Buoyant plume above a line source of heat in an anisotropic porous medium

Buoyancy-induced convection arising from a horizontal line heat source embedded in an anisotropic porous medium is investigated analytically. The porous medium is anisotropic is permeability with its principal axes oriented in a direction that is oblique to the gravity vector. Assuming the boundary layer approximation, closed-form exact similarity solutions for both flow and temperature fields are presented and compared with those of isotropic case. Scale analysis is applied to predict the order of magnitudes involved in the boundary layer regime for which the conditions of validity are obtained. Effects of both anisotropic parameters (K* and %) and Rayleigh number Ra_L are observed to be strongly significant. It is demonstrated that a minimum (maximum) intensity of the thermal convective plume above the line source of heat can be obtained if the porous matrix is oriented with its principal axis with higher permeability parallel (perpendicular) to the vertical direction.     

Photonic tunneling between two wires

The tunneling of a well-defined propagating state from one wire to another, leaving all other neighbor states unaffected, is of great importance for photons, electrons and phonons. We present a simple coupling structure enabling such a transfer to be realized, for any given state energy and state width. We illustrate the analytic study of this structure by an application to the transmission of a telecommunication signal. We discuss then how such a structure could be realized with the help of current surface technologies.     

On global motions of a compressible barotropic and selfgravitating gas with density-dependent viscosities

We consider the Cauchy problem for the equations of selfgravitating motions of a barotropic gas with density-dependent viscosities μ(ρ), and λ(ρ) satisfying the Bresch–Desjardins condition, when the pressure P(ρ) is not necessarily a monotone function of the density. We prove that this problem admits a global weak solution provided that the adiabatic exponent γ associated with P(ρ) satisfies ${gamma > frac{4}{3}}${gamma > frac{4}{3}}.     

DNA-Based Asymmetric Inverse Electron-Demand Hetero-Diels–Alder

While artificial cyclases hold great promise in chemical synthesis, this work presents the first example of a DNA-catalyzed inverse electron-demand hetero-Diels–Alder (IEDHDA) between dihydrofuran and various α,β-unsaturated acyl imidazoles. The resulting fused bicyclic O,O-acetals containing three contiguous stereogenic centers are obtained in high yields (up to 99 %) and excellent diastereo- (up to >99:1 dr) and enantioselectivities (up to 95 % ee) using a low catalyst loading. Most importantly, these results show that the concept of DNA-based asymmetric catalysis can be expanded to new synthetic transformations offering an efficient, sustainable, and highly selective tool for the construction of chiral building blocks.     

Synthesis and preliminary evaluation of pro-RNA 2'-O-masked with biolabile pivaloyloxymethyl groups in an RNA interference assay

The cellular delivery of bioactive nucleic acid-based drugs such as small interfering RNA (siRNA) represents a major technical hurdle for their pharmaceutical application. Prodrug-like approaches provide an attractive concept to address the delivery problem. With the aim to prepare RNA-based prodrugs bearing biolabile protections which facilitate cellular uptake and are prone to be removed enzymatically inside cells in order to release functional RNA, we synthesized pro-RNA totally or partially masked in 2'-OH position with pivaloyloxymethyl (PivOM) groups. A suitable strategy has been developed to synthesize and to purify base-sensitive mixed 2'-OH/2'-O-PivOM oligoribonucleotides, and to include them in siRNA. In this strategy, the fluoride labile [(triisopropylsilyl)oxy]-benzyloxycarbonyl group (tboc) as nucleobase protection (for A and C), the TBS group as 2'-OH protection and the Q-linker to solid-support were compatible with the PivOM groups masking some 2'-OH. We have taken advantage of the specific stability of the PivOM group to apply selected acidic, basic, and fluoride ions treatment for the deprotection and release of pro-RNA. This kind of pro-siRNA was studied in a human cell culture-based RNAi assay and preliminary promising data are discussed.     

Efficient guanidination of the phosphate linkage towards cationic phosphoramidate oligonucleotides

An efficient postsynthesis method of guanidination of oligonucleotides was employed to introduce several guanidinium groups into internucleotide phosphoramidate linkages. The amino functions of aminobutylphosphoramidate links were converted to guanidine butylphosphoramidates using a solution of O-methylisourea hemisulfate in aqueous ammonia, in a short reaction time. The synthesis of various fully guanidylated oligonucleotides was successfully performed to provide a new class of cationic phosphoramidate oligonucleotides.     

Delocalization due to correlations in two-dimensional disordered systems

We study the spectral statistics of interacting spin-less fermions in a two-dimensional disordered lattice. Within a full quantum treatment for small few-particle systems, we compute the low-energy many-body states numerically. While at weak disorder the interactions reduce spectral correlations and lead to localization, for the case of strong disorder we find that a moderate Coulomb interaction has a delocalizing effect. In addition, we observe a non-universal structure in the level-spacing distribution which we attribute to a mechanism reinforcing spectral correlations taking place in small systems at strong disorder.Received: 1 July 2004, Published online: 14 December 2004PACS: 71.27. + a Strongly correlated electron systems; heavy fermions - 73.20.Jc Delocalization processes - 72.15.Rn Localization effects (Anderson or weak localization)     

Microstubs resonators integrated to bent Y-branch waveguide

We report numerical simulations, based on a finite difference time domain (FDTD) method, of light propagation in two-dimensional semiconductor micro-optical waveguides coupled to one or several lateral stubs. It is shown that when the stub is covered with a perfectly metallic thin layer, the transmission spectrum contains several narrow dips. Such simulation of the metallic coating can be used in the far infrared frequency domain, far from the optical regime. We propose a selective filtering device based on the interaction between several stubs. Inserting an appropriate defect stub between a set of periodical stubs leads to a tunnelling transmission, with a narrow peak inside the gap. This filtering phenomenon is used to propose a demultiplexer based on a Y-shaped waveguide for separating signals with different frequencies. Finally, we show that the filtering effect of a stub can also be reproduced when the metal is described in the frame of a Drude model instead of being perfect, which makes plausible the realization of the above devices in the near optical regime.     

Evidence of fano-like interference phenomena in locally resonant materials

Sonic crystals consisting of three-dimensional arrays of units which exhibit localized resonances have been discovered recently. Here, it is shown that their two-dimensional counterparts behave in a similar manner. Particularly, it is observed that the transmittance spectra show very asymmetric peaks which are explained as a Fano-like interference phenomenon. A finite difference time domain method is employed to perform a comprehensive study of the resonance line shape as a function of the mass density of the structural units. Also, a simple analytical model is introduced to give an intuitive account of the origin of the interference phenomenon.