Fluoride-selective polymeric membrane electrodes based on Zr(IV)- and Al(III)-salen ionophores of various structures

Al(III)- and Zr(IV)-salophens of novel structures were tested as anion-selective ionophores. It was shown that these compounds are highly selective to fluoride and give selectivity greatly deviating from classical Hofmeister pattern, when doped into the polymeric membrane of ion-selective electrode (ISE). The following selectivity sequence has been recorded for both ionophores: F⁻ > ClO₄⁻ > SCN⁻ > NO₃⁻ ≈ Br⁻ ≈ Cl⁻. The results of potentiometric and spectrophotometric measurements allow to conclude that the nature and structure of salophen ligands influence stability of ISE working parameters. An increase in salophen ligands lipophilicity results in prolongation of the ISE lifetime, most likely due to slower ionophore decomposition caused by the hydrolysis of imine bonds in salophen structure. Ion-selective electrodes (ISEs) with the most successful Al(III)-salophen exhibited a stable, fast and near-Nernstian fluoride response and a functional lifetime near 3 weeks and selectivity coefficients with as follows: −2.8 (Y⁻ = Br⁻), −2.7 (Cl⁻), −2.8 (NO₃⁻), −1.5 (SCN⁻), −1.3 (ClO₄⁻), which is better than for other ones based on Zr(IV)- and Al(III)-salophens and salens described to date.     

Interplay between the Ribosomal Tunnel,Nascent Chain,and Macrolides Influences Drug Inhibition

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Highlights? Macrolides exhibit nascent polypeptide chain–dependent inhibition ? Peptides can form specific interactions with the ribosomal tunnel ? A covalent bond between tylosin and the ribosome is necessary for inhibitory activity     

Domino Staudinger/aza‐Wittig/Mannich Reaction: An Approach to Diversity of Di‐ and Tetrahydropyrrole Scaffolds

A highly efficient and selective domino reaction producing valuable di‐ and tetrahydropyrrole‐based skeletons from azidoethyl‐substituted CH‐acids and (thio)carbonyl compounds has been developed. By involving the additional functional groups in starting compounds into the domino reaction or postmodification of the primary reaction products, the simple construction of the pharmaceutically relevant three‐ and polycyclic azaheterocyclic scaffolds was demonstrated.     

Development of Polar Order in the Liquid Crystal Phases of a 4‐Cyanoresorcinol‐Based Bent‐Core Mesogen with Fluorinated Azobenzene Wings

A bent‐core mesogen consisting of a 4‐cyanoresorcinol unit as the central core and laterally fluorinated azobenzene wings forms four different smectic LC phase structures in the sequence SmA–SmC_s–SmC_sP_AR–M, all involving polar SmC_sP_S domains with growing coherence length of tilt and polar order on decreasing temperature. The SmA phase is a cluster‐type de Vries phase with randomized tilt and polar direction; in the paraelectric SmC_s phase the tilt becomes uniform, although polar order is still short‐range. Increasing polar correlation leads to a new tilted and randomized polar smectic phase with antipolar correlation between the domains (SmC_sP_AR) which then transforms into a viscous polar mesophase M. As another interesting feature, spontaneous symmetry breaking by formation of a conglomerate of chiral domains is observed in the non‐polar paraelectric SmC_s phase.     

Synthesis and Isolation of the Titanium–Scandium Endohedral Fullerenes—Sc2TiC@Ih‐C80, Sc2TiC@D5h‐C80 and Sc2TiC2@Ih‐C80: Metal Size Tuning of the TiIV/TiIII Redox Potentials

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed‐metal Sc–Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH₄ and Ti/CH₄ systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH₄ system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH₄ system are Sc₄C₂@C₈₀ (the most abundant EMF from this synthesis), Sc₃C₂@C₈₀, isomers of Sc₂C₂@C₈₂, and the family Sc₂C_{2 n} (2 n=74, 76, 82, 86, 90, etc.), as well as Sc₃CH@C₈₀. The Sc–Ti/CH₄ system produces the mixed‐metal Sc₂TiC@C_{2 n} (2 n=68, 78, 80) and Sc₂TiC₂@C_{2 n} (2 n=80) clusterfullerene families. The molecular structures of the new, transition‐metal‐containing endohedral fullerenes, Sc₂TiC@I_h‐C₈₀, Sc₂TiC@D_5h‐C₈₀, and Sc₂TiC₂@I_h‐C₈₀, were characterized by NMR spectroscopy. The structure of Sc₂TiC@I_h‐C₈₀ was also determined by single‐crystal X‐ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc₂TiC‐ and Sc₂TiC₂‐containing clusterfullerenes have Ti‐localized LUMOs. Encapsulation of the redox‐active Ti ion inside the fullerene cage enables analysis of the cluster–cage strain in the endohedral fullerenes through electrochemical measurements.     

Side chain flexibility and the pore dimensions in the GABA<Subscript>A</Subscript> receptor

Permeation of ions through open channels and their accessibility to pore-targeting drugs depend on the pore cross-sectional dimensions, which are known only for static X-ray and cryo-EM structures. Here, we have built homology models of the closed, open and desensitized α₁β₂γ₂ GABA_A receptor (GABA_AR). The models are based, respectively, on the X-ray structure of α₃ glycine receptor (α₃ GlyR), cryo-EM structure of α₁ GlyR and X-ray structure of β₃ GABA_AR. We employed Monte Carlo energy minimizations to explore how the pore lumen may increase due to repulsions of flexible side chains from a variable-diameter electroneutral atom (an expanding sphere) pulled through the pore. The expanding sphere computations predicted that the pore diameter averaged along the permeation pathway is larger by approximately 3 Å than that computed for the models with fixed sidechains. Our models predict three major pore constrictions located at the levels of ?2′, 9′ and 20′ residues. Residues around the ?2′ and 9′ rings are known to form the desensitization and activation gates of GABA_AR. Our computations predict that the 20′ ring may also serve as GABA_AR gate whose physiological role is unclear. The side chain flexibility of residues ?2′, 9′ and 20′ and hence the dimensions of the constrictions depend on the GABA_AR functional state.     

Donor-acceptor complexes of borazines

Donor-acceptor complexes of borazine (BZ) and its substituted derivatives with Lewis acids (A = MCl(3), MBr(3); M = B, Al, Ga) and Lewis bases (D = NH(3), Py) have been theoretically studied at the B3LYP/TZVP level of theory. The calculations showed that complexes with Lewis bases only are unstable with respect to dissociation into their components, while complexes with Lewis acids only (such as aluminum and gallium trihalides) are stable. It was shown that formation of ternary D→BZ→A complexes may be achieved by subsequent introduction of the Lewis acid (acceptor A) and the Lewis base (donor D) to borazine. The nature of substituents in the borazine ring, their number, and position were shown to have only minor influence on the stability of ternary D→BZ→A complexes due to the compensation effect. Much weaker acceptor properties of borazine are explained in terms of large endothermic pyramidalization energy of the boron center in the borazine ring. In contrast to borazine, binary complexes of the isoelectronic benzene were predicted to be weakly bound even in the case of very strong Lewis acids; ternary DA complexes of benzene were predicted to be unbound. The donor-acceptor complex formation was predicted to significantly reduce both the endothermicity (by 70-95 kJ mol(-1)) and the activation energy (by 40-70 kJ mol(-1)) for the borazine hydrogenation. Thus, activation of the borazine ring by Lewis acids may be a facile way for the hydrogenation of borazines and polyborazines.