Total Synthesis of Albicidin: A Lead Structure from Xanthomonas albilineans for Potent Antibacterial Gyrase Inhibitors

The peptide antibiotic albicidin, which is synthesized by the plant pathogenic bacterium Xanthomonas albilineans, displays remarkable antibacterial activity against various Gram‐positive and Gram‐negative microorganisms. The low amounts of albicidin obtainable from the producing organism or through heterologous expression are limiting factors in providing sufficient material for bioactivity profiling and structure–activity studies. Therefore, we developed a convergent total synthesis route toward albicidin. The unexpectedly difficult formation of amide bonds between the aromatic amino acids was achieved through a triphosgene‐mediated coupling strategy. The herein presented synthesis of albicidin confirms the previously determined chemical structure and underlines the extraordinary antibacterial activity of this compound. The synthetic protocol will provide multigram amounts of albicidin for further profiling of its drug properties.     

Para‐hydrogen induced polarization of Si‐29 NMR resonances as a potentially useful tool for analytical applications

Para‐hydrogen–induced polarization effects have been observed in the ²⁹Si NMR spectra of trimethylsilyl para‐hydrogenated molecules. The high signal enhancements and the long T₁ values observed for the ²⁹Si hyperpolarized resonances point toward the possibility of using ²⁹Si for hyperpolarization applications. A method for the discrimination of multiple compounds and/or complex mixtures of hydroxylic compounds (such as steroids), consisting of the silylization of alcoholic functionalities with an unsaturated silylalkyl moiety and subsequent reaction with para‐H₂, is proposed. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and Conformation of 3,6‐Connected Cyclohexadiene Chains

3,6‐Connected cyclohexadienes as precursors for polyphenylenes are synthesized and characterized by mass spectrometry and NMR spectroscopy. Pure fractions of trimers, hexamers, and nonamers are collected after separation of the product mixture by recycling GPC. The anticipated formation of rigid linear structures, due to the trans‐configuration of the monomeric units, is supported by density functional theory and experimentally confirmed by dynamic light scattering from dilute solution at low scattering angles. The obtained translational diffusion coefficients are represented by rigid rod‐like or prolate ellipsoid‐like molecular shapes. The measurements of diffusion coefficients reveal a length‐dependent ratio of 1:2:3 between the three oligomers, which directly correlates to the expected length extension from trimer to nonamer.

     

Zirconium Borohydride - a Versatile Reducing Agent for the Reduction of Electrophilic and Nucleophilic Substrates

Zirconium borohydride, a potential reducing agent, reduces acids, esters, imines to the corresponding alcohols and secondary amines in good yield at room temperature within two hours. This facile reducing property was taken advantage off in the synthesis of pheromones and some novel chiral precursors for asymmetric synthesis.     

密度泛函活性理论中的信息论方法

密度泛函活性理论(DFRT)运用简单的密度泛函探讨和定量化分子的反应活性,是近来发展起来的一个关于分子活性理论的新方法。在新近的文献中,这样的简单密度泛函的例子包括香农熵,费舍尔信息以及其它来自信息论中的密度泛函。本文综述了DFRT信息论方法的原理,包括物理信息极小原理、最小信息增益原理和信息守恒原理。总结了DFRT信息论方法在电子密度、形态密度和分子中的原子三种表述下的理论框架。此外,还介绍了运用信息论方法在定量描述空间位阻效应、亲电性、亲核性和区域选择性中的突出应用,以及对亲电芳香取代反应的邻对间位取代效应的起源和本质提供的一个全新诠释。最后简要地展望了该领域的几个可能的未来发展方向。     

Improved Thin‐Film Transistor Performance Through a Melt of Poly(para‐phenyleneethynylene)

The performance of polymer field‐effect transistors (PFETs) based on short rigid rod semiconducting poly(2,5‐didodecyloxy‐p‐phenyleneethynylene) (D‐OPPE) is highlighted. The controlled heating and cooling of thin films of D‐OPPE allows for a recrystallization from the melt, boosting the performance of D‐OPPE‐based transistors. The improved film properties induced by controlled annealing lead to a hole field‐effect mobility around 0.014 cm² V⁻¹ s⁻¹, an on/off ratio of 10⁶, a sub‐threshold swing of 3 V dec⁻¹ and a threshold voltage of −35 V, employing a poly(methyl methacrylate) (PMMA) gate dielectric. Thus, PFETs out of D‐OPPE compete now with spin‐coated, polycrystalline poly(3‐hexylthiophene)‐based PFETs.

     

三角柱直眼掏槽爆破参数研究

通过对三角柱直眼掏槽爆破进行试验研究 ,认为爆破槽腔的形成过程可分成两个阶段 ,即岩石破坏阶段和破碎岩石抛出阶段 ;进而 ,依照岩石压缩破坏与剪切破坏的力学准则 ,导出三角柱直眼掏槽炮眼间距与装药系数的计算式 ;根据计算结果 ,在井下巷道工程实际应用中进行对比 ,证实了它的可靠性。     

A comparative electrochemical study of diffusion in room temperature ionic liquid solvents versus acetonitrile.

Measurements on the diffusion coefficient of the neutral molecule N,N,N',N'-tetramethyl-para-phenylenediamine and the radical cation and dication generated by its one- and two-electron oxidation, respectively, are reported over the range 298-348 K in both acetonitrile and four room temperature ionic liquids (RTILs). Data were collected using single and double potential step chronoamperometry at a gold disk electrode of micrometer dimension, and analysed via fitting to the appropriate analytical expression or, where necessary, to simulation. The variation of diffusion coefficient with temperature was found to occur in an Arrhenius-type manner for all combinations of solute and solvent. For a given ionic liquid, the diffusional activation energies of each species were not only closely equivalent to each other, but also to the RTIL's activation energy of viscous flow. In acetonitrile supported with 0.1 M tetrabutylammonium perchlorate, the ratio in diffusion coefficients of the radical cation and dication to the neutral molecule were calculated as 0.89 +/- 0.05 and 0.51 +/- 0.03, respectively. In contrast, amongst the ionic liquids the same ratios were determined to be on average 0.53 +/- 0.04 and 0.33 +/- 0.03. The consequences of this dissimilarity are considered in terms of the modelling of voltammetric data gathered within ionic liquid solvents.     

A computational study of unique properties of pillar[n]quinones: self-assembly to tubular structures and potential applications as electron acceptors and anion recognizers

Density functional theory has been used to calculate the thermodynamic properties and molecular orbitals of pillar[n]quinones. Pillar[n]quinones are expected to be effective electron acceptors and the ability to accept more than one electron increases with the size of the interior cavity. Pillar[5]quinone and pillar[7]quinone show a great intramolecular charge transfer upon the electron excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) as indicated by a large difference of electron distributions between their HOMO and LUMO and a notable dipole moment difference between the ground and first triplet excited state. The aggregation of pillar[n]quinones leads to tubular dimeric structures joined by 2n C? H···O nonclassical hydrogen bonds (HBs) with binding energies about 2 kcal/mol per HB. The longitudinal extension of the supramolecular self‐assembly of pillar[n]quinone may be adjustable through forming and breaking their HBs by controlling the surrounding environment. The tunability of the diameter of the tubular structures can be achieved by changing the number of quinone units in the pillar[n]quinone. The electrostatic potential maps of pillar[n]quinones indicate that the positive charge in the interior cavity decreases as the number of quinone units increases. Chloride and bromide anions are chosen to examine the noncovalent anion‐π interactions between pillar[n]quinones and captured anions. The calculations show that the better compatibility of the effective radius of the anions with the interior dimension of pillar[n]quinone leads to larger stabilization energy. The selectivity of spatial matching and specific interaction of pillar[n]quinone is believed to possibly serve as a candidate for ionic and molecular recognition. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011