide-sodium dimer metallacrowns has been synthesized and characterized by single-crystal X-ray analysis: {LnNa[12-MC_{Mn(III)N(shi)}-4]}₂(iph)₄, where Ln^III?=?La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14), and Y (15); MC is metallacrown; shi^3? is salicylhydroximate; and iph^2? is isophthalate. The manganese(III) ions and shi^3? ligands generate the 12-MC-4 framework with one Ln^III and Na⁺ ion bound to each [12-MC_{Mn(III)N(shi)}-4] on opposite sides of the central MC cavity. The carboxylate groups of the isophthalate ligands bridge between the central Ln^III ion and each ring Mn^III ion, and the meta-arrangement of the carboxylate groups joins two LnNa[12-MC_{Mn(III)N(shi)}-4] units together to form the dimer through the Ln^III ions, which reside on the interior of the molecule. The identity of the central Ln^III ion slightly impacts the size the [12-MC_{Mn(III)N(shi)}-4] framework. As the crystal radius of the Ln^III ion increases from Lu^III (1.02 Å) to La^III (1.19 Å), the 12-MC-4 framework expands to accommodate the larger Ln^III ion as the MC cavity increases in size (0.53 Å for Lu^III to 0.58 Å for La^III) and the average cross cavity Mn^III-Mn^III and oxime oxygen-oxime oxygen distances also increase (Mn^III-Mn^III distances: 6.48 Å for Lu^III to 6.52 Å for La^III; O_oxime-O_oxime distances: 3.66 Å for Lu^III to 3.75 Å for La^III). In addition, the larger Ln^III ions reside further from the MC cavity as indicated by the Ln^III-oxime oxygen mean plane (O_oxMP) distance. The Ln^III-O_oxMP distance steadily decreases from La^III (1.7527(12) Å) to Lu^III (1.5575(15) Å).

Graphic Abstract

The complex {LaNa[12-MC_{Mn(III)N(shi)}-4]}₂(iph)₄(DMF)₆(H₂O)₂ is a dimer of [12-MC-4] molecules linked by four isophthalate anions

     

Relaxation effects in the quantification of fat using gradient echo imaging

Quantification of fat has been investigated using images acquired from multiple gradient echoes. The evolution of the signal with echo time and flip angle was measured in phantoms of known fat and water composition and in 21 research subjects with fatty liver. Data were compared to different models of the signal equation, in which each model makes different assumptions about the T1 and/or T2* relaxation effects. A range of T1, T2*, fat fraction and number of echoes was investigated to cover situations of relevance to clinical imaging. Results indicate that quantification is most accurate at low flip angles (to minimize T1 effects) with a small number of echoes (to minimize spectral broadening effects). At short echo times, the spectral broadening effects manifest as a short apparent T2 for the fat component.     

A novel assay to probe heparin-peptide interactions using pentapeptide-stabilized gold nanoparticles

In this article, we present a novel assay to probe the interactions between heparin and heparin-binding peptides based on CALNN pentapeptide-stabilized gold nanoparticles. This assay relies on rapid aggregation of gold nanoparticles and dramatic retardation in the presence of a large excess of heparin due to the binding of peptides to heparin. Using this method, the dissociation constant ( K d) and melting temperature ( T m) of three different peptides against heparin were determined. The results from capillary electrophoresis demonstrated that K d values measured by this method were comparatively accurate. It was found that the peptide with the lowest K d did not have the highest T m. Structural analysis by circular dichroism was performed to explain this phenomenon. A comparison with the results from affinity chromatography indicates that electrostatic interactions only are not the major determinant of the affinity between heparin and peptide, but other interactions such as hydrogen-bonding and hydrophobic interactions may play important roles in the overall interactions. This novel assay is inexpensive, label-free, and easy to implement in the laboratories, does not suffer precipitation of the heparin-peptide complex or their conformational changes caused by surface immobilization, and is expected to be a useful complement to other existing methods.     

Benchmarking the accuracy of coverage-dependent models: adsorption and desorption of benzene on Pt (1 1 1) and Pt3Sn (1 1 1) from first principles

Bimetallic catalysts have demonstrated properties favorable for upgrading biofuel through catalytic hydrodeoxygenation. However, the design and optimization of such bimetallic catalysts requires the ability to construct accurate, predictive models of these systems. To generate a model that predicts the kinetic behavior of benzene adsorbed on Pt (1 1 1) and a Pt₃Sn (1 1 1) surface alloy (Pt₃Sn (1 1 1)), the adsorption of benzene was studied for a wide range of benzene coverages on both surfaces using density functional theory (DFT) calculations. The adsorption energy of benzene was found to correlate linearly with benzene coverage on Pt (1 1 1) and Pt₃Sn (1 1 1); both surfaces exhibited net repulsive lateral interactions. Through an analysis of the d-band properties of the metal surface, it was determined that the coverage dependence is a consequence of the electronic interactions between benzene and the surface. The linear coverage dependence of the adsorption energy allowed us to quantify the influence of the lateral interactions on the heat of adsorption and temperature programmed desorption (TPD) spectra using a mean-field model. A comparison of our simulated TPD to experiment showed that this mean-field model adequately reproduces the desorption behavior of benzene on Pt (1 1 1) and Pt₃Sn (1 1 1). In particular, the TPD correctly exhibits a broadening desorption peak as the initial coverage of benzene increases on Pt (1 1 1) and a low temperature desorption peak on Pt₃Sn (1 1 1). However, due to the sensitivity of the TPD peak temperature to the desorption energy, precise alignment of experimental and theoretical TPD spectra demands an accurate calculation of the adsorption energy. Therefore, an analysis of the effect of the exchange-correlation functional on TPD modeling is presented. Through this work, we show the necessity of incorporating lateral interactions into theoretical models in order to correctly predict experimental behavior.     

Stereoselective synthesis of acortatarins a and B

Acortatarins A and B have been synthesized via stereoselective spirocyclizations of glycals. Mercury-mediated spirocyclization of a pyrrole monoalcohol side chain leads to acortatarin A. Glycal epoxidation and reductive spirocyclization of a pyrrole dialdehyde side chain leads to acortatarin B. Acid equilibration and crystallographic analysis indicate that acortatarin B is a contrathermodynamic spiroketal with distinct ring conformations compared to acortatarin A.