Low-temperature rapid-scan detection of reactive intermediates in epoxidation reactions catalyzed by a new enzyme mimic of cytochrome p450

The use of synthetic iron(III) porphyrins as models for heme-type catalysts in biomimetic cytochrome P450 research has provided valuable information on the nature and reactivity of intermediates produced in the "peroxide shunt" pathway. This article reports spectroscopic detection of reactive intermediates formed in the epoxidation reaction of cis-stilbene with m-chloroperoxybenzoic acid catalyzed by a new mimic of cytochrome P450 with a substituted RSO3- group (1). The application of low-temperature rapid-scan stopped-flow techniques enabled the determination of equilibrium and rate constants for the formation and decay of all intermediates in the catalytic cycle of 1, including the rate constant for the formation (1*+)FeIV=O and for oxygen transfer to the substrate. Noteworthy, the reaction of (1*+)FeIV=O with cis-stilbene leads to an almost complete re-formation (95%) of the starting complex 1. The results show that complex 1 is a valuable catalyst with promising properties for further applications in a biomimetic approach toward mimicking oxygenation reactions of cytochrome P450.     

Modified interfacial statistical associating fluid theory: a perturbation density functional theory for inhomogeneous complex fluids

A density functional theory based on Wertheim's first order perturbation theory is developed for inhomogeneous complex fluids. The theory is derived along similar lines as interfacial statistical associating fluid theory [S. Tripathi and W. G. Chapman, J. Chem. Phys. 122, 094506 (2005)]. However, the derivation is more general and applies broadly to a range of systems, retaining the simplicity of a segment density based theory. Furthermore, the theory gives the exact density profile for ideal chains in an external field. The general avail of the theory has been demonstrated by applying the theory to lipids near surfaces, lipid bilayers, and copolymer thin films. The theoretical results show excellent agreement with the results from molecular simulations.     

Wave packet driven dissociation and concerted elimination in CH2I2

We follow the evolution of a vibrational wave packet in a highly excited state of the halogenated methane CH(2)I(2). We observe how the wave packet modulates both dissociation and concerted elimination to form CH(2)I(+) and I(2) (+), respectively. We present a simple and intuitive interpretation of the molecular dynamics leading to the formation of the products.     

In situ surface-enhanced infrared absorption spectroscopy for the analysis of the adsorption and desorption process of Au nanoparticles on the SiO2/Si surface

The adsorption and desorption of Au nanoparticles (AuNP) in colloidal D2O suspension on the (3-aminopropyl)triethoxysilane treated SiO2/Si surface was investigated by in situ attenuated total reflection surface enhanced infrared absorption (ATR-SEIRA) spectroscopy with a liquid flow cell. With increasing surface density of AuNP, the absorption of the vibrational modes of D2O and of the citrate molecules covering the AuNP increases due to SEIRA. Repulsive electrostatic Coulomb forces between the AuNP lead to the saturation of the AuNP surface density at submonolayer coverage. We show that the adsorption kinetics can be investigated by monitoring in situ the molecular vibrational modes of D2O and the citrate molecules. Furthermore, we clarify that the adsorption process can be described very well by a diffusion-limited first-order Langmuir kinetics model. When exposing a saturated AuNP submonolayer to 2-aminoethanethiol (AET)/D2O solution, the AuNP are removed from the surface and the IR absorption of the D2O vibrational modes become weaker again. Taking into account the time dependencies of the OD and the CH peaks, we propose a microscopic model where the AET molecules quickly adsorb on the AuNP by replacing most of the precovering citrate molecules exposed to the AET solution. As this takes place, the AuNP agglomerate-as we could detect with scanning electron microscopy-and are finally removed from the surface.     

Computation of Material Forces in Thermoplasticity Based on Alternative Smoothing Algorithms

Different approaches to the computation of material forces in inelastic structures are investigated. Dissipative effects in inelastic materials are described by internal variables. The formulation of balance equations in the material space requires the computation of gradients of these internal variables. The computational evaluation of these gradients in the context of finite element simulations needs a global representation of the internal variable fields. On the one side, this request can be carried out by a global formulation that discretizes the internal variable fields in terms of nodal degrees additional to the displacements. A numerically more effective approach applies smoothing algorithms which project the internal variables of a typical local formulation from the integration points onto the nodal points. In detail, the implementation of two smoothing algorithms for the computation of material forces is dicussed. The L2–projection necessiates the solution of a system of equations on the global level. A patch recovery yields a smoothed solution from an element patch surrounding the nodal point of interest. The performance of both algorithms is compared for the material force computation in finite thermoplasticity. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Total synthesis and mechanism of action of the antibiotic armeniaspirol A

Emerging antimicrobial resistance urges the discovery of antibiotics with unexplored, resistance-breaking mechanisms. Armeniaspirols represent a novel class of antibiotics with a unique spiro[4.4]non-8-ene scaffold and potent activities against Gram-positive pathogens. We report a concise total synthesis of (±) armeniaspirol A in six steps with a yield of 20.3% that includes the formation of the spirocycle through a copper-catalyzed radical cross-coupling reaction. In mechanistic biological experiments, armeniaspirol A exerted potent membrane depolarization, accounting for the pH-dependent antibiotic activity. Armeniaspirol A also disrupted the membrane potential and decreased oxygen consumption in mitochondria. In planar lipid bilayers and in unilamellar vesicles, armeniaspirol A transported protons across membranes in a protein-independent manner, demonstrating that armeniaspirol A acted as a protonophore. We provide evidence that this mechanism might account for the antibiotic activity of multiple chloropyrrole-containing natural products isolated from various origins that share a 4-acylphenol moiety coupled to chloropyrrole as a joint pharmacophore. We additionally describe an efflux-mediated mechanism of resistance against armeniaspirols.

The antibiotic armeniaspirol A depolarized bacterial and mammalian cell membranes through a protonophore activity, that accounts for its potent antibiotic effects. A total synthesis of (±) armeniaspirol A was achieved in six steps.     

Perfluoro‐Tagged Benzyloxycarbonyl Protecting Group and Its Application in Fluorous Biphasic Systems

The synthesis of a new perfluoro‐tagged benzyloxycarbonyl protecting group is reported, as well as its application in the parallel protection of amines. Isolation of the protected amines was performed by simple liquid‐liquid extraction between perfluorinated and organic solvents. Deprotection was achieved by standard hydrogenolysis. The novel protecting group was also applied to cyclization protocols leading to quinazoline‐2,4‐diones. These products were isolated by simple extraction procedures     

Embedded-ring migration on graphene zigzag edge

Reaction pathways are presented for hydrogen-mediated isomerization of a zigzag graphene edge containing a five-member carbon ring surrounded by six-member rings. A new reaction sequence in which this embedded five-member ring moves, or migrates, through the edge has been identified. The elementary steps of the pathways were analyzed using density functional theory (DFT). Rate coefficients were obtained by classical transition state theory utilizing the DFT energies, frequencies, and geometries. The results indicate that this new reaction sequence is competitive with the other important zigzag edge reactions allowing embedded five-member rings to move freely within a zigzag edge. The embedded rings have slight thermodynamic preference for the interior of the edge over the corner for large substrates.