A single residue switch converts abietadiene synthase into a pimaradiene specific cyclase

Terpene synthases often catalyze complex cyclization reactions that typically represent the committed step in particular biosynthetic pathways, leading to great interest in their enzymatic mechanisms. We have recently demonstrated that substitution of a specific Ile with Thr was sufficient to "short circuit" the complex cyclization reaction normally catalyzed by ent-kaurene synthases to instead produce ent-pimaradiene. Here we report that the complex cyclization/rearrangement reaction catalyzed by abietadiene synthase can be similarly cut short to produce pimaradienes by an analogous Ser for Ala change, albeit with a slight shift in active site location to accommodate the difference in substrate stereochemistry. This result has mechanistic implications for enzymatic catalysis of abietadiene cyclization, and terpene synthases more broadly. Furthermore, these defined single residue switches may be useful in engineering product outcome in diterpene synthases more generally.     

Cyclopropanation of diazoesters with styrene derivatives catalyzed by magnetically recoverable copper-plated iron nanoparticles

Simple bare copper-plated iron nanoparticles catalyzed the cyclopropanation of diazoesters with styrene derivatives. The reaction proceeded smoothly and provided the desired products in moderate to good yields, with selectivity for the trans isomer in neat conditions. The reaction scope was explored and di or tri-substituted cyclopropanes were synthesized. The catalysts could be magnetically separated and reusable up to five times. It was also characterized by TEM, XPS, and ICP-MS. A gram-scale reaction was performed with a yield of 72%.     

Shape effect of diamagnetic susceptibility of a hydrogenic donor in a nano structured semiconductor systems

The diamagnetic susceptibility of a hydrogenic donor in GaAs/ Al_xGa_1-xAs semiconductor nanostructured systems like Quantum Well Wire and Quantum Dot has been computed for various cross-sectional geometries of these systems for various width of the confining potential in the effective mass approximation using variational method. The non parabolicity of the conduction band has also been included. This calculation may throw some light on the effect of geometries on the semiconductor–metal transition in these systems.     

Ring strain effects: VI—benzocyclobutadienoquinone radical anion

Electron paramagnetic resonance studies were carried out on the radical anion of 4-deuterobenzocyclobutadienoquinone. The hyperfine coupling constants which had been assigned previously were found to be in error. The effects of the strained 4-membered ring on the spin density distribution are discussed.     

Field-driven super/subradiant lasing without inversion in three-level ladder scheme

A model of a superradiant laser comprising of N three-level systems driven by two pumping lasers is considered in semiclassical approximation. Steady-state equations for density matrix elements are solved both analytically and numerically. Various properties of generated light are investigated. It is found that such a laser can exhibit both superradiance, when the intensity scales as N(2), and subradiance when the intensity does not depend on N. The phase of the laser field is locked to the relative phase of the pumping lasers.     

Effect of isotopically sensitive branching on product distribution for pentalenene synthase: support for a mechanism predicted by quantum chemistry

Mechanistic proposals for the carbocation cascade reaction leading to the tricyclic sesquiterpene pentalenene are assessed in light of the results of isotopically sensitive branching experiments with the H309A mutant of pentalenene synthase. These experimental results support a mechanism for pentalenene formation involving a 7-protoilludyl cation whose intermediacy was first predicted using quantum-chemical calculations.     

Determination of Total Petroleum Hydrocarbon (TPH) and Polycyclic Aromatic Hydrocarbon (PAH) in Soils: A Review of Spectroscopic and Nonspectroscopic Techniques

Abstract: In the analysis of petroleum hydrocarbon–contaminated soils for total petroleum hydrocarbons (TPHs) and polycyclic aromatic hydrocarbons (PAHs), the roles of spectroscopic and nonspectroscopic techniques are inseparable. Therefore, spectroscopic techniques cannot be discussed in isolation. In this report, spectroscopic techniques including Raman, fluorescence, infrared, and visible and near-infrared (Vis-NIR) spectroscopies, as well as mass spectroscopy (coupled to a gas chromatograph) and nonspectroscopic techniques such as gravimetry, immunoassay, and gas chromatography with flame ionization detection are reviewed. To bridge the perceived gap in coverage of the quantitative applications of Vis-NIR spectroscopy in the rapid determination of TPHs and PAHs in soils, a detailed review of studies from the period 1999–2012 are presented. This report also highlights the strengths and limitations of these techniques and evaluates their performance from the perspective of their attributes of general applicability, namely economic portability, operational time, accuracy, and occupational health and safety considerations. Overall, the fluorescence spectroscopic technique had the best performance (85% total score) in comparison to the others, and the gravimetric technique performed the least (60% total score). Method-specific solutions geared toward performance improvement are also suggested.