Slow Step Research Articles

Mechanism and Catalysis of Oxidative Degradation of Fiber-Reinforced Epoxy Composites.

Carbon fiber-reinforced polymer (CFRP) materials are widely used in aerospace and recreational equipment, but there is no efficient procedure for their end-of-life recycling. Ongoing work in the chemistry and engineering communities emphasizes recovering carbon fibers from such waste streams by dissolving or destroying the polymer binding. By contrast, our goal is to depolymerize amine-cured epoxy CFRP composites catalytically, thus enabling not only isolation of high-value carbon fibers, but simultaneously opening an approach to recovery of small molecule monomers that can be used to regenerate precursors to new composite resin. To do so will require understanding of the molecular mechanism(s) of such degradation sequences. Prior work has shown the utility of hydrogen peroxide as a reagent to affect epoxy matrix decomposition [1]. Herein we describe the chemical transformations involved in that sequence: the reaction proceeds by oxygen atom transfer to the polymer's linking aniline group, forming an N-oxide intermediate. The polymer is then cleaved by an elimination and hydrolysis sequence. We find that elimination is the slower step. Scandium trichloride is an efficient catalyst for this step, reducing reaction time in homogeneous model systems and neat cured matrix blocks. The conditions can be applied to composed composite materials, from which pristine carbon fibers can be recovered.

Polyethylene Glycols as Efficient Catalysts for the Oxidation of Bicyclic Monoterpenes by Ceric Ammonium Nitrate in Acetonitrile under Acid‐Free Conditions: Kinetic and Mechanistic Approach

ABSTRACTPolyethylene glycols (PEG) acts as efficient catalysts for the oxidation of bicyclic monoterpenes such as borneol, isoborneol, and camphor by ceric ammonium nitrate (CAN), a laboratory desktop reagent, in acetonitrile medium under mineral acid‐free conditions. The kinetics of the reactions revealed first‐order dependence on in both [CAN] and [bicyclic terpene]. The rate of oxidation is accelerated with an increase in [PEG] linearly, which could be explained by considering PEG‐bound oxidant (PEG‐CAN) as more reactive species than (CAN) itself. The mechanism of oxidation in PEG media has been explained through the participation of PEG‐bound oxidant (PEG‐CAN) and bicyclic monoterpene in the slow step.

Do Skeletal Dynamics Mediate Sugar Uptake and Transport in Human Erythrocytes?

We explore, herein, the hypothesis that transport of molecules or ions into erythrocytes may be affected and directly stimulated by the dynamics of the spectrin/actin skeleton. Skeleton/actin motions are driven by thermal fluctuations that may be influenced by ATP hydrolysis as well as by structural alterations of the junctional complexes that connect the skeleton to the cell’s lipid membrane. Specifically, we focus on the uptake of glucose into erythrocytes via glucose transporter 1 and on the kinetics of glucose disassociation at the endofacial side of glucose transporter 1. We argue that glucose disassociation is affected by both hydrodynamic forces induced by the actin/spectrin skeleton and by probable contact of the swinging 37-nm-long F-actin protofilament with glucose, an effect we dub the “stickball effect.” Our hypothesis and results are interpreted within the framework of the kinetic measurements and compartmental kinetic models of Carruthers and co-workers; these experimental results and models describe glucose disassociation as the “slow step” (i.e., rate-limiting step) in the uptake process. Our hypothesis is further supported by direct simulations of skeleton-enhanced transport using our molecular-based models for the actin/spectrin skeleton as well as by experimental measurements of glucose uptake into cells subject to shear deformations, which demonstrate the hydrodynamic effects of advection. Our simulations have, in fact, previously demonstrated enhanced skeletal dynamics in cells in shear deformations, as they occur naturally within the skeleton, which is an effect also supported by experimental observations.

High-efficient energy recovery from organics degradation for neutral wastewater treatment based on radicals catalytic reaction of Fe2+/Fe3+-EDTA complexes

A high-efficient photo-induced wastewater fuel cell (WFC) for neutral wastewater treatment was proposed in this paper based on radicals catalytic reaction of Fe3+/Fe2+- EDTA complexes in the system which has a lower redox potential than Fe3+/Fe2+ and can accelerate easily the conversion of slow catalytic step from Fe (III) to Fe (II). The results indicated that the WFC shows an excellent performance in a wide pH range of 5–9 and achieves optimal efficiency for organic degradation and electricity generation at pH 7.0. The removal ratio of organic pollutants (Rhodamine b, Phenol, and Methylene Blue) increased to 69.42%, 53.99% and 82.7% from 29.87%, 16.25% and 39% respectively after 3 h operation at an initial pH of 7.0. Meanwhile, the short-circuit current is up to 1.24–2.16 times that of the WFC without EDTA-ferrous complex. Furthermore, the system almost does not produce any sludge. The proposed WFC system can keep stable of Fe3+/Fe2+ in neutral solution and generate more intermediate active free radicals to treat neutral wastewater and recover the abundant chemical energy in organics.

Structural basis of transcriptional stalling and bypass of abasic DNA lesion by RNA polymerase II

Abasic sites are among the most abundant DNA lesions and interfere with DNA replication and transcription, but the mechanism of their action on transcription remains unknown. Here we applied a combined structural and biochemical approach for a comprehensive investigation of how RNA polymerase II (Pol II) processes an abasic site, leading to slow bypass of lesion. Encounter of Pol II with an abasic site involves two consecutive slow steps: insertion of adenine opposite a noninstructive abasic site (the A-rule), followed by extension of the 3'-rAMP with the next cognate nucleotide. Further studies provided structural insights into the A-rule: ATP is slowly incorporated into RNA in the absence of template guidance. Our structure revealed that ATP is bound to the Pol II active site, whereas the abasic site is located at an intermediate state above the Bridge Helix, a conserved structural motif that is cirtical for Pol II activity. The next extension step occurs in a template-dependent manner where a cognate substrate is incorporated, despite at a much slower rate compared with nondamaged template. During the extension step, neither the cognate substrate nor the template base is located at the canonical position, providing a structural explanation as to why this step is as slow as the insertion step. Taken together, our studies provide a comprehensive understanding of Pol II stalling and bypass of the abasic site in the DNA template.

Kinetic and Mechanistic Evaluation of Inorganic Arsenic Species Adsorption onto Humic Acid Grafted Magnetite Nanoparticles

Humic acid coated magnetic iron oxide nanoparticles (HA-MNPs) were synthesized, characterized, and studied for the removal of toxic inorganic arsenic species from aqueous media. The adsorption of As(III) and As(V) followed pseudo-second-order kinetics, and the observed data were accurately modeled employing the Freundlich adsorption isotherm. Application of the Weber and Morris intraparticle diffusion model to the observed kinetic data suggests that the adsorption occurs in three distinct stages, fast, intermediate, and slow steps. We propose the initial stage is governed by surface association, followed by intraparticle diffusion of arsenic through the HA matrix and, finally, chemical reaction or bonding between the arsenic species and HA functionality. The HA-MNP nanoadsorbent absorbs >95% of the inorganic arsenic species with an absorption capacity of 12.2–12.6 mg/g from aqueous media and is effective under a variety of conditions. Post arsenic adsorption characterization of the nanoparticles suggests ...

Gel-emulsions prepared using a low-molecular-weight gelator and their use in the synthesis of porous polymers

Water-in-oil (W/O) gel-emulsions consisting of water and a monomer were successfully prepared using N-3-hydroxypropylcarbonyl-L-isoleucyl-aminooctadecane as a gelator. Low-temperature polymerization of the gel-emulsions with a redox initiator was performed to obtain the corresponding porous polymers. Furthermore, polymerization of gel-emulsions containing bifunctional monomers gave crosslinked porous PMMA-HDODA, PMMA-EGDMA, PMMA-DVB, and PSt-DVB, which were found to be mechanically robust and solvent resistant. The microstructures of the porous polymers and crosslinked porous polymers were observed by scanning electron microscopy. The adsorption capacities of the polymers toward methanol, dichloromethane, acetone, tetrahydrofuran, toluene, hexane, and kerosene were assessed and rationalized in terms of the surface microstructures of the polymers. The time courses of the adsorptions were investigated, revealing a two-step adsorption process comprising rapid permeation into the cavities of the porous polymers followed by a slow swelling step. The high water-repellency of the crosslinked porous polymers was confirmed by contact angle measurement. The reusabilities of the polymers as solvent absorbents were evaluated through repeated adsorption and drying. Water-in-oil (W/O) gel-emulsions consisting of water and a monomer were successfully prepared using N-3-hydroxypropylcarbonyl-L-isoleucyl-aminooctadecane as a gelator. Low-temperature polymerization of the gel-emulsions with a redox initiator was performed to obtain the corresponding porous polymers. Polymerization of gel-emulsions containing bifunctional monomers gave crosslinked porous polymers, which were found to be mechanically robust and solvent resistant. The adsorption capacities of the polymers toward several liquids were assessed and rationalized in terms of the surface microstructures of the polymers. The time courses of the adsorptions were investigated, revealing a two-step adsorption process comprising rapid permeation into the cavities of the porous polymers followed by a slow swelling step.

A single nucleotide incorporation step limits human telomerase repeat addition activity.

Human telomerase synthesizes telomeric DNA repeats (GGTTAG)n onto chromosome ends using a short template from its integral telomerase RNA (hTR). However, telomerase is markedly slow for processive DNA synthesis among DNA polymerases. We report here that the unique template-embedded pause signal restricts the first nucleotide incorporation for each repeat synthesized, imparting a significantly greater KM This slow nucleotide incorporation step drastically limits repeat addition processivity and rate under physiological conditions, which is alleviated with augmented concentrations of dGTP or dGDP, and not with dGMP nor other nucleotides. The activity stimulation by dGDP is due to nucleoside diphosphates functioning as substrates for telomerase. Converting the first nucleotide of the repeat synthesized from dG to dA through the telomerase template mutation, hTR-51U, correspondingly shifts telomerase repeat addition activity stimulation to dATP-dependent. In accordance, telomerase without the pause signal synthesizes DNA repeats with extremely high efficiency under low dGTP concentrations and lacks dGTP stimulation. Thus, the first nucleotide incorporation step of the telomerase catalytic cycle is a potential target for therapeutic enhancement of telomerase activity.

Transition-Metal-Free Suzuki-Type Cross-Coupling Reaction of Benzyl Halides and Boronic Acids via 1,2-Metalate Shift.

Cross-coupling of organoboron compounds with electrophiles (Suzuki-Miyaura reaction) has greatly advanced C-C bond formation and has been well received in medicinal chemistry. During the past 50 years, transition metals have played a central role throughout the catalytic cycle of this important transformation. In this process, chemoselectivity among multiple carbon-halogen bonds is a common challenge. In particular, selective oxidative addition of transition metals to alkyl halides rather than aryl halides is difficult due to unfavorable transition states and bond strengths. We describe a new approach that uses a single organic sulfide catalyst to activate both C(sp3) halides and arylboronic acids via a zwitterionic boron "ate" intermediate. This "ate" species undergoes a 1,2-metalate shift to afford Suzuki coupling products using benzyl chlorides and arylboronic acids. Various diaryl methane analogues can be prepared, including those with complex and biologically active motifs. The reactions proceed under transition-metal-free conditions, and C(sp2) halides, including aryl bromides and iodides, are unaffected. The orthogonal chemoselectivity is demonstrated in the streamlined synthesis of highly functionalized diaryl methane scaffolds using multi-halogenated substrates. Preliminary mechanistic experiments suggest both the sulfonium salt and the sulfur ylide are involved in the reaction, with the formation of sulfonium salt being the slowest step in the overall catalytic cycle.

New Insights into the Kinetics of Structural Transformation and Hydrogenation Activity of Nano-crystalline Molybdenum Carbide

Transformation of cubic molybdenum carbide into hexagonal β-Mo2C was found to follow the Avrami–Erofeev kinetic model with an n parameter of four indicating a slow nucleation step with a polyhedral growth of the transformed phase. The transformation not only occurred at 773 K under atmospheric H2 pressure, but also imperceptibly occurred under the employed hydrogenation reaction conditions (413 K and 70 psig of H2 pressure) when using toluene or 1-octene as reactants, not affecting the activity under the steady state. Our computational studies also revealed that both crystal structures share common atomic cluster arrangements with small differences in angles and interatomic distances. The transformation was triggered by local distortions which favored shorter and more homogeneous interatomic distances as present in the hexagonal phase. The prepared molybdenum carbide materials were selective to the hydrogenation of 1-octene under the mild hydrogenation conditions tested. Hydrogenation of 1-octene was preferential in a 50 wt% mixture of toluene and 1-octene with minimal toluene hydrogenation under the tested conditions.

CoFFEE: Corrections For Formation Energy and Eigenvalues for charged defect simulations

Charged point defects in materials are widely studied using Density Functional Theory (DFT) packages with periodic boundary conditions. The formation energy and defect level computed from these simulations need to be corrected to remove the contributions from the spurious long-range interaction between the defect and its periodic images. To this effect, the CoFFEE code implements the Freysoldt–Neugebauer–Van de Walle (FNV) correction scheme. The corrections can be applied to charged defects in a complete range of material shapes and size: bulk, slab (or two-dimensional), wires and nanoribbons. The code is written in Python and features MPI parallelization and optimizations using the Cython package for slow steps. Program summaryProgram title: CoFFEEProgram Files doi:http://dx.doi.org/10.17632/s2x4d542dc.1Licensing provisions: BSD 3-clauseProgramming language: PythonExternal routines/libraries: numpy, scipy, mpi4py, matplotlibNature of problem: Most electronic structure codes based on Density Functional Theory use periodic boundary conditions. This leads to spurious electrostatic interactions during simulation of charged defects, which affects the computed defect formation energy and the defect eigenvalue.Solution method: We implement the Freysoldt–Neugebauer–Van de Walle (FNV) correction scheme to correct the defect formation energy and eigenvalues. Our implementation can be applied to charged defects in 3D bulk materials as well as materials having 2D and 1D geometries.

The 17-residue-long N terminus in huntingtin controls stepwise aggregation in solution and on membranes via different mechanisms

Aggregation of huntingtin protein arising from expanded polyglutamine (polyQ) sequences in the exon-1 region of mutant huntingtin plays a central role in the pathogenesis of Huntington's disease. The huntingtin aggregation pathways are of therapeutic and diagnostic interest, but obtaining critical information from the physiologically relevant htt exon-1 (Httex1) protein has been challenging. Using biophysical techniques and an expression and purification protocol that generates clean, monomeric Httex1, we identified and mapped three distinct aggregation pathways: 1) unseeded in solution; 2) seeded in solution; and 3) membrane-mediated. In solution, aggregation proceeded in a highly stepwise manner, in which the individual domains (N terminus containing 17 amino acids (N17), polyQ, and proline-rich domain (PRD)) become ordered at very different rates. The aggregation was initiated by an early oligomer requiring a pathogenic, expanded Gln length and N17 α-helix formation. In the second phase, β-sheet forms in the polyQ. The slowest step is the final structural maturation of the PRD. This stepwise mechanism could be bypassed by seeding, which potently accelerated aggregation and was a prerequisite for prion-like spreading in vivo Remarkably, membranes could catalyze aggregation even more potently than seeds, in a process that caused significant membrane damage. The N17 governed membrane-mediated aggregation by anchoring Httex1 to the membrane, enhancing local concentration and promoting collision via two-dimensional diffusion. Considering its central roles in solution and in membrane-mediated aggregation, the N17 represents an attractive target for inhibiting multiple pathways. Our approach should help evaluate such inhibitors and identify diagnostic markers for the misfolded forms identified here.

Hyperriesig leitende nanogranulare Materialen

SummaryHypergiant conductance of nanogranular materials ‐ Ampacity in the GA/cm2 range by boson current transport at room temperatureUsing focused electron beam induced processing a new class of hypergiant conducting materials has been discovered. The production is possible with slow step focused electron beam deposition using organometallic compounds. Applications are in electronics and IR radiation harvesting.

Kinetika adsorpcije dvovalentnih metala na aminofunkcionalizovanom ugljeničnom nanomaterijalu

The subject of this study was to investigate the adsorption behavior of selected divalent metals (Cu (II), Cd (II), Pb (II)) on amino-functionalized multiwalled carbon nanotubes, MWCNT-NH2, at four selected pH values (3; 4.5, 6, 11), in order to estimate the possibility of using MWCNT-NH2 to remove ion metal from aqueous solutions at relatively low concentrations (0.01-0.1 mg L-1). The focus of the study was to determine the adsorption mechanism at the experimental conditions. The adsorption of divalent metal ions on MWCNT-NH2 was best described by the pseudo-second order kinetics model, which indicates that adsorption can be attributed to the chemical interactions between the adsorbates and the binding groups on the surface of the adsorbent. Modeling the adsorption rate by the Weber-Morris diffusion model indicated that intra-particle diffusion, although a slower step than external diffusion, is not the only limiting step in the adsorption process, and that the interaction of the investigated ions with the binding sites on the adsorbent surface also control the adsorption rate. Assessment of the impact of pH on the adsorption rate has shown that the pH value, in the investigated range, has the highest effect on the adsorption efficiency of Cu (II) and the lowest for the removal of Cd (II).

Activated release of bioactive aldehydes from their precursors embedded in electrospun poly(lactic acid) nonwovens.

Hexanal and benzaldehyde are naturally-occurring aroma compounds from plants with enzyme-inhibition and antimicrobial properties. Although useful for food preservation applications, the end-use of these compounds can be challenging due to their volatility and susceptibility to oxidative degradation. In this study, stable precursors for benzaldehyde and hexanal were synthetized via reversible condensation reactions with N,N′-dibenzylethane-1,2-diamine. The molecular structures of the resulting 1,3-dibenzylethane-2-phenyl and 1,3-dibenzylethane-2-pentyl imidazolidines were confirmed by NMR analyses. The precursors were encapsulated in poly(lactic acid) fibers via electrospinning, using a 90 : 10 ethyl formate : dimethyl sulfoxide blend as a solvent. Triggered release of benzaldehyde and/or hexanal from the resulting active nonwovens was achieved by the addition of 1 N citric acid, which can be described using a pseudo first order kinetic equation involving rapid and slow release steps.

SsDNA hybridization facilitated by T7 ssDNA binding protein (gp2.5) rapidly initiates from the strand terminus or internally followed by a slow zippering step

The hybridization of two complementary ssDNA is essential for molecular biology and biological physics. T7 ssDNA binding protein (gp2.5) can rapidly facilitate this hybridization, but the mechanism and kinetic process remain unknown. As determined by fluorescence resonance energy transfer (FRET) and rapid kinetic analysis methods, gp2.5 binding coiling ssDNA to form gp2.5-ssDNA complex is the rate-limiting step for the entire DNA hybridization process. Afterward, the hybridization initiates from either the terminus or the internal part of ssDNA at a nucleation rate of 0.45 s−1. The remaining DNA strands are hybridized in a zippering mode at a rate of 0.07 s−1, limited by the dissociation of gp2.5 from ssDNA. The gp2.5-faciliated hybridization rate constant (74 μM−1s−1) is much higher than the spontaneous hybridization rate (1.7 μM−1s−1) in the absence of gp2.5. These hybridization mechanism and kinetic process are different from those by other ssDNA binding proteins, such as T4 gp32, Escherichia coli SSB protein and recA protein. Compare with E. coli SSB, the relatively slower association of gp2.5 to ssDNA and faster dissociation of gp2.5 from ssDNA are probably the major reasons to facilitate the rapid hybridization. This study provides valuable insights into the molecular mechanism of the recombination and repair processes and proposes a basis for improvement in their associated biotechnologies.

Enabling Efficient Late‐Stage Functionalization of Drug‐Like Molecules with LC‐MS and Reaction‐Driven Data Processing

Late‐stage functionalization (LSF) through C–H functionalization of drug leads is a powerful synthetic strategy for drug discovery. A key challenge in LSF is that multiple regioisomeric products are often generated, which requires slow and laborious product isolation and structure confirmation steps. To address this, an analytical approach using LC‐HR‐MS/MS coupled with automated chemically aware data processing was developed. Using this method to analyse reaction screening arrays based on three common C–H functionalization chemistries with a set of marketed drugs, the relative amount and localization of chemical modification could be determined for each regioisomeric product generated in the screening. This approach allows one to construct a workflow in which the various regioisomeric products of a given transformation are triaged according to their site of modification, allowing downstream isolation and structure elucidation efforts to focus on those analogues of highest interest, leading to an overall increase in productivity of the LSF strategy.

A Distal Loop Controls Product Release and Chemo- and Regioselectivity in Cytochrome P450 Decarboxylases.

Cytochrome P450 OleT utilizes hydrogen peroxide (H2O2) to catalyze the decarboxylation or hydroxylation of fatty acid (FA) substrates. Both reactions are initiated through the abstraction of a substrate hydrogen atom by the high-valent iron-oxo intermediate known as Compound I. Here, we specifically probe the influence of substrate coordination on OleT reaction partitioning through the combined use of fluorescent and electron paramagnetic resonance (EPR)-active FA probes and mutagenesis of a structurally disordered F-G loop that is distal from the heme-iron active site. Both probes are efficiently metabolized by OleT and efficiently trigger the formation of Compound I. Transient fluorescence and EPR reveal a slow product release step, mediated by the F-G loop, that limits OleT turnover. A single-amino acid change or excision of the loop reveals that this region establishes critical interactions to anchor FA substrates in place. The stabilization afforded by the F-G loop is essential for regulating regiospecific C-H abstraction and allowing for efficient decarboxylation to occur. These results highlight a regulatory strategy whereby the fate of activated oxygen species can be controlled at distances far removed from the site of chemistry.

Kinetic and Computational Studies of Rhenium Catalysis for Oxygen Atom Transfer Reactions

The rhenium(v)oxo dimer {MeReO(edt)}2 (edt = 1,2-ethanedithiolate) is an effective catalyst for the oxygen atom transfer (OAT) reaction from pyridine oxide and picoline oxide to triphenylarsine (Ph3As) as oxygen acceptor. Kinetics measurements were carried out by the initial rate method because of the monomerization reaction of the pyridine product with the {MeReO(edt)}2 catalysts. The derived rate is R = k[Re][NO] (where NO is picoline oxide or pyridine oxide) and independent of the Ph3As concentration. The rate constant at room temperature in chloroform is k(PicNO) = 268.1 ± 3.5 L mol−1 s−1 and k(PyNO) = 155.3 ± 2.3 L mol−1 s−1. The analogue rhenium(v)oxo dimer {MeReO(pdt)}2 (pdt = 1,3-propanedithiolate) does not monomerize with pyridine. However, {MeReO(edt)}2 rapidly monomerizes with pyridine. Density functional theory study of the enthalpy of the monomerization reaction shows that the {MeReO(edt)}2 reaction with pyridine is more thermodynamically favoured than {MeReO(pdt)}2 and this is attributed to the higher angle strain on the {MeReO(edt)}2 bridging sulfur. The computational study of the proposed slow step shows that enthalpy of activation (ΔH‡) of ReV oxidation to ReVII is unchanged by varying the substituent on the pyridine oxide.

Strong influence of polaron-polaron interaction on the magnetoresistance effect in La0.7A0.3MnO3 thin films

The colossal magnetoresistance effect endows La0.7A0.3MnO3 manganites distinctive fascination. Both theoretical and experimental studies demonstrated that the interplay among polarons could significantly influence magnetoresistance. However, the underlying microscopic mechanism of the influence remains elusive due to the lack of experimental evidences. Utilizing ultrafast optical spectroscopy to track the polaron dynamics around Curie temperatures, we observed a diverse two-step recovery process in three sibling manganite thin films with various magnetoresistance effects and Curie temperatures, while the slow step was proposed to be the formation evolution of correlated polarons through the polaron-polaron interaction. Polarons in La0.7Ca0.3MnO3 equilibrate much faster than those in La0.7(Ca0.58Sr0.42)0.3MnO3 and La0.7Sr0.3MnO3, indicating a comparatively tighter interaction between polarons and subsequently a stronger magnetoresistance effect.