Apparent Constant Research Articles

Influence of dislocations on the apparent elastic constants in single metallic crystallites: an analytical approach

Intricate knowledge of dislocation networks in metals has proven paramount in understanding the constitutive behaviour of these materials but current experimental methods yield limited information on the characteristics of these networks. Recently, the isotropic anelastic response of metals has been used to investigate complex dislocation networks through the well-known phenomenon that the observed elastic constants are influenced by dislocations. Considering the dependence of the behaviour of a Frank-Read (FR) source on its initial dislocation character and using discerning characteristics of dislocations, i.e. Burgers vector, line sense and slip system, the present paper takes dislocation character, crystal structure and dislocation network geometry into account and obtains the anisotropic mechanical response for a generic Poisson’s ratio. In this work, the tensile test tangent moduli and yield points are presented for spatially uniform and nonuniform dislocation distributions across slip systems. First, the reversible shear strain of the FR source is derived as a function of initial dislocation character. The area swept by a mobile and initially straight dislocation segment pinned at both ends is given as an explicit function of the line stress. Secondly, the anisotropic anelastic strain contribution of FR sources to the total pre- and at-yield strain in single crystallites is calculated. For a given normal stress and superposition of the principal infinitesimal linear elastic lattice strain and anelastic dislocation strain, the tangent moduli are presented. The moduli and the inception of plastic flow have a notable dependence on initial dislocation character, spatial dislocation distribution and loading direction.

Photocatalytic degradation of Novacron blue and Novacron yellow textile dyes by the TiO2/palygorskite nanocomposite.

The photocatalytic discoloration of industrial dyes, Novacron blue (NB) and Novacron yellow (NY), was investigated using composites based on TiO2 and natural palygorskite (Pal-Ti10 and Pal-Ti30). The method consisted of synthesizing the composites starting from a physical mixture of TiO2 and natural palygorskite in the presence of alcohol, for impregnation through calcination under conditions of temperature equal to 450 °C and atmospheric air. The characterization techniques used in this work were FTIR, XRD, XRF, SEM, particle size analysis and zeta potential. The photocatalysis for the NB dye was investigated through the application of a factorial 24 experimental design, aiming at the best experimental conditions and finally applying them in another NY industrial dye. The investigated concentrations of NB were 10 ppm and 30 ppm, the composites were synthesized using 10 and 30% (p/p) of titanium dioxide in palygorskite, the two pH values were 2.0 and 6.0 and the light intensities 9 and 18 W were used. Tests performed at pH 2.0, Pal-Ti30 composite, power 18 W and 10 ppm of dye showed 100% color removal of both dyes in 90 min. The bleaching process followed the pseudo-first order kinetic model, and the apparent constants (Kapp) were 0.0216 min-1 and 0.0193 min-1 for NB and NY dyes, respectively. The results of total organic carbon (TOC) showed mineralization of 61.70% and 58.06% for NB and NY, respectively, in 90 min of treatment, and the by-products were detected by GC-MS.

Shear modulus of single wood pulp fibers from torsion tests

The shear modulus of pulp fibers is difficult to measure and only very little literature is available on this topic. In this work we are introducing a method to measure this fiber property utilizing a custom built instrument. From the geometry of the fiber cross section, the fiber twisting angle and the applied torque, the shear modulus is derived by de Saint Venant’s theory of torsion. The deformation of the fiber is applied by a moving coil mechanism. The support of the rotating part consists of taut bands, making it nearly frictionless, which allows easy control of the torque to twist the fiber. A permanent magnet moving coil meter was fitted with a sample holder for fibers and torque references. Measurements on fine metal bands were performed to validate the instrument. The irregular shape of the fibers was reconstructed from several microtome cuts and an apparent torsion constant was computed by applying de Saint Venant’s torsion theory. Fibers from two types of industrial pulp were measured: thermomechanical pulp (TMP) and Kraft pulp. The average shear modulus was determined as (2.13 {pm } 0.36) GPa for TMP and (2.51 {pm } 0.50) GPa for Kraft fibers, respectively. The TMP fibers showed a smaller shear modulus but, due to their less collapsed state, a higher torsional rigidity than the kraft fibers.

Corn’s dextrin, a novel environmentally friendly promoter of methane hydrate formation

Gas hydrate has the potential to be used as a high capacity storage of natural gas. Methane is the primary component of natural gas in nature. Therefore, it is of interest to evaluate the thermodynamic and kinetic behavior of methane hydrate for industrialization of natural gas storage through hydrate formation. In this study, the main kinetic parameters of methane hydrate formation in the presence of Corn’s dextrin were measured and its performance as a promoter was compared to sodium dodecyl sulfate (SDS). The experiments were conducted in a laboratory-reactor at an initial temperature of 275.15 K, and an initial pressure of 7.5 MPa. The kinetic results showed that the use of Corn’s dextrin, SDS, and their mixture significantly reduced the induction time and semicompletion time, and also increased the gas consumption and the rate of gas uptake during hydrate formation, and the apparent rate constant of hydrate growth compared to that of pure water. Corn’s dextrin with a concentration of 1 wt% had the best performance to promote the kinetics of methane hydrate formation. Dextrin (1 wt%) declines the induction time from 1256 down to 18 min. The utilization of dextrin also enhanced the final storage capacity of methane hydrate by up to 12.5%. The storage capacity of methane hydrate after 60 min of hydrate crystallization showed the values of 121.6 and 124.5 V/V for 1 and 7 wt% dextrin, respectively, which is greater than that of pure water (32.3 V/V). The average apparent constant of hydrate growth (kapp) of 7 wt% dextrin until 2 min after the beginning of growth stage was reported approximately 28 times larger than pure water. By comparison of the kinetic results of dextrin with SDS, it was concluded that dextrin could compete against SDS in promoting methane hydrate formation at a specific concentration.

Universal capability of 3-ketosteroid \u03941-dehydrogenases to catalyze \u03941-dehydrogenation of C17-substituted steroids

Background3-Ketosteroid Δ1-dehydrogenases (KSTDs) are the enzymes involved in microbial cholesterol degradation and modification of steroids. They catalyze dehydrogenation between C1 and C2 atoms in ring A of the polycyclic structure of 3-ketosteroids. KSTDs substrate spectrum is broad, even though most of them prefer steroids with small substituents at the C17 atom. The investigation of the KSTD’s substrate specificity is hindered by the poor solubility of the hydrophobic steroids in aqueous solutions. In this paper, we used 2-hydroxpropyl-β-cyclodextrin (HBC) as a solubilizing agent in a study of the KSTDs steady-state kinetics and demonstrated that substrate bioavailability has a pivotal impact on enzyme specificity.ResultsMolecular dynamics simulations on KSTD1 from Rhodococcus erythropolis indicated no difference in ΔGbind between the native substrate, androst-4-en-3,17-dione (AD; − 8.02 kcal/mol), and more complex steroids such as cholest-4-en-3-one (− 8.40 kcal/mol) or diosgenone (− 6.17 kcal/mol). No structural obstacle for binding of the extended substrates was also observed. Following this observation, our kinetic studies conducted in the presence of HBC confirmed KSTD1 activity towards both types of steroids. We have compared the substrate specificity of KSTD1 to the other enzyme known for its activity with cholest-4-en-3-one, KSTD from Sterolibacterium denitrificans (AcmB). The addition of solubilizing agent caused AcmB to exhibit a higher affinity to cholest-4-en-3-one (Ping-Pong bi bi KmA = 23.7 μM) than to AD (KmA = 529.2 μM), a supposedly native substrate of the enzyme. Moreover, we have isolated AcmB isoenzyme (AcmB2) and showed that conversion of AD and cholest-4-en-3-one proceeds at a similar rate. We demonstrated also that the apparent specificity constant of AcmB for cholest-4-en-3-one (kcat/KmA = 9.25∙106 M−1 s−1) is almost 20 times higher than measured for KSTD1 (kcat/KmA = 4.71∙105 M−1 s−1).ConclusionsWe confirmed the existence of AcmB preference for a substrate with an undegraded isooctyl chain. However, we showed that KSTD1 which was reported to be inactive with such substrates can catalyze the reaction if the solubility problem is addressed.

Revealing the degree and impact of inhomogeneous electrolyte distributions on silver based gas diffusion electrodes

Silver based oxygen depolarized cathodes (ODC) are a promising technology to reduce the specific electrical energy demand of chlor-alkali electrolysis by around 30 %. Here we present the first modeling approach of an ODC for evaluating the influence of inhomogeneities on electrochemical performance and electrode dynamics. The focus is laid on the electrolyte distribution in the electrode because it is crucial for the gas-electrolyte interface and thus electrode performance and dynamics. To model the inhomogeneities, several one-dimensional three-phase models with different flooding degree of the electrode are coupled in parallel. The model is validated against polarization curves and electrochemical impedance spectra for ODC with silver content of 92, 97, 98 and 99 wt-% Ag. The electrochemical impedance spectroscopy measurements show flattened semicircles in the Nyquist plot with one apparent fast time constant, which ranges from 0.0015s to 0.03s depending on the respective ODC composition. The fast time constants result from electrochemical reaction and double layer; no time constant for a mass transport was identified. According to the simulations, the distorted form of the semi-circles is caused by local distribution of the liquid electrolyte within the ODC. It is revealed that there are strong gradients in local performance inside the electrode. Domains with a low intrusion of liquid electrolyte seem to be electrochemically highly active whereas flooded domains are almost inactive. The comparison of ODC with varying silver/polytetrafluoroethylene ratio suggests that the size and location of the gas-liquid interface, rather than the wetted catalyst surface area, is the key to achieving high ODC performance. We reveal that the electrochemical impedance spectroscopy is sensitive to the electrolyte distribution, while polarization curves do not contain sufficient information about the location and distribution of the electrolyte.

Effect of Methane, CO2, and H2S on the Solubility of Methyl and Ethyl Mercaptans in a 25 wt % Methyldiethanolamine Aqueous Solution at 333 and 365 K

We have investigated the solubility of methanethiol and ethanethiol (methyl and ethyl mercaptans in an aqueous methyldiethanolamine solution (25 wt%) by using static-analytic method at 333 and 365K. The measurements were done for different pressure of methane in the absence of acid gas with individual acid gas present, and with mixture of acid gases present (CO2 and/with H2S). Additional measurement of Henry's law constant were realized by considering gas stripping method. Effect of total pressure (realized by addition of methane) and effect of acid gas loading for a constant total pressure around 7 MPa were studied. The increasing of pressure and acid gas loading increase the apparent Henry's law constant of the mercaptan, highlighting a drop out effect of the mercaptans.

Using chemical modeling for designing of optimal pH sensor based on analytical sensitivity enhancement

The behavior of chemical systems can be predicted based on the chemical model or the law of mass action in combining with Beer’s Law. The information about chemical behavior of different species in an equilibrium condition and pure responses of active species propound the idea of using this information for designing of sensors, systematically.In this contribution, we applied chemical modeling for designing pH sensors using common pH indicators. Because individually considered they display a limited dynamic range, two or more pH indicators are usually required for the pH measurements in a broad pH range. Two different aspects were considered: 1) finding the optimum approach for the combination of pH indicators (i.e. an array or a mixture of indicators in solution), in this regard, analytical sensitivity is used as an optimization criterion and 2) tuning equilibrium constants of indicators to produce new desired pH indicators. The tuning of apparent constants was performed using a surfactant solution. An array of a certain indicator in the aqueous and micellar phase increases its dynamic pH range. Three pH indicators including phenol red, neutral red, and methyl orange were selected and their interactions with the micellar phase of Brij-35 were investigated. Based on the obtained information about the behavior of the indicators in micellar solution, the optimum pH sensor was then designed and pH was predicted with the root mean square error of pH prediction value of 0.1.

Changes in alpine grassland type drive niche differentiation of nitrifying communities on the Qinghai‒Tibetan Plateau

Nitrification is a critical process of nitrogen (N) cycling performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB), nitrite-oxidizing bacteria (NOB), and complete ammonia oxidation (comammox). However, the performance of these nitrifying groups in alpine grasslands has rarely been documented. This study investigated their abundance and community structure in four alpine grasslands on the Qinghai‒Tibetan Plateau with quantitative PCR and high-throughput sequencing. Potential ammonia oxidation (PAO) was measured and partitioned by inhibitor method. PAO was mainly dominated by AOA. PAO and potential nitrite oxidation (PNO) significantly decreased, while the apparent half-saturation constant (Km) had no significant change along a grassland degradation gradient. Phylogenetic analyses revealed that the predominant AOA, AOB, and NOB were Nitrososphaera, Nitrosospira, and Nitrospira lineages, respectively. Network analyses revealed that the alpine nitrifying co-occurrence network had a modular structure that was mainly shaped by taxonomic relatedness and environmental relationships, indicating different nitrifying species occupy disparate niches. Soil pH, ammonium, and moisture were the main drivers for variation in AOA, AOB, and NOB communities, respectively. Here, we provide evidence of niche differentiation in nitrifying communities with changes in grassland type. Moreover, these findings support a theoretical foundation for predicting the repercussions of grassland degradation on soil nitrifying communities in alpine ecosystems.

PH Overpotential for Unveiling the pH Gradient Effect of H + /OH − Transport in Electrode Reaction Kinetics

pH Overpotential for Unveiling the pH Gradient Effect of H ⁺ /OH ⁻ Transport in Electrode Reaction Kinetics

Sensitivity of the turbulent Schmidt number and the turbulence models to simulate catalytic and photocatalytic processes with surface reaction limited by mass transfer

Based on CFD’s simulations using RANS and SGDH (Standard Gradient Diffusion Hypothesis), this study presents an alternative to better predict turbulent catalytic systems with surface reaction limited by mass transfer selecting an optimal turbulent Schmidt number (Sct). Simulations of H2O2 degradation were performed in an annular reactor with Mn/Al oxide immobilized into the wall using the AKN, RSM and standard k-ε turbulence models with the Sct varying from 0.2 to 1.1. The numerical apparent reaction constant (kapp) was compared with literature data to verify the prediction errors. The ideal Sct was applied to simulate the benzoic acid photocatalytic degradation in photocatalytic reactor coated with TiO2 for different flow rates. The results showed that the Sct of 0.8 for AKN, 0.7 for RSM, and 0.2 for standard k-ε are the best for systems with high mass transfer limitations and the Sct of 1.1 for AKN, 1.0 for RSM, and 0.4 for standard k-ε are the best for systems with low mass transfer limitations. It was concluded that all turbulence models predicted well the mass transfer when the ideal Sct was used and the standard-k-ε model could be an alternative since the prediction error was less than 6.5% with low computational cost.

PH Dependence of T2 for Hyperpolarizable 13C-Labelled Small Molecules Enables Spatially Resolved pH Measurement by Magnetic Resonance Imaging.

Hyperpolarized 13C magnetic resonance imaging often uses spin-echo-based pulse sequences that are sensitive to the transverse relaxation time T2. In this context, local T2-changes might introduce a quantification bias to imaging biomarkers. Here, we investigated the pH dependence of the apparent transverse relaxation time constant (denoted here as T2) of six 13C-labelled molecules. We obtained minimum and maximum T2 values within pH 1–13 at 14.1 T: [1-13C]acetate (T2,min = 2.1 s; T2,max = 27.7 s), [1-13C]alanine (T2,min = 0.6 s; T2,max = 10.6 s), [1,4-13C2]fumarate (T2,min = 3.0 s; T2,max = 18.9 s), [1-13C]lactate (T2,min = 0.7 s; T2,max = 12.6 s), [1-13C]pyruvate (T2,min = 0.1 s; T2,max = 18.7 s) and 13C-urea (T2,min = 0.1 s; T2,max = 0.1 s). At 7 T, T2-variation in the physiological pH range (pH 6.8–7.8) was highest for [1-13C]pyruvate (ΔT2 = 0.95 s/0.1pH) and [1-13C]acetate (ΔT2 = 0.44 s/0.1pH). Concentration, salt concentration, and temperature alterations caused T2 variations of up to 45.4% for [1-13C]acetate and 23.6% for [1-13C]pyruvate. For [1-13C]acetate, spatially resolved pH measurements using T2-mapping were demonstrated with 1.6 pH units accuracy in vitro. A strong proton exchange-based pH dependence of T2 suggests that pH alterations potentially influence signal strength for hyperpolarized 13C-acquisitions.

Dibenzyl trisulfide binds to and competitively inhibits the cytochrome P450 1A1 active site without impacting the expression of the aryl hydrocarbon receptor

The toxicological manifestation of many pollutants relies upon their binding to the aryl hydrocarbon receptor (AHR), and it follows a cascade of reactions culminating in an elevated expression of cytochrome P450 (CYP) 1 enzymes. CYP1A1 and CYP1B1 are associated with enhanced carcinogenesis when chronically exposed to certain polyaromatic hydrocarbons, and their inhibition may lead to chemoprevention. We evaluated dibenzyl trisulfide (DTS), expressed in the ethnomedical plant, Petiveria alliacea, for such potential chemoprevention. Using recombinant human CYP1A1 and CYP1B1 bactosomes on a fluorogenic assay, we first demonstrated that DTS moderately inhibited both enzymes with half maximal inhibitory concentration (IC50) values of 1.3 ± 0.3 and 1.7 ± 0.3 μM, respectively. Against CYP1A1, DTS was a reversible, competitive inhibitor with an apparent inhibitory constant (Ki) of 4.55 ± 0.37 μM. In silico molecular modeling showed that DTS binds with an affinity of −39.8 kJ·mol−1, situated inside the binding pocket, approximately 4.3 Å away from the heme group, exhibiting interactions with phenylalanine residue 123 (Phe-123), Phe-224, and Phe-258. Lastly, zebrafish (Danio rerio) embryos were exposed to 0.08–0.8 μM DTS from 24 to 96 h post fertilization (hpf) with the in vivo ethoxyresorufin-O-deethylase (EROD) assay, and, at 96 hpf, DTS significantly suppressed EROD CYP1A activity in a dose-dependent manner, with up to 60% suppression in the highest 0.8 μM exposure group. DTS had no impact on gene transcription levels for cyp1a and aryl hydrocarbon receptor 2 (ahr2). In co-exposure experiments, DTS suppressed CYP1A activity induced by both B[a]P and PCB-126, although these reductions were not significant. Taken together, these results demonstrate that DTS is a direct, reversible, competitive inhibitor of the carcinogen-activating CYP1A enzyme, binding in the active site pocket close to the heme site, and shows potential in chemoprevention.

A green nanocomposite based modified cellulose/TiO2/Cinnamon bark for the reduction of toxic organic compounds using spectrophotometric technique

The current study, reported the fabrication of bio-nanocomposite catalyst based on sulfated carboxymethyl cellulose (S-CMC) and TiO2 nanoparticles composited with cinnamon bark (CIN) (CIN/S-CMC/TiO2). The prepared CIN/S-CMC/TiO2 bio-nanocomposite was produced as supporting material for the preparation of highly active zero valent metal nanoparticles (MNPs) The CIN/S-CMC/TiO2 loaded with mono Co, Cu and bimetallic Cu-Co metal ions were then reacted with aqueous solution of sodium borohydride in order to reduce the metal ions into their respective zero-valent MNPs (M0). The CIN/S-CMC/TiO2 bio-nanocomposite loaded with mono and bimetallic M0 were analyzed by FTIR, XRD, EDX and FE-SEM, which confirms the successful fabrication of M0 on the surface of CIN/S-CMC/TiO2 bio-nanocomposite. The M0/CIN/S-CMC/TiO2 bio-nanocomposites were utilized as an effective catalyst for the successful reduction of 4-nitrophenol (4NP) and azo dyes such as Congo Red (CR), Methyl Orange (MO) And Acid Red (AR). Among all the fabricated catalysts, Co-Cu/CIN/S-CMC/TiO2 displayed better catalytic activity for the reduction of 4-NP and toxic azo dyes in the existence of sodium borohydride, with apparent constants of 0.99 min−1, 0.747 min−1, 0.621 min−1 and 0.342 min−1 respectively. Despite of having enhanced catalytic capability, the proposed bio-nanocomposite catalyst could be easily recoverable from the reaction solution after complete reduction process and can be used several times for the reduction of new samples.

Comparative Biochemistry of Four Polyester (PET) Hydrolases*.

The potential of bioprocessing in a circular plastic economy has strongly stimulated research into the enzymatic degradation of different synthetic polymers. Particular interest has been devoted to the commonly used polyester, poly(ethylene terephthalate) (PET), and a number of PET hydrolases have been described. However, a kinetic framework for comparisons of PET hydrolases (or other plastic-degrading enzymes) acting on the insoluble substrate has not been established. Herein, we propose such a framework, which we have tested against kinetic measurements for four PET hydrolases. The analysis provided values of kcat and KM , as well as an apparent specificity constant in the conventional units of M-1 s-1 . These parameters, together with experimental values for the number of enzyme attack sites on the PET surface, enabled comparative analyses. A variant of the PET hydrolase from Ideonella sakaiensis was the most efficient enzyme at ambient conditions; it relied on a high kcat rather than a low KM . Moreover, both soluble and insoluble PET fragments were consistently hydrolyzed much faster than intact PET. This suggests that interactions between polymer strands slow down PET degradation, whereas the chemical steps of catalysis and the low accessibility associated with solid substrate were less important for the overall rate. Finally, the investigated enzymes showed a remarkable substrate affinity, and reached half the saturation rate on PET when the concentration of attack sites in the suspension was only about 50 nM. We propose that this is linked to nonspecific adsorption, which promotes the nearness of enzyme and attack sites.

Mineralization and Self‐assembly of Gold Nanoparticles using Sulfur Amino Acid Modified Hierarchically Porous Metal‐Organic Frameworks

AbstractIn this work, we modified the hierarchically porous metal‐organic frameworks (HP‐MOFs) HP‐UiO‐66 with sulfur‐containing amino acid cysteine and methionine, respectively. The cysteine or methionine modified HP‐MOFs, HP‐UiO‐66‐C and HP‐UiO‐66‐M, can be used for the adsorption and mineralization of Gold nanoparticles. Au nanoparticles will form in HP‐MOFs when the concentration of chloroauric acid is relatively low. Intriguingly, when the concentration of the chloroauric acid solution was increased to 5 mg ml−1, hexagonal gold micro flakes appeared in the system after adsorption by the HP‐UiO‐66‐C and HP‐UiO‐66‐M. Transmission electron microscope revealed that the hexagonal gold micro flakes are formed by the ordered self‐assembly of gold nanoparticles induced by the HP‐UiO‐66‐C and HP‐UiO‐66‐M. The obtained AuNP@HP‐UiO‐66‐C and AuNP@HP‐UiO‐66‐M can catalyze the reduction of 4‐NP. The apparent reaction rates constant of unit mass catalyst kcat are 0.0356 mg−1 min−1 and 0.0265 mg−1 min−1, respectively.

Biosynthesis of Cylindrospermopsin in Cyanobacteria: Characterization of CyrJ the Sulfotransferase.

7-Deoxy-desulfo-cylindrospermopsin was purified at small-scale from the supernatant of a culture of the cyanobacterium Oscillatoria sp. PCC 10702. This metabolite was obtained in a pure form using a three-step chromatographic procedure, and its identity was confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). LC-MS quantification showed that this metabolite was excreted in the culture medium of Oscillatoria sp. PCC 10702. Isotopic incorporation studies using [2-13C,15N]glycine, a cylindrospermopsin precursor, and Oscillatoria sp. PCC 10702 cells showed that glycine was incorporated into 7-deoxy-desulfo-cylindrospermopsin, 7-deoxy-cylindrospermopsin, 7-epi-cylindrospermopsin, and cylindrospermopsin. The isotopic incorporation rate was consistent with the following metabolic flux: 7-deoxy-desulfo-cylindrospermopsin → 7-deoxy-cylindrospermopsin → 7-epi-cylindrospermopsin and cylindrospermopsin. We have cloned the cyrJ gene into an expression vector and overproduced the putative sulfotransferase CyrJ in Escherichia coli. The purified protein CyrJ catalyzed, in vitro, the transfer of a sulfonate group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to 7-deoxy-desulfo-cylindrospermopsin to give 7-deoxy-cylindrospermopsin. Kinetic analysis afforded the following apparent constants: KMapp. (PAPS) = 0.12 μM, Vmaxapp. = 20 nM/min, KMapp. (7-deoxy-desulfo-cylindrospermopsin) = 0.12 μM, and KIapp. (7-deoxy-desulfo-cylindrospermopsin) = 4.1 μM. Preliminary data suggested that CyrJ catalyzed the reaction through a ternary-complex kinetic mechanism. All these data confirmed that CyrJ catalyzed a sulfotransfer during the penultimate step of the biosynthesis of cylindrospermopsin.

The active form of the influenza cap-snatching endonuclease inhibitor baloxavir marboxil is a tight binding inhibitor

Baloxavir marboxil (BXM) is an FDA-approved antiviral prodrug for the treatment of influenza A and B infection and postexposure prophylaxis. The active form, baloxavir acid (BXA), targets the cap-snatching endonuclease (PA) of the influenza virus polymerase complex. The nuclease activity delivers the primer for transcription, and previous reports have shown that BXA blocks the nuclease activity with high potency. However, biochemical studies on the mechanism of action are lacking. Structural data have shown that BXA chelates the two divalent metal ions at the active site, like inhibitors of the human immunodeficiency virus type 1 (HIV-1) integrase or ribonuclease (RNase) H. Here we studied the mechanisms underlying the high potency of BXA and how the I38T mutation confers resistance to the drug. Enzyme kinetics with the recombinant heterotrimeric enzyme (FluB-ht) revealed characteristics of a tight binding inhibitor. The apparent inhibitor constant (Kiapp) is 12 nM, while the I38T mutation increased Kiapp by ∼18-fold. Order-of-addition experiments show that a preformed complex of FluB-ht, Mg2+ ions and BXA is required to observe inhibition, which is consistent with active site binding. Conversely, a preformed complex of FluB-ht and RNA substrate prevents BXA from accessing the active site. Unlike integrase inhibitors that interact with the DNA substrate, BXA behaves like RNase H inhibitors that compete with the nucleic acid at the active site. The collective data support the conclusion that BXA is a tight binding inhibitor and the I38T mutation diminishes these properties.

Of Pots and Porridge: Food, Cooking, and Serving in Old Norse Sources

This article explores the ways in which medieval Icelanders imagined the food and culinary practices of their Viking-Age ancestors. Through a critical synthesis of vocabulary and narrative in sagas, poetry, and law texts, it becomes possible to move beyond isolated references to food and culinary practice, and to begin to consider attitudes to particular foods and technologies. The survey identifies some apparent constants in culinary memory - boiling, the use of kettles - as well as contrasting cooking and eating practices in elite and non-elite circles, and in foreign and domestic settings. The textual evidence thus usefully augments and nuances what can be gleaned from the material remains of culinary practice in Viking and medieval Iceland.

Electrostatically mediated layer-by-layer assembly of a bioinspired thymine polycation and gold nanoparticles

In this work, we built self-assembled structures of gold nanoparticles (AuNPs) using a new polyelectrolyte formed by copolymerization of the monomers 4-vinylbenzyl thymine (VBT) and 4-vinylbenzyl triethylammonium chloride (VBA). For the synthesis, a monomer ratio of 1 mol of VBT to 16 mol of VBA was used to obtain the polycation [(VBT)1:(VBA)16]16+ with a charge density suitable for the electrostatic adsorption of citrate stabilized AuNPs. The objective of this work was to explore the optical and electrochemical properties of the structure built on gold and quartz substrates with a view to its potential application as a (bio)sensor for optical and electrochemical detection. The polycation adsorption process was studied by surface plasmon resonance, which allowed us to select the appropriate concentration and adsorption time for the polymer and verify the formation of the self-assembly. Characterization of the system by atomic force microscopy allowed determining that AuNPs are distributed in isolation, such as dimers, trimers, tetramers, or bigger aggregates. The analysis of the resonance of the localized surface plasmons (LSPR) of the AuNPs, as well as the increase in the intensity of the polycation Raman signals revealed the electromagnetic coupling between close nanoparticles. In addition, the electrochemical impedance spectroscopy study of gold electrodes modified with the ([(VBT)1:(VBA)16]16+/AuNPs)n multilayer showed that incorporation of the AuNPs in the structure produced an increase in the apparent heterogeneous charge transfer constant. This fact indicates that the structure of the film and the nanoparticles distribution favor the electronic conduction mediated by AuNPs. The observed properties indicate that this multilayer represents a very interesting platform for the development of (bio)sensors based on optical or electrochemical detection.