Short New Route to the Chiral Spiro‐Tetrahydrofuran Subunit Common to Some Terpenoids

Synthetic strategy based on a stereoselective iodoetherification reaction of a carvone‐derived hydroxyalkene unit has been developed for the asymmetric synthesis of the key spiro‐tetrahydrofuran subunit common to some naturally occurring terpenoids.     

Sequential assembly of an active RNA polymerase molecule at the air-water interface

At the heart of understanding cellular processes lies our ability to explore the specific nature of communication between sequential information carrying biopolymers. However, the data extracted from conventional solution phase studies may not reflect the dynamics of communication between recognized partners as they occur in the crowded cellular milieu. We use the principle of immobilization of histidine-tagged biopolymers at a Ni(II)-encoded Langmuir monolayer to study sequence-specific protein-protein interactions in an artificially crowded environment. The advantage of this technique lies in increasing the surface density of one of the interacting partners that allows us to study macromolecular interactions in a controlled crowded environment, but without compromising the speed of the reactions. We have taken advantage of this technique to follow the sequential assembly process of the multiprotein complex Escherichia coli RNA polymerase at the interface and also deciphered the role of one of the proteins, omega (ω), in the assembly pathway. Our reconstitution studies indicate that in the absence of molecular chaperones or other cofactors, omega (ω) plays a decisive role in refolding the largest protein beta prime (β') and its recruitment into the multimeric assembly to reconstitute an active RNA polymerase. It was also observed that the monolayer had the ability to distinguish between sequence-specific and -nonspecific interactions despite the immobilization of one of the biomacromolecules. The technique provides a universal two-dimensional template for studying protein-ligand interactions while mimicking molecular crowding.     

Hydrogen‐Bonding Interactions Trigger a Spin‐Flip in Iron(III) Porphyrin Complexes

A key step in cytochrome P450 catalysis includes the spin‐state crossing from low spin to high spin upon substrate binding and subsequent reduction of the heme. Clearly, a weak perturbation in P450 enzymes triggers a spin‐state crossing. However, the origin of the process whereby enzymes reorganize their active site through external perturbations, such as hydrogen bonding, is still poorly understood. We have thus studied the impact of hydrogen‐bonding interactions on the electronic structure of a five‐coordinate iron(III) octaethyltetraarylporphyrin chloride. The spin state of the metal was found to switch reversibly between high (S=⁵/₂) and intermediate spin (S=³/₂) with hydrogen bonding. Our study highlights the possible effects and importance of hydrogen‐bonding interactions in heme proteins. This is the first example of a synthetic iron(III) complex that can reversibly change its spin state between a high and an intermediate state through weak external perturbations.     

Benzannulation of Triynes to Generate Functionalized Arenes by Spontaneous Incorporation of Nucleophiles

The thermal reaction of ester‐tethered 1,3,8‐triynes provides novel benzannulation products with concomitant incorporation of a nucleophile. Evidence suggests that this reaction proceeds via an allene‐enyne intermediate generated by an Alder‐ene reaction in the first step. Depending on the substituent of the alkyne moiety on the allene‐enyne intermediate, the subsequent transformation can take one of two different paths, each leading to discrete aromatization products. The benzannulation of a silane‐substituted 1,3,8‐triynes provides arene products with a nucleophile incorporated onto the newly formed benzene core, whereas an aryl substituent leads to nucleophile trapping at the benzylic carbon atom connected to the aryl substituent. The formation of these two different products results from the involvement of two regioisomeric allene‐enyne intermediates.     

High-affinity DNA targeting using readily accessible mimics of N2'-functionalized 2'-amino-α-L-LNA

N2'-Pyrene-functionalized 2'-amino-α-L-LNAs (locked nucleic acids) display extraordinary affinity toward complementary DNA targets due to favorable preorganization of the pyrene moieties for hybridization-induced intercalation. Unfortunately, the synthesis of these monomers is challenging (~20 steps, <3% overall yield), which has precluded full characterization of DNA-targeting applications based on these materials. Access to more readily accessible functional mimics would be highly desirable. Here we describe short synthetic routes to a series of O2'-intercalator-functionalized uridine and N2'-intercalator-functionalized 2'-N-methyl-2'-aminouridine monomers and demonstrate, via thermal denaturation, UV-vis absorption and fluorescence spectroscopy experiments, that several of them mimic the DNA-hybridization properties of N2'-pyrene-functionalized 2'-amino-α-L-LNAs. For example, oligodeoxyribonucleotides (ONs) modified with 2'-O-(coronen-1-yl)methyluridine monomer Z, 2'-O-(pyren-1-yl)methyluridine monomer Y, or 2'-N-(pyren-1-ylmethyl)-2'-N-methylaminouridine monomer Q display prominent increases in thermal affinity toward complementary DNA relative to reference strands (average ΔT(m)/mod up to +12 °C), pronounced DNA-selectivity, and higher target specificity than 2'-amino-α-L-LNA benchmark probes. In contrast, ONs modified with 2'-O-(2-napthyl)uridine monomer W, 2'-O-(pyren-1-yl)uridine monomer X or 2'-N-(pyren-1-ylcarbonyl)-2'-N-methylaminouridine monomer S display very low affinity toward DNA targets. This demonstrates that even conservative alterations in linker chemistry, linker length, and surface area of the appended intercalators have marked impact on DNA-hybridization characteristics. Straightforward access to high-affinity building blocks such as Q, Y, and Z is likely to accelerate their use in DNA-targeting applications within nucleic acid based diagnostics, therapeutics, and material science.     

Magnetic and electrical properties of oxygen stabilized nickel nanofibers prepared by the borohydride reduction method

Fine nickel fibers have been synthesized by chemical reduction of nickel ions in aqueous medium with sodium borohydride. The thermal stability and relevant properties of these fibers, as-prepared as well as air-annealed, have been investigated by structural, magnetic and electrical measurements. As-prepared samples appear to have a novel crystal structure due to the presence of interstitial oxygen. Upon annealing in air, the fcc-Ni phase emerges out initially and develops into a nanocomposite subsequently by retaining its fiber-like structure in nano phase. The as-prepared sample is observed to be weakly magnetic at room temperature, but attains surprisingly high magnetization values at low temperatures. This is attributed to the modified spin structure, presumably due to the presence of interstitial oxygen in the lattice. Development of a weakly ferromagnetic and electrically conducting phase upon annealing in air is attributed to the formation of the fcc-Ni phase. The structural phase transformations corroborate well with magnetic and electrical measurements.     

Kinetic Study of the Interaction of Three Glycine-Containing Dipeptides with Hydroxopentaaquarhodium(III) Ion in Aqueous Medium

The kinetics of the interaction of three glycine-containing dipeptides, namely, glycyl-L-valine (L¹-L??H), glycyl-glycine (L²-L??H) and glycyl-L-glutamine (L³-L??H), with [Rh(H₂O)₅OH]²⁺ has been studied spectrophotometrically in aqueous medium as a function of the Rh(H₂O)₅OH²⁺ and dipeptide concentrations, pH and temperature, at constant ionic strength. At pH = 4.3, the substrate complex exists predominantly as the hydroxopentaaqua species and dipeptides as zwitterions. The reaction has been found to proceed via two parallel paths: both processes are ligand dependent. The rate constants for the processes are of the order: k ₁??10^?3 s^?1 and k ₂??10^?5 s^?1. The activation parameters for both steps were evaluated from Eyring plots. Based on the kinetic and activation parameters an associative interchange mechanism is proposed for both of the interaction processes. The low $\Delta H_{1}^{\neq}$ and $\Delta H_{2}^{\neq}$ values and large negative values of $\Delta S_{1}^{\neq}$ and $\Delta S_{2}^{\neq}$ support the associative mode of activation for both processes. The product of the reaction has been characterized using IR and ESI-mass spectroscopic analysis.     

Mixed convection heat transfer from three heated square cylinders in cross-flow at low Reynolds numbers

This paper presents the effects of cross buoyancy and Prandtl number on the flow and heat transfer characteristics around three equal isothermal square cylinders arranged in a staggered configuration within an unconfined medium. Transient two-dimensional numerical simulations are performed with a finite volume code based on the SIMPLEC algorithm in a collocated grid system. The pertinent dimensionless parameters, such as Reynolds, Prandtl and Richardson numbers are considered in the range of 1 ≤ Re ≤ 30, 0.7 ≤ Pr ≤ 100 and 0 ≤ Ri ≤ 1. The representative streamlines, vortex structures and isotherm patterns are presented and discussed. In addition, the overall drag and lift coefficients and average Nusselt numbers are determined to elucidate the effects of Reynolds, Prandtl and Richardson numbers on flow and heat transfer. The flow is observed to be steady for all the ranges of parameters considered. The drag coefficient is found to decrease with Re (for Ri = 0) and Ri at low Pr, whereas it increases with Pr at higher Ri. The lift coefficient decreases with Ri at low Pr and increases with Pr at higher Ri. The time and surface average cylinder Nusselt number is found to increase monotonically with Re as well as Pr while it remains almost insensitive to Ri at low Pr.     

Dynamic modeling of soil–tool interaction: An overview from a fluid flow perspective

The study of tillage tool interaction centers on soil failure patterns and development of force prediction models for design optimization. The force-deformation relationships used in models developed to date have been considering soil as a rigid solid or elasto-plastic medium. Most of the models are based on quasi-static soil failure patterns. In recent years, efforts have been made to improve the conventional analytical and experimental models by numerical approaches. This paper aims at reviewing the existing methods of tillage tool modeling and exploring the use of computational fluid dynamics to deal with unresolved aspects of soil dynamics in tillage. The discussion also focuses on soil rheological behaviour for its visco-plastic nature and its mass deformation due to machine interaction which may be analyzed as a Bingham plastic material using a fluid flow approach. Preliminary results on visco-plastic soil deformation patterns and failure front advancement are very encouraging. For a tool operating speed of 5.5 m s⁻¹, the soil failure front was observed to be about 100-mm forward of the tool.     

g-B3C2N3: A Potential Two Dimensional Metal-free Photocatalyst for Overall Water Splitting**

Based on hybrid density functional theory (DFT) calculations, we propose a new two-dimensional (2D) B-C-N material, graphitic- (g- ), with the promising prospect of metal-free photocatalysis. We find it to be a near ultraviolet (UV) absorbing direct band gap (3.69 eV) semiconductor with robust dynamical and mechanical stability. Estimating the band positions with respect to water oxidation and hydrogen reduction potential levels along with a detailed analysis of reaction mechanism of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), we observe that g- monolayer can be efficiently used for hydrogen fuel generation over entire pH range as well as for spontaneous water splitting at basic pH range. Upon biaxial strain application, band positions get realigned along with the free energy change that is involved in HER and OER. Consequently, operational range of pH for OER gets broadened and the proposed material exhibits the ability to perform spontaneous and simultaneous oxidation and reduction even in neutral pH. The combination of pH variation and applied strain can be used as a key to control the reducing and/or oxidizing abilities precisely for diverse photocatalytic reactions to attain environmental sustainability.