Stability of a nonlinear oscillator with random damping

A noisy damping parameter in the equation of motion of a nonlinear oscillator renders the fixed point of the system unstable when the amplitude of the noise is sufficiently large. However, the stability diagram of the system can not be predicted from the analysis of the moments of the linearized equation. In the case of a white noise, an exact formula for the Lyapunov exponent of the system is derived. We then calculate the critical damping for which the nonlinear system becomes unstable. We also characterize the intermittent structure of the bifurcated state above threshold and address the effect of temporal correlations of the noise by considering an Ornstein-Uhlenbeck noise.     

Lipid Biomarkers of Lens Aging

Lipids are important structural components of cell membranes and have profound effect on membrane fluidity. Lipid profiling and lipidomics have captured increased attention due to the well-recognized roles of lipids in numerous human diseases. Investigating lipid profiles not only provides insights into the specific roles of lipid molecular species in health and diseases, but can also help in identifying potential preventive or therapeutic biomarkers. Cataract, the loss of transparency of eye lens, is a disease of protein aggregation. There are several factors contributing to the stability in protein conformation. Age-related changes in lipid composition could be a contributing factor for altered protein–lipid interaction leading to protein aggregation and cataract. Keeping this in view, in the present study, fatty acid profiling from different age groups of lenses was carried out, using a freshwater catfish as the model. Total lipids were extracted from lenses of three different age groups of fishes (young, adult, and aged) and fatty acid methyl esters (FAME) were prepared and FAME analysis was carried out using gas chromatography–mass spectrometry. The results showed that three fatty acids viz. heneicosylic acid (C21), docosahexaenoic acid (C22:6), nervonic acid (C24:1) which were not present in the adult lens, appeared in the aged lens. On the other hand, eicosenoic acid (C20:1) present in the adult lens was found to be absent in the aged lens. The appearance or disappearance of these fatty acids can possibly serve as biomarkers of aging lens which is the most vulnerable stage for cataract development.     

Keteniminium‐Driven Umpolung Difunctionalization of Ynamides

A three‐component Pd‐catalyzed coupling of ynamides, aryl diazonium salts, and aryl boronic acids for the synthesis of novel triaryl‐substituted enamides is described. This transformation represents the first example of an umpolung regioselective unsymmetrical syn‐1,2‐diarylation/aryl‐olefination of ynamides. The aryl moieties of the diazonium salt (electrophile) and boronic acid (nucleophile) are explicitly incorporated in the electrophilic α‐ and nucleophilic β‐position, respectively, of the ynamide, resulting in a single isomer of the N‐bearing tetrasubstituted olefin. The scope is broad (68 examples), showing excellent functional‐group tolerance. DFT calculations substantiate the rationale of the mechanistic cycle and the regioselectivity. The chemoselectivity and synthetic potential of the enamide products were also studied.     

TITAN: A Code for Modeling and Generating Electric Fields—Features and Applications to Enzymatic Reactivity

We present here a versatile computational code named “elecTric fIeld generaTion And maNipulation (TITAN),” capable of generating various types of external electric fields, as well as quantifying the local (or intrinsic) electric fields present in proteins and other biological systems according to Coulomb's Law. The generated electric fields can be coupled with quantum mechanics (QM), molecular mechanics (MM), QM/MM, and molecular dynamics calculations in most available software packages. The capabilities of the TITAN code are illustrated throughout the text with the help of examples. We end by presenting an application, in which the effects of the local electric field on the hydrogen transfer reaction in cytochrome P450 OleT_JE enzyme and the modifications induced by the application of an oriented external electric field are examined. We find that the protein matrix in P450 OleT_JE acts as a moderate catalyst and that orienting an external electric field along the Fe─O bond of compound I has the biggest impact on the reaction barrier. The induced catalysis/inhibition correlates with the calculated spin density on the O-atom. © 2019 Wiley Periodicals, Inc.     

A Roadmap to Uranium Ionic Liquids: Anti‐Crystal Engineering

In the search for uranium‐based ionic liquids, tris(N,N‐dialkyldithiocarbamato)uranylates have been synthesized as salts of the 1‐butyl‐3‐methylimidazolium (C₄mim) cation. As dithiocarbamate ligands binding to the UO₂²⁺ unit, tetra‐, penta‐, hexa‐, and heptamethylenedithiocarbamates, N,N‐diethyldithiocarbamate, N‐methyl‐N‐propyldithiocarbamate, N‐ethyl‐N‐propyldithiocarbamate, and N‐methyl‐N‐butyldithiocarbamate have been explored. X‐ray single‐crystal diffraction allowed unambiguous structural characterization of all compounds except N‐methyl‐N‐butyldithiocarbamate, which is obtained as a glassy material only. In addition, powder X‐ray diffraction as well as vibrational and UV/Vis spectroscopy, supported by computational methods, were used to characterize the products. Differential scanning calorimetry was employed to investigate the phase‐transition behavior depending on the N,N‐dialkyldithiocarbamato ligand with the aim to establish structure–property relationships regarding the ionic liquid formation capability. Compounds with the least symmetric N,N‐dialkyldithiocarbamato ligand and hence the least symmetric anions, tris(N‐methyl‐N‐propyldithiocarbamato)uranylate, tris(N‐ethyl‐N‐propyldithiocarbamato)uranylate, and tris(N‐methyl‐N‐butyldithiocarbamato)uranylate, lead to the formation of (room‐temperature) ionic liquids, which confirms that low‐symmetry ions are indeed suitable to suppress crystallization. These materials combine low melting points, stable complex formation, and hydrophobicity and are therefore excellent candidates for nuclear fuel purification and recovery.     

Behavior of Highly Diluted Electrolytes in Strong Electric Fields—Prevention of Alumina Deposition on Grading Electrodes in HVDC Transmission Modules by CO2‐induced pH‐Control

Alumina deposition on platinum grading electrodes in high voltage direct current (HVDC) transmission modules is an unsolved problem that has been around for more than three decades. This is due to the unavoidable corrosion of aluminum heat sinks that causes severe damage to electrical power plants and losses in the range of a million Euro range per day in power outage. Simple experiments in a representative HV test setup showed that aluminates at concentrations even below 10^?8 mol L^?1 can deposit on anodes through neutralization by protons produced in de‐ionized water (κ≤0.15 μS cm^?1) at 20–35 kV (8 mA) per electrode. In this otherwise electrolyte‐poor aqueous environment, the depositions are formed three orders of magnitude below the critical precipitation concentration at pH 7! In the presence of an inert electrolyte such as TMAT (tetramethylammonium‐p‐toluenesulfonate), at a concentration level just above that of the total dissolved aluminum, no deposition was observed. Deposition can be also prevented by doping with CO₂ gas at a concentration level that is magnitudes lower than that of the dissolved aluminum. From an overview of aqueous aluminum chemistry, the mystery of the alumina deposition process and its inhibition by CO₂ is experimentally resolved and fully explained by field accumulation and repulsion models in synergism with acid–base equilibria. The extraordinary size of the alumina depositions is accounted for in terms of proton tunneling through “hydrated” alumina, which is supported by quantum chemical calculations. As a consequence, pulse‐purging with pure CO₂ gas is presented as a technical solution to prevent the deposition of alumina.     

Synthesis of l-3-epi-isofagomine,its homo-, n-butyl and bicyclic analogues from d-glucose as glycosidase inhibitors

Synthesis of l-3-epi-isofagomine, its homo-, n-butyl derivatives and its bicyclic analogue as potent glycosidase inhibitor has been achieved from readily available d-glucose. Inhibition of some commercially available glycosidases was also carried out with the newly synthesized inhibitors which showed reasonably good inhibitions (9.4–198.2 μM). One of them (compound 11) showed selective inhibition of β-galactosidase.     

Oriented (Local) Electric Fields Drive the Millionfold Enhancement of the H-Abstraction Catalysis Observed for Synthetic Metalloenzyme Analogues

This contribution follows the recent remarkable catalysis observed by Groves et al. in hydrogen-abstraction reactions by a) an oxoferryl porphyrin radical-cation complex [Por^⋅+Fe^IV(O)L_ax] and b) a hydroxoiron porphyrazine ferric complex [PyPzFe^III(OH)L_ax], both of which involve positively charged substituents on the outer circumference of the respective macrocyclic ligands. These charge-coronated complexes are analogues of the biologically important Compound I (Cpd I) and synthetic hydroxoferric species, respectively. We demonstrate that the observed enhancement of the H-abstraction catalysis for these systems is a purely electrostatic effect, elicited by the local charges embedded on the peripheries of the respective macrocyclic ligands. Our findings provide new insights into how electrostatics can be employed to tune the catalytic activity of metalloenzymes and can thus contribute to the future design of new and highly efficient hydrogen-abstraction catalysts.     

A Simple Dual-Beam Time-Multiplexed Photon Doppler Velocimeter for Pressure-Shear Plate Impact Experiments

Experimental Mechanics - Pressure-shear plate impact experiments generate normal and transverse particle velocities during high strain rate deformations. Traditionally, freespace lenscoupled...     

In situ synthesis of copper nanoparticles and poly(o-toluidine): A metal-polymer composite material

We report here a novel in situ synthetic method for the preparation of poly(o-toluidine) and copper nanoparticle composite material. In this experiment, o-toluidine and cupric sulfate were used as the precursor; during the reaction o-toluidine was oxidized and forms poly(o-toluidine), while on the other hand cupric sulfate gets reduced and forms copper nanoparticle. IR, UV-vis and Raman spectra provide the information on the structure of the polymer. The TEM and SEM show the size of the nanoparticles and the morphology of the polymer, respectively.