How metallylenes activate small molecules

We have studied the activation of dihydrogen by metallylenes using relativistic density functional theory (DFT). Our detailed activation strain and Kohn–Sham molecular orbital analyses have quantified the physical factors behind the decreased reactivity of the metallylene on going down Group 14, from carbenes to stannylenes. Along this series, the reactivity decreases due to a worsening of the back-donation interaction between the filled lone-pair orbital of the metallylene and the σ*-orbital of H₂, which, therefore, reduces the metallylene–substrate interaction and increases the reaction barrier. As the metallylene ligand is varied from nitrogen to phosphorus to arsenic a significant rate enhancement is observed for the activation of H₂ due to (i) a reduced steric (Pauli) repulsion between the metallylene and the substrate; and (ii) less activation strain, as the metallylene becomes increasingly more predistorted. Using a rationally designed metallylene with an optimal Group 14 atom and ligand combination, we show that a number of small molecules (i.e. HCN, CO₂, H₂, NH₃) may also be readily activated. For the first time, we show the ability of our H₂ activated designer metallylenes to hydrogenate unsaturated hydrocarbons. The results presented herein will serve as a guide for the rational design of metallylenes toward the activation of small molecules and subsequent reactions.

Quantum chemical analyses reveal how model metallylene catalysts activate H₂. This is the first step towards the rational design of metallylenes for the activation of small molecules and subsequent reactions.     

Dielectric behavior of some small ketones as ideal polar molecules

The dielectric behaviors of some small symmetric ketone molecules, including acetone, 3-pentanone, cyclopentanone, 4-heptanone, and cyclohexanone, were investigated as a function of temperature (T) over a wide frequency range from 50 MHz (3.14 × 10(8) s(-1), in angular frequency) to 3 THz (1.88 × 10(13) s(-1)). The temperature dependencies of the rotational diffusion times (τ(r)) determined using (17)O NMR spin-lattice relaxation time (T(1)) measurements and viscosities of the ketones were also examined. The obtained temperature dependencies of the parameters for the ketones were compared with those of ideal polar molecules, which obey the Stokes-Einstein-Debye (SED) relationship without the formation of intermolecular dimeric associations and without orientational correlations between dipoles (molecular axes), that is, free rotation. Kirkwood correlation factors (g(K)) of only acetone and 3-pentanone were close to unity over a wide temperature range, whereas those of other ketones were obviously less than unity. These results revealed that no correlations exist between the rotational motions of dipoles in acetone and 3-pentanone, as expected in ideal polar molecules. However, other ketones exhibited orientational correlations in their dipoles because of dipole-dipole interactions via antiparallel configurations. Furthermore, because acetone and 3-pentanone satisfied the SED relationship and because their microscopic dielectric relaxation times (τ(μ)), which were calculated from the determined dielectric relaxation times (τ(D)) via the relationship τ(μ) = τ(D)g(K)(-1), were identical to 3τ(r) and were proportional to Vη(k(B)T)(-1) over the wide temperature range examined, where V, k(B), and η represent the effective molecular volume, Boltzmann's constant, and the viscosity of the liquid molecules, respectively, these two ketone molecules behave as ideal polar molecules. In addition, other ketones not significantly larger than acetone and 3-pentanone in molecular size likely form dimeric intermolecular associations with antiparallel cyclic configurations, which demonstrates the g(K) values less than unity.     

On the angular shape of van der Waals dimers of small polar molecules

Ab initio calculations on twenty van der Waals dimers of small polar molecules at the experimentally observed intermolecular separation R, using Hartree-Fock molecular moments, show that the minima in the electrostatic interaction expanded up to R⁻⁶ converges to angular structures which are close to those observed experimentally for such complexes.     

Open-tubular capillary electrochromatography of small polar molecules using etched, chemically modified capillaries

The migration characteristics of small polar molecules are evaluated on etched, chemically modified capillaries with four different moieties (C5, C18, diol and cholesterol) bonded onto a silica hydride surface. The effects of pH on migration are used to determine the possible contributions of eletrophoretic mobility, electroosmotic flow (EOF) and analyte/bonded phase interactions. The EOF on etched capillaries is more complicated than on ordinary fused capillaries because it changes from anodic to cathodic as the pH is raised. A mixture of neurotransmitters and related compounds is used to further evaluate the effects of the bonded moiety on the separation properties of this particular electrophoretic format.     

A REMPI study of solvation of small Hg n clusters by polar molecules

Free Hg _n (DME) _m clusters (where DME=dimethyl-ether,n=1, 2, 3,m=1÷5) formed in a supersonic expansion were studied by the REMPI (Resonance-Enhanced Multi-Photon Ionization) technique. A large decrease of ionization energies due to solvation of Hg _n clusters is observed. Preliminary results are discussed in terms of different equilibrium configurations of the electronic ground, excited and ionic states of clusters.     

Critical number density in polar liquids: orientational polarizability of polar molecules in n-decanol/n-heptanol and n-heptanol/heptane systems

The orientational polarizability of polar molecules in n-decanol/n-heptanol and n-heptanol/heptane systems has been determined. Experimental permittivities show that both systems (together with the family of aliphatic normal alcohols) are described by a unique function of the number density ϱ and the absolute temperature T. The experimental data show that the polarizability increases rapidly as a function of ϱ from a critical number densityϱ_c.     

Chemical dimerizers and three-hybrid systems: scanning the proteome for targets of organic small molecules

The integration of technological advances in areas as diverse as chemical biology, proteomics, genomics, automation, and bioinformatics has led to the emergence of novel screening paradigms for analyzing the molecular basis of drug action. This review summarizes recent advances in three-hybrid technologies and their application to the characterization of small molecule-protein interactions and proteome-wide identification of drug receptors.     

复杂样品痕量极性小分子化合物的样品前处理及分析方法研究进展

核苷、胺、氨基酸等极性小分子化合物是生物、食品、环境等领域重要的研究对象,但各种复杂基体中痕量极性小分子的分离分析需要高效的前处理介质和技术以及快速灵敏的分析方法。本文综述了硅胶材料、有机聚合物、炭材料和硼酸材料等样品前处理分离介质及反相液相色谱、亲水作用色谱等分析方法在复杂样品痕量极性小分子化合物分析中的应用,并展望了其发展趋势。     

Evaluation of chlorophyll fluorescence and membrane injury in the leaves of barley cultivars under osmotic stress

Two physiological tests for screening drought tolerance of barley (Hordeum vulgare, L.) plants are compared in this work. Water deficit is induced by treating the plants' roots with polyethylene glycol (PEG 8000). The relative water content (RWC) of the plants is used as a measure of the water status. Conductometrically determined electrolyte leakage from the leaf tissue demonstrates the membrane injury caused by dehydration. It is shown that the injury index increases with the decrease of the RWC of the leaves. The F(v)/F(m) ratio is employed to assess changes in the primary photochemical reactions of the photosynthetic apparatus after dehydration. The results suggest that PSII is weakly affected by the imposed osmotic stress. The fluorescence behaviour of the examined cultivars is related to their RWC.     

Substrate-selective aqueous organometallic catalysis. How small water-soluble organic molecules enhance the supramolecular discrimination

Selective decarboxylation of two alkylallylurethane isomers into alkylallylamines has been performed in a biphasic system by using randomly hydroxypropylated and methylated cyclodextrins as discriminating agents. Surprisingly, the presence of small organic hydrosoluble molecules such as amine or alcohol derivatives appeared to be crucial in the discriminating process. Indeed, it was clearly proved that the presence of such additives enhances greatly the substrate selectivity. For instance, the addition of triethylamine to the reaction medium allows to improve the discriminating power of methylated-β-cyclodextrin by a factor 7. These unexpected results were explained by considering the formation of ternary cyclodextrin/substrate/additive complexes.     

Mobility of small molecules in membrane materials based on copolyimide P84

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Anthocyanins protect against A2E photooxidation and membrane permeabilization in retinal pigment epithelial cells

The pyridinium bisretinoid A2E, an autofluorescent pigment that accumulates in retinal pigment epithelial cells with age and in some retinal disorders, can mediate a detergent-like perturbation of cell membranes and light-induced damage to the cell. The photodynamic events initiated by the sensitization of A2E include the generation of singlet oxygen and the oxidation of A2E at carbon-carbon double bonds. To assess the ability of plant-derived anthocyanins to modulate adverse effects of A2E accumulation on retinal pigment epithelium (RPE) cells, these flavylium salts were isolated from extracts of bilberry. Nine anthocyanin fractions reflecting monoglycosides of delphinidin, cyanidin, petunidin and malvidin were obtained and all were shown to suppress the photooxidation of A2E at least in part by quenching singlet oxygen. The anthocyanins tested exhibited antioxidant activity of variable efficiency. The structural characteristics relevant to this variability likely included the ability to form a stable quinonoidal anhydro base at neutral pH, a conjugated diene structure in the C (pyrane) ring, the presence of hydroxyl groups on the B (benzene) ring and the relative hydrophobicity conferred by the arrangement of substituents on the B ring. Cells that had taken up anthocyanins also exhibited a resistance to the membrane permeabilization that occurs as a result of the detergent-like action of A2E.     

Charge separation reaction in clusters of polar molecules: MD simulations

Rate constant of intermolecular electron transfer (ET) in a photoexcited donor-acceptor model system solvated by a cluster of polar molecules has been expressed in terms of the statistical distribution of the electrostatic potential energy difference between the reacting sites. This distribution has been calculated for a particular case of acetonitrile clusters a ≈120 K by MD computer simulation. The MD values of the cluster reorganization energy and the ET rate constant have been compared with the corresponding MD results for the donor-acceptor pair solvated in bulk acetonitrile and with theoretical predictions based on the continuum model.     

Interaction of cis-diastereomers of dicyclohexano-18-crown-6 with maleic anhydride and some “small” polar molecules

The cis-anti-cis diastereomer of dicyclohexano-18-crown-6 with maleic anhydride or formamide gives a crystalline complex of ratio 12. The three-dimensional structure of the former was found from x-ray studies. The formation of complexes was used for the separation of the individual cis-syn-cis and cis-anti-cis diastereomers from a mixture. Crystallization of a cis-anti-cis diastereomer from DMFA, DMSO, and N-methylpyrrolidine gave a high melting form.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1329–1332, October, 1989.     

Donor-acceptor systems of Pt(II) and redox-induced reactivity towards small molecules

Oxidation at a redox-active ligand is shown to enhance reactivity at the metal center and makes it susceptible to chemical reactions which also include H(2) activation.     

Structure formation in systems involving poly(ethylene oxide) and small molecules

The structure of the intercalates in which poly(ethylene oxide), PEO, is the host component and para-disubstituted benzenes are the guest molecules is completely elucidated by X-ray diffractometry, polarized FTIR spectrography, and calorimetry. It is shown that the association of these molecules results from the adaptation of the shape of their external surfaces. On this basis the possibility of formation of structure involving PEO and small molecules is discussed.