Estimation Of Rate Constants Research Articles

Microscopic Phage Adsorption Assay: High-throughput quantification of virus particle attachment to host bacterial cells.

Phages, viruses of bacteria, play a pivotal role in Earth's biosphere and hold great promise as therapeutic and diagnostic tools in combating infectious diseases. Attachment of phages to bacterial cells is a crucial initial step of the interaction. The classic assay to quantify the dynamics of phage attachment involves co-culturing and enumeration of bacteria and phages, which is laborious, lengthy, hence low-throughput, and only provides ensemble estimates of model-based adsorption rate constants. Here, we utilized fluorescence microscopy and particle tracking to obtain trajectories of individual virus particles interacting with cells. The trajectory durations quantified the heterogeneity in dwell time, the time that each phage spends interacting with a bacterium. The average dwell time strongly correlated with the classically-measured adsorption rate constant. We successfully applied this technique to quantify host-attachment dynamics of several phages including those targeting key bacterial pathogens. This approach should benefit the field of phage biology by providing highly quantitative, model-free readouts at single-virus resolution, helping to uncover single-virus phenomena missed by traditional measurements. Owing to significant reduction in manual effort, our method should enable rapid, high-throughput screening of a phage library against a target bacterial strain for applications such as therapy or diagnosis.

N-Perfluorooctane versus n-Octane: Pyrene Fluorescence to Compare and Contrast Solute Solvation.

Fluorous solvents may offer distinctly different solvation environments to a solute compared to their hydrocarbon analogues due to the inherently high electronegativity associated with fluorine. Solute solvation within n-perfluorooctane (PFO) is compared with that in n-octane using the well-established polycyclic aromatic hydrocarbon (PAH) fluorescence probe pyrene in the temperature range of 288 to 318 K. Both density (ρ) and dynamic viscosity (η) of PFO are considerably higher than those of n-octane. UV-vis molecular absorbance, fluorescence emission/excitation, and excited-state emission intensity decay reveal the cybotactic region of pyrene to be more nonpolar in PFO than that in n-octane. Bimolecular quenching rate constants (kq) for the pyrene-nitrobenzene fluorophore-quencher pair adhere to the Stokes-Einstein formulation; however, they are considerably higher than the estimated rate constants for the diffusion-controlled process (kdiff). This is due to the high electron affinity of nitrobenzene leading to aromatic π-π interactions between pyrene and nitrobenzene. For a nonaromatic low electron affinity quencher, such as nitromethane, while kq < kdiff in n-octane, kq > kdiff in PFO. This is due to the fact that highly electronegative fluorines on PFO stabilize the partial positive charge (δ+) that develops on excited pyrene during electron/charge transfer to the quencher nitromethane, facilitating quenching in the process. Exciplex formation between pyrene and triethylamine (TEA) is more favored in PFO as opposed to n-octane although ηPFO > ηn-octane. The developing charge on the exciplex is stabilized by the electronegative fluorines of the PFO. The pyrene-TEA exciplex appears to form exclusively in the excited state of pyrene, and the kinetics of exciplex formation is in the subnanosecond regime. On the contrary, the efficiency of exciplex formation between pyrene and N,N-dimethylaniline (DMA) is comparable in PFO and n-octane, and the kinetics is slower in comparison to that of the pyrene-TEA exciplex. Certain ground-state heterogeneity is detected for the pyrene-DMA system in PFO due to the low solubilizing ability of the fluorous solvent. Highly electronegative fluorines on perfluorohydrocarbon solvents are found to offer unusual and unique solvation characteristics.

222 nm causes greater protein damage and repair inhibition of E. coli than 254 nm for water disinfection

Germicidal ultraviolet (UV) light has been widely used to inactivate pathogens in water. Emerging alternatives to conventional low pressure (LP) mercury lamps emitting at 254 nm, such as krypton chloride (KrCl) excimer lamps emitting at 222 nm, are gaining acceptance and popularity due to advantages in human safety and disinfection mechanisms. Cyclobutane pyrimidine dimer (CPD) formation kinetics and photolyase damage kinetics were quantified in E. coli for 222 nm and 254 nm UV. Molecular damage and cell regrowth were also quantified after UV irradiation under photorepair and dark repair incubation conditions using a standardized photorepair fluence response protocol. CPDs and photolyase were measured using enzyme linked immunosorbent assays (ELISA). A novel ELISA for photolyase was developed for this study. Culture-based log inactivation UV fluence responses were similar for 254 nm and 222 nm, with Geeraerd model estimates for rate constants of 1.18±0.09 and 1.24±0.08 cm2 mJ−1 for LP and KrCl lamps, respectively. Molecular UV fluence kinetics showed that the rate of CPD formation was greater by LP, but the rate of photolyase damage was greater by KrCl, as supported by the intercepts of repair kinetics. Compared to LP irradiated samples, KrCl irradiated samples exhibited less repair overall. For a given lamp, similar repair was observed between light and dark repair incubations. Percent reactivation rates with respect to photorepair fluence were (3.7±1.4)×10−5 and (–1.3±2.5)×10−5 cm2 mJ-1 for LP and KrCl lamps, respectively. CPDs decreased at a higher rate during repair incubations in LP samples than KrCl samples, and photolyase concentration increased in LP samples but decreased in KrCl samples. The results quantify contributions of photolyase protein damage to disinfection and repair prevention mechanism of KrCl lamps. This study mechanistically demonstrates why KrCl lamps can be applied for UV water disinfection to limit photorepair after treatment. Synopsis: This study used a novel photolyase assay to demonstrate photolyase damage inflicted by krypton chloride excimer lamps contributes to disinfection of bacteria to prevent bacterial photorepair of damaged DNA and regrowth in drinking water treatment.

One-Pot Phyto-Mediated Synthesis of Fe2O3/Fe3O4 Binary Mixed Nanocomposite Efficiently Applied in Wastewater Remediation by Photo-Fenton Reaction

A binary Fe2O3/Fe3O4 mixed nanocomposite was prepared by phyto-mediated avenue to be suited in the photo-Fenton photodegradation of methylene blue (MB) in the presence of H2O2. XRD and SEM analyses illustrated that Fe2O3 nanoparticles of average crystallite size 8.43 nm were successfully mixed with plate-like aggregates of Fe3O4 with a 15.1 nm average crystallite size. Moreover, SEM images showed a porous morphology for the binary Fe2O3/Fe3O4 mixed nanocomposite that is favorable for a photocatalyst. EDX and elemental mapping showed intense iron and oxygen peaks, confirming composite purity and symmetrical distribution. FTIR analysis displayed the distinct Fe-O assignments. Moreover, the isotherm of the developed nanocomposite showed slit-shaped pores in loose particulates within plate-like aggregates and a mesoporous pore-size distribution. Thermal gravimetric analysis (TGA) indicated the high thermal stability of the prepared Fe2O3/Fe3O4 binary nanocomposite. The optical properties illustrated a narrowing in the band gab (Eg = 2.92 eV) that enabled considerable absorption in the visible region of solar light. Suiting the developed binary Fe2O3/Fe3O4 nanocomposite in the photo-Fenton reaction along with H2O2 supplied higher productivity of active oxidizing species and accordingly a higher degradation efficacy of MB. The solar-driven photodegradation reactions were conducted and the estimated rate constants were 0.002, 0.0047, and 0.0143 min−1 when using the Fe2O3/Fe3O4 nanocomposite, pure H2O2, and the Fe2O3/Fe3O4/H2O2 hybrid catalyst, respectively. Therefore, suiting the developed binary Fe2O3/Fe3O4 nanocomposite and H2O2 in photo-Fenton reaction supplied higher productivity of active oxidizing species and accordingly a higher degradation efficacy of MB. After being subjected to four photo-Fenton degradation cycles, the Fe2O3/Fe3O4 nanocomposite catalyst still functioned admirably. Further evaluation of Fe2O3/Fe3O4 nanocomposite in photocatalytic remediation of contaminated water using a mixture of MB and pyronine Y (PY) dyestuffs revealed substantial dye photodegradation efficiencies.

Linear Mixed Model of Virus Disinfection by Free Chlorine to Harmonize Data Collected across Broad Environmental Conditions.

Despite the critical importance of virus disinfection by chlorine, our fundamental understanding of the relative susceptibility of different viruses to chlorine and robust quantitative relationships between virus disinfection rate constants and environmental parameters remains limited. We conducted a systematic review of virus inactivation by free chlorine and used the resulting data set to develop a linear mixed model that estimates chlorine inactivation rate constants for viruses based on experimental conditions. 570 data points were collected in our systematic review, representing 82 viruses over a broad range of environmental conditions. The harmonized inactivation rate constants under reference conditions (pH = 7.53, T = 20 °C, [Cl-] < 50 mM) spanned 5 orders of magnitude, ranging from 0.0196 to 1150 L mg-1 min-1, and uncovered important trends between viruses. Whereas common surrogate bacteriophage MS2 does not serve as a conservative chlorine disinfection surrogate for many human viruses, CVB5 was one of the most resistant viruses in the data set. The model quantifies the role of pH, temperature, and chloride levels across viruses, and an online tool allows users to estimate rate constants for viruses and conditions of interest. Results from the model identified potential shortcomings in current U.S. EPA drinking water disinfection requirements.

Methoxylated bis(pentafluorophenyl)boron‐β‐diketonates: Synthesis, Optical Properties and Hydrolytic Stability

AbstractA series of methoxylated bis(pentafluorophenyl)boron‐β‐diketonates (dkB(C6F5)2) with intense emission has been synthesized and characterized by a variety of physicochemical methods. The photophysical properties of the synthesized compounds were thoroughly investigated by electronic absorption, steady‐state, and time‐resolved fluorescence spectroscopy in both solution and solid state. The studied compounds exhibit solvatochromic behavior attributed to the intramolecular charge transfer (ICT) nature of the first excited state. The formation of exciplexes between DBMB(C6F5)2 and benzene derivatives was also studied. Notably, the studied compounds display a complex luminescence profile in the solid state, featuring fluorescence, delayed fluorescence and phosphorescence emission. Furthermore, the hydrolytical stability of the complexes was assessed, demonstrating excellent resistance to hydrolysis. Estimated rate constants, determined at 60 °C, are significantly lower (90‐2000 times) than those observed for the well‐studied dibenzoylmethanatoboron difluoride (DBMBF2). The incorporation of pentafluorophenyl substituents at the boron atom enables the synthesis of highly fluorescent and hydrolytically stable boron chelate complexes, suitable for sensing and bioimaging applications in water‐containing environments.

A pharmacokinetic model for hyperpolarized 13C-pyruvate MRI when using metabolite-specific bSSFP sequences.

Metabolite-specific balanced SSFP (MS-bSSFP) sequences are increasingly used in hyperpolarized [1-13C]Pyruvate (HP 13C) MRI studies as they improve SNR by refocusing the magnetization each TR. Currently, pharmacokinetic models used to fit conversion rate constants, kPL and kPB, and rate constant maps do not account for differences in the signal evolution of MS-bSSFP acquisitions. In this work, a flexible MS-bSSFP model was built that can be used to fit conversion rate constants for these experiments. The model was validated in vivo using paired animal (healthy rat kidneys n = 8, transgenic adenocarcinoma of the mouse prostate n = 3) and human renal cell carcinoma (n = 3) datasets. Gradient echo (GRE) acquisitions were used with a previous GRE model to compare to the results of the proposed GRE-bSSFP model. Within simulations, the proposed GRE-bSSFP model fits the simulated data well, whereas a GRE model shows bias because of model mismatch. For the in vivo datasets, the estimated conversion rate constants using the proposed GRE-bSSFP model are consistent with a previous GRE model. Jointly fitting the lactate T2 with kPL resulted in less precise kPL estimates. The proposed GRE-bSSFP model provides a method to estimate conversion rate constants, kPL and kPB, for MS-bSSFP HP 13C experiments. This model may also be modified and used for other applications, for example, estimating rate constants with other hyperpolarized reagents or multi-echo bSSFP.

The Michaelis–Menten Reaction at Low Substrate Concentrations: Pseudo-First-Order Kinetics and Conditions for Timescale Separation

We demonstrate that the Michaelis–Menten reaction mechanism can be accurately approximated by a linear system when the initial substrate concentration is low. This leads to pseudo-first-order kinetics, simplifying mathematical calculations and experimental analysis. Our proof utilizes a monotonicity property of the system and Kamke’s comparison theorem. This linear approximation yields a closed-form solution, enabling accurate modeling and estimation of reaction rate constants even without timescale separation. Building on prior work, we establish that the sufficient condition for the validity of this approximation is s0≪K\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$s_0 \\ll K$$\\end{document}, where K=k2/k1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$K=k_2/k_1$$\\end{document} is the Van Slyke–Cullen constant. This condition is independent of the initial enzyme concentration. Further, we investigate timescale separation within the linear system, identifying necessary and sufficient conditions and deriving the corresponding reduced one-dimensional equations.

Microelectrode investigation of iron and copper surfaces aged in presence of monochloramine.

Ductile iron and copper coupons were aged 137-189 days and 2 days, respectively, with 2 mg Cl2 L-1 monochloramine under four water chemistries (pH 7 or 9 and 0 or 3 mg L-1 orthophosphate). Subsequently, microelectrode profiles of monochloramine concentration, oxygen concentration, and pH were measured from the bulk water to near the coupon reactive surface, allowing estimation of flux and apparent surface reaction rate constants for monochloramine and oxygen. Both metals showed similar trends with orthophosphate where orthophosphate decreased metal reactivity with monochloramine (pH 9) and oxygen (pH 7). Comparing iron and copper coupons, apparent surface reaction rate constants for monochloramine and oxygen were one and two orders of magnitude greater, respectively, for iron coupons under all conditions. Overall, this research provides the first insights into monochloramine concentration, oxygen concentration, and pH by direct measurement near ductile iron and copper reactive surfaces aged in the presence of monochloramine.

Steady-State and transient kinetic investigations of the oxidation of NO over Pt/SiO2

The reaction mechanism of the oxidation of NO under conditions of high NO concentration over a 2 wt% Pt/SiO2 was studied using both kinetic and transient isotopic tracing experiments. The reaction mechanism was found to involve both NO adsorption and oxygen adsorption, contrary to the situation under low NO concentration where NO reacts from the gas phase. The kinetic rate data could be fitted to the Langmuir-Hinshelwood model permitting the estimation of rate and equilibrium constants of elementary steps. These steps were investigated using transient responses following independent reactant gas isotopic switches for tracing the nitrogen path and the oxygen path during NO oxidation. These transients could be sufficiently described by a microkinetic model based on two pools of NO intermediates, one pool each for the adsorbed molecular oxygen and atomically adsorbed oxygen. Surface atomic oxygen was found to be formed by two different routes: via a direct dissociation of molecularly adsorbed oxygen and also via an assisted pathway that leads to NO2 formation. The latter surface reaction between adsorbed NO and adsorbed molecular oxygen comprised the kinetically relevant step. Insights from the two experimental and model approaches were in good agreement and provided a coherent reaction mechanism for NO oxidation under high NO reactant concentration.

Kinetics, products and mechanisms of unsaturated alcohols and NO3 radicals

Unsaturated alcohols are emitted in the atmosphere from anthropogenic and biological sources. Although NO3 radical-initiated reaction of unsaturated alcohols constitute a significant portion to SOA formation, less studies have been reported. In this study, the rate constants of the reactions between NO3 radicals with 2-methyl-2-propen-1-ol (2M2P1O), (E)-4-hexen-1-ol (t-4H1O) and 4-penten-2-ol (4P2O) were measured at 298 ± 2 K by using relative rate methods. The rate constants were 3.41 ± 0.51 (2M2P1O), 6.45 ± 0.66 (t-4H1O) and 0.56 ± 0.15 (4P2O) (in units of 10−13 cm3molecule−1s−1), respectively. The reaction rates were also estimated by structure-activity relationships (SARs) method and compared with the experimental results. It showed that the estimated rate constant of 4-penten-2-ol by SARs was unreasonable due to the lack of group parameters. In addition, large amounts of organic nitrates were identified based on fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). The MS results showed that dinitrates were primary SOA components. The obtained reaction rate constants and the formation mechanisms of organic nitrates provide an important experimental basis for future atmospheric chemical models.

Kinetic Model for Predicting Perfluoroalkyl Acid Degradation During UV-Sulfite Treatment.

Hydrated electron (eaq-) treatment processes show great potential in remediating recalcitrant water contaminants, including perfluoroalkyl and polyfluoroalkyl substances (PFAS). However, treatment efficacy depends upon many factors relating to source water composition, UV light source characteristics, and contaminant reactivity. Here, we provide critical insights into the complex roles of solution parameters on contaminant abatement through application of a UV-sulfite kinetic model that incorporates first-principles information on eaq- photogeneration and reactivity. The model accurately predicts decay profiles of short-chain perfluoroalkyl acids (PFAAs) during UV-sulfite treatment and facilitates quantitative interpretation of the effects of changing solution composition on PFAS degradation rates. Model results also confirm that the enhanced degradation of PFAAs observed under highly alkaline pH conditions results from changes in speciation of nontarget eaq- scavengers. Reverse application of the model to UV-sulfite data collected for longer chain PFAAs enabled estimation of bimolecular rate constants (k2, M-1 s-1), providing an alternative to laser flash photolysis (LFP) measurements that are not feasible due to the water solubility limitations of these compounds. The proposed model links the disparate means of investigating eaq- processes, namely, UV photolysis and LFP, and provides a framework to estimate UV-sulfite treatment efficacy of PFAS in diverse water sources.

Abstract 2577: Analyzing specific and nonspecific tissue uptake of antibodies by SPECT imaging and non-linear compartmental modeling

Abstract Understanding the mechanisms driving specific target engagement versus off-target nonspecific cellular uptake of antibodies can help inform design and/or delivery strategies that maximize therapeutic indices. Receptor binding kinetics, rates of receptor-mediated internalization and nonspecific pinocytosis are often measured through in vitro cell culture models, but there have been fewer efforts to collectively measure these cellular parameters in vivo. Single photon emission computed tomography (SPECT) imaging is a powerful method of estimating these parameters by quantifying both free/bound (intact) and internalized/catabolized antibody using non-residualizing halogen labels and residualizing radiometal-chelates, respectively. Modeling longitudinal in vivo imaging data of non-residualizing antibody can inform receptor occupancy (RO), while residualizing tracer kinetics can provide additional insights into catabolism due to pinocytosis and receptor-mediated internalization in the intracellular spaces. Here, we describe SPECT imaging of tumor-targeting trastuzumab (anti-HER2) and non-targeting anti-glycoprotein D (anti-gD) antibody, each labeled with non-residualizing ([125I]SIB) and residualizing radiometal-chelate (111In-DOTA) radionuclides in HER2+ xenograft-bearing mice. A non-linear compartmental model was fitted to SPECT imaging data to characterize RO of 10 mg/kg of anti-HER2 out to 7 days post dosing as well as to estimate rate constants of pinocytosis and receptor-mediated internalization of anti-HER2 in tumor. Longitudinal SPECT imaging demonstrated statistically significant differences in tumor drug exposure between 125I- and 111In-labeled anti-HER2 relative to anti-gD (p &amp;lt; 0.01). Tracer kinetics of anti-HER2 and associated RO simulations in tumor showed rapid extravasation from blood to tumor with peak RO of 25% at 13 hours post administration followed by a reduction in RO, reaching ~10% by day 7. Simulations for free, bound and catabolized tracer kinetics from our fitted model showed that anti-HER2 in the tumor interstitium was primarily receptor-bound with minimal pinocytosis of unbound anti-HER2 as well as gradual receptor-mediated catabolism, reaching a peak concentration of 113 µg/mL 7 days post dosing. Rate constants of nonspecific pinocytosis and HER2-mediated receptor internalization were estimated to be 0.0145 and 0.036 hours−1, respectively. Our model allowed us to successfully capture anti-HER2 antibody receptor binding and specific and nonspecific internalization over time in vivo. These data and analyses demonstrate the power of SPECT imaging using both non-residualizing and residualizing radiolabels combined with compartmental modeling to better characterize the different biological states (free, bound, and catabolized) of antibody within interstitial and intracellular compartments. Citation Format: Nicholas Cho, Phillip Spinosa, Danielle Mandikian, Geoffrey Del Rosario, Shang-Fan Yu, Jason Ho, Greg Ferl, Charles A. Boswell. Analyzing specific and nonspecific tissue uptake of antibodies by SPECT imaging and non-linear compartmental modeling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2577.

A detailed kinetic submechanism for OH[formula omitted] chemiluminescence in hydrogen combustion revisited. Part 2

In this series of two papers, we present a novel physically sound kinetic submechanism aimed at modeling the formation and decay of chemiluminescent (electronically excited) OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-based mixtures over a wide range of temperatures and pressures. The present Part describes the construction of this detailed reaction submechanism based on the groundwork and findings of the preceding paper. In so doing, we relied both on critical evaluation of known and at times conflicting experimental and theoretical data on the elementary reaction kinetics of OH∗ and on our quantum chemical calculations and rate constant estimates for the key processes. The kinetic model under development was validated against the representative dataset of the observed OH(A2Σ+→X2Π) chemiluminescent emission accompanying the high-temperature (post-shock) oxidation of various H2-containing mixtures. To improve its performance against the collected OH∗ emission data, the rate constants of some reactive and quenching processes, to which the OH∗ kinetics is the most sensitive and for which there is a particular uncertainty, were jointly varied within the limits imposed by theoretical considerations and/or the scatter in the available experimental evidence. Thus, by refining the rate constants of these elementary processes, we obtained a reasonable reaction submechanism for OH∗ that describes the known experimental data better than previous models.Novelty and significance statement Although ultraviolet OH∗ chemiluminescence as an optical signature of the combustion process and the underlying chemistry is known to provide unique capabilities for combustion diagnostics, the development of the detailed reaction mechanisms intended for quantitative interpretation of the OH∗ emission measurements has now almost stagnated. Indeed, existing kinetic models contain a discouragingly small number of elementary processes and, apparently, do not account for the full variety of possible reaction pathways of OH∗ formation and depletion. Therefore, in this serial work, we have intended to revise the current ideas about the kinetics of OH∗ production and consumption during ignition and combustion and focused first on the OH∗ kinetics in a reacting H2/O2/Ar/N2 mixture. The principal outcome of this Part of the series is a new physically based kinetic submechanism aimed at modeling the formation and decay of electronically excited OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-containing mixtures. Its incorporation into any conventional kinetic model of hydrogen oxidation enables quantitative interpretation of the OH(A2Σ+−X2Π) chemiluminescent emission observed experimentally in flames and shock-heated reacting flows with an accuracy higher than that of previous analogous mechanisms. No less importantly, the rate constant approximations for a number of individual elementary processes with OH∗, recommended here based on the critical review of literature data and on our quantum chemical calculations, are also of independent interest for the kinetics of electronically excited OH∗ particles. Finally, the OH∗ reaction subset we suggested for hydrogen flames can form the basis of more complex OH∗ chemiluminescence modeling reaction mechanisms for other fuels and fuel compositions.

A detailed kinetic submechanism for OH[formula omitted] chemiluminescence in hydrogen combustion revisited. Part 1

In this series of two papers, we present a novel physically sound kinetic submechanism aimed at modeling the formation and decay of chemiluminescent (electronically excited) OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-based mixtures over a wide range of thermodynamic parameters and mixture compositions. The present part of this series describes the preparatory stages of the work, such as the aggregation of the data set for the OH∗ chemiluminescent emission (including the OH∗ emission profiles obtained in our shock-tube experiments), the choice of the basic kinetic model of hydrogen oxidation, and, importantly, quantum chemical calculations and theoretical estimates for the rate constant of the nonadiabatic preassociation reaction H + O + M ⇆ OH∗ + M, recognized as a major source of excited OH∗ molecules in hydrogen-air flames. These rate constant estimates are carried out in a wide range of temperatures (200–4000 K) and pressures (10−3−100 bar). It is shown that the rate constant of this key reaction exhibits distinct non-Arrhenius temperature behavior and, moreover, has a complex fall-off pressure dependence, with the transition from a third to a second-order, high-pressure regime occurring at pressures about 10 atm. The obtained dependence on temperature and pressure is in excellent agreement with the known experimental data and can be recommended (as part of the appropriate reaction mechanisms) for further kinetic modeling of OH∗ chemiluminescence accompanying the high-temperature oxidation of hydrogen and other fuels.Novelty and Significance StatementAlthough ultraviolet OH∗ chemiluminescence as an optical signature of the combustion process and the underlying chemistry is known to provide unique capabilities for combustion diagnostics, the development of the detailed reaction mechanisms intended for quantitative interpretation of the OH∗ emission measurements has now almost stagnated. Indeed, existing kinetic models contain a discouragingly small number of elementary processes and, apparently, do not account for the full variety of possible reaction pathways of OH∗ formation and depletion. Therefore, in this serial work, we have revised the current ideas about the kinetics of OH∗ production and consumption during ignition and combustion and focused first on the OH∗ kinetics in a reacting H2/O2/Ar/N2 mixture. The fundamentally significant and novel results of this Part of the present series are (i) the rate constant of the nonadiabatic preassociation reaction H + O + M ⇆ OH∗ + M (the primary source of OH∗ in hydrogen combustion), theoretically obtained for the first time as a function of temperature and pressure, and (ii) the OH∗ emission profiles obtained in our shock-tube experiments. The other aspects described here, such as the aggregation of the literature-based data set for the post-shock OH∗ chemiluminescent emission and the choice of the basic kinetic model of hydrogen oxidation, are also of interest and relevance.

Monte Carlo simulation of air kerma rate constants for different radionuclides

The air kerma rate constant values for different radionuclides are often used in radiation shielding calculations, calibration of instruments, estimating the absorbed dose from brachytherapy sources, etc. Usually, air kerma rate constant values are calculated in a deterministic way. This work evaluates the possibility of air kerma rate constant estimation through Monte Carlo simulations. Ten different radionuclides were considered in this preliminary study, covering low and high-energy gamma spectra. Mean percentage differences of less than 10% relative to the results of reference works were found. Most of our results present less than a 5% relative percentage difference considering the literature results. The air kerma rate constants calculated in this work agree with reference results, thus validating the methodology, the photon spectra database used, and the Monte Carlo simulations with EGSnrc. Future work can extend the calculation of air kerma rate constants for other radionuclides and study the influence of different sources of uncertainty in its results.

Computational study of the solvation effect of supercritical water on o-phthalic acid decarboxylation by varying the solvent dielectric constant

This study investigates the solvation effect on the decarboxylation of o-phthalic acid using quantum calculations with a continuum solvation model with varying dielectric constants. Two mechanisms were considered: with and without a catalytic water molecule. The latter had a lower reaction barrier. Increasing the dielectric constant facilitates the hydrogen transfer but impedes C–C bond breaking in both mechanisms. The change in the free energies due to solvation of the transition state is generally greater than the reactant. The reaction barrier predominantly increased with the dielectric constant in both mechanisms. The Kirkwood slope estimated by plotting estimated rate constants against the Kirkwood polarity parameter, (ε−1)/(2ε+1), was close to the experimental value (only for the assisted mechanism). The simulations indicate that the solvation effect on the decarboxylation rate can be elucidated by considering the effect of dielectric constant of supercritical water.

Insight into magnetically induced ring currents and photophysics of six-porphyrin nanorings.

The series of nanorings based on Zn-porphyrins and tetraoxa-isophlorins in different oxidation states (Q = 0, 2+, 4+, 6+) have been studied studied computationally at density functional theory level (DFT) using BHandHLYP functional combined with def2-SVP basis sets. Magnetically induced ring currents of nanorings have been calculated using the GIMIC method and the Ampère-Maxwell integration scheme. Ring current calculations show that neutral nanorings sustain equal diatropic and paratropic currents of 8 nA T-1, resulting in zero net ring current strengths. The charged nanorings sustain strong ring currents with tropicity depending on the oxidation state Q. Among the considered nanorings, the nanoring composed of 6 isophlorins c-Iso66+ is the most aromatic with a ring current of IGIMIC = 81.6 nA T-1. The structure c-P62+ with a ring current of IGIMIC = 54.9 nA T-1 can be considered as the most aromatic among the synthesized porphyrin nanorings. Spin-orbit coupling matrix elements, oscillator strengths, and excitation energies calculated at the CAM-B3LYP/def2-SVP level of theory were used to estimate rate constants for radiative and nonradiative processes. The algorithm based on X-H approximation were used to calculate the internal conversion rates (kIC). The main channel for the deactivation of the excitation energy in the studied nanorings is the process of internal conversion. The deactivation of excited energy occurs due to the vibrations of certain groups of C-H bonds in the nanorings. The nanoring c-Iso6 has magnetically allowed low-lying transitions that contributes significantly to the paratropic ring current, resulting in strong local antiaromaticity in the tetraoxa-isophlorin units.

Integrated Advanced Molecular Tools Predict In Situ cVOC Degradation Rates: Field Demonstration.

Chlorinated volatile organic compound (cVOC) degradation rate constants are crucial information for site management. Conventional approaches generate rate estimates from the monitoring and modeling of cVOC concentrations. This requires time series data collected along the flow path of the plume. The estimates of rate constants are often plagued by confounding issues, making predictions cumbersome and unreliable. Laboratory data suggest that targeted quantitative analysis of Dehalococcoides mccartyi (Dhc) biomarker genes (qPCR) and proteins (qProt) can be directly correlated with reductive dechlorination activity. To assess the potential of qPCR and qProt measurements to predict rates, we collected data from cVOC-contaminated aquifers. At the benchmark study site, the rate constant for degradation of cis-dichloroethene (cDCE) extracted from monitoring data was 11.0 ± 3.4 yr-1, and the rate constant predicted from the abundance of TceA peptides was 6.9 yr-1. The rate constant for degradation of vinyl chloride (VC) from monitoring data was 8.4 ± 5.7 yr-1, and the rate constant predicted from the abundance of TceA peptides was 5.2 yr-1. At the other study sites, the rate constants for cDCE degradation predicted from qPCR and qProt measurements agreed within a factor of 4. Under the right circumstances, qPCR and qProt measurements can be useful to rapidly predict rates of cDCE and VC biodegradation, providing a major advance in effective site management.

Clustering micropollutants and estimating rate constants of sorption and biodegradation using machine learning approaches

Effluent from wastewater treatment plants is considered an important source of micropollutants (MPs) in aquatic environments. However, monitoring MPs in effluents is often inefficient owing to the variety in their types. Thus, this study derived marker constituents to estimate the behavior of MPs in each cluster using the self-organizing map (SOM), a machine learning-based clustering analysis method. In SOM analysis, the physicochemical properties, functional groups, and the initial biotransformation rules of 29 out 42 MPs were used to ultimately estimate the degradation rate constants of 13 MPs. Consequently, when the physicochemical properties and functional groups were considered, SOM analysis showed outstanding performance to label MPs with an accuracy value of 0.75 for each aerobic and anoxic condition. Based on the clustering results, 11 MPs were determined to be marker constituents under each aerobic and anoxic condition. Moreover, an estimation method for the rate constants of unlabeled MPs was successfully developed using the identified markers with the random forest classifier. The proposed algorithm could estimate both sorption and biotransformation of MPs regardless of dominant removal mechanisms, whether the MPs were removed by sorption or biotransformation. An accuracy of 0.77 was calculated for estimating rate constants under both aerobic and anoxic conditions, which is remarkably higher than those reported previously. The proposed procedure could be extended further to efficiently monitor MPs in effluents.