Kinetic Exchange Research Articles

Fractionation of Dissolved Organic Matter by (Oxy)Hydroxide‐Coated Sands: Competitive Sorbate Displacement during Reactive Transport

Sorptive retention of dissolved organic matter (DOM) at soil particle surfaces controls C flux through the critical zone. Prior studies have shown that pristine Al‐ and Fe‐(oxy)hydroxide surfaces are especially reactive toward DOM sorptive stabilization. However, the impact of progressive and/or preexisting organic surface coatings on further surficial uptake and exchange during repeated DOM infusion episodes remains unclear. In this study, DOM solutions were extracted from organic horizons in grassland (G) and mixed conifer forest (F) vegetation types in the Jemez River Basin Critical Zone Observatory. Extracted DOM solutions were used to sequentially irrigate columns packed with either quartz sand (Qtz), Al‐hydroxide‐coated quartz sand (Al‐Qtz), or Fe‐hydroxide‐coated quartz sand (Fe‐Qtz). Use of distinct DOM sources enabled investigation of how sorption, fractionation, and exchange ensued during reactive transport through mineral media progressively coated with sorbate organic matter (SOM). During initial irrigation of fresh mineral media with G‐DOM, the magnitude of DOM sorption (per unit sorbent mass) followed the trend: Al‐Qtz ≥ Fe‐Qtz > Qtz. Effluent solutions showed diminished molar absorptivity and humification index (HIX) values, indicating preferential uptake of high‐molar‐mass aromatic constituents. Introduction of F‐DOM to G‐SOM‐coated surfaces revealed competitive desorption of G‐SOM from the organo–mineral interface. During F‐DOM irrigation, high HIX values were observed in effluent solutions, indicating remobilization of G‐SOM by displacement. According to spectroscopic analyses, the displaced G‐SOM consisted of aromatic phenolic acids with high excitation–emission “fingerprints” characteristic of fulvic‐ and humic‐acid‐like compounds, providing evidence for kinetic DOM exchange reactions.

Dispersion in oscillatory electro-osmotic flow through a parallel-plate channel with kinetic sorptive exchange at walls

Dispersion in time-oscillatory electro-osmotic flows in a slit micro-channel under the effect of kinetic sorptive exchange at walls is theoretically investigated using the homogenization method. The two walls of the channel are considered to be made up of different materials, and therefore have different zeta potentials and sorption coefficients. A general expression for the Taylor dispersion coefficient under different zeta potentials as well as various sorption conditions at the walls is derived analytically. The dispersion coefficient is found to be dependent on the oscillation frequency, the Debye parameter, the species partition coefficient, the reaction kinetics and the ratio of the wall potentials. The results demonstrate that the presence of wall sorption tends to enhance the dispersion when the oscillation frequency is low, but the effect is negligible in high-frequency oscillatory flows. Moreover, it is found that the dispersion coefficient could be significantly changed by adjusting the relative wall potentials for low-frequency flows.

NO<sub>x</sub> cycle and the tropospheric ozone isotope anomaly: an experimental investigation

Abstract. The oxygen isotope composition of nitrogen oxides (NOx) in the atmosphere is a useful tool for understanding the oxidation of NOx into nitric acid / nitrate in the atmosphere. A set of experiments was conducted to examine change in isotopic composition of NOx due to NOx–O2–O3 photochemical cycling. At low NOx / O2 mixing ratios, NOx became progressively and nearly equally enriched in 17O and 18O over time until it reached a steady state with Δ17O values of 39.3 ± 1.9‰ and δ18O values of 84.2 ± 4‰, relative to the isotopic composition of the initial O2 gas. As the mixing ratios were increased, the isotopic enrichments were suppressed by isotopic exchange between O atoms, O2, and NOx. A kinetic model was developed to simulate the observed data and it showed that the isotope effects occurring during O3 formation play a dominant role in controlling NOx isotopes and, in addition, secondary kinetic isotope effects or isotope exchange reactions are also important during NOx cycling. The data and model were consistent with previous studies which showed that the NO + O3 reactions occur mainly via the transfer of the terminal atoms of O3. The model predicts that under tropospheric concentrations of NOx and O3, the timescale of NOx–O3 isotopic equilibrium ranges from hours (for ppbv NOx / O2 mixing ratios) to days (for pptv mixing ratios) and yields steady state Δ17O and δ18O values of 45‰ and 117‰ respectively (relative to Vienna Standard Mean Ocean Water (VSMOW)) in both cases. Under atmospheric conditions when O3 has high concentrations, the equilibrium between NOx and O3 should occur rapidly (h) but this equilibrium cannot be reached during polar winters and/or nights if the NOx conversion to HNO3 is faster. The experimentally derived rate coefficients can be used to model the major NOx–O3 isotopologue reactions at various pressures and in isotope modeling of tropospheric nitrate.

Phase transition in kinetic exchange opinion models with independence

In this work we study the critical behavior of a three-state ($+1$, $-1$, $0$) opinion model with independence. Each agent has a probability $q$ to act as independent, i.e., he/she can choose his/her opinion independently of the opinions of the other agents. On the other hand, with the complementary probability $1-q$ the agent interacts with a randomly chosen individual through a kinetic exchange. Our analytical and numerical results show that the independence mechanism acts as a noise that induces an order-disorder transition at critical points $q_{c}$ that depend on the individuals' flexibility. For a special value of this flexibility the system undergoes a transition to an absorbing state with all opinions $0$.

A Model for Extracting Fundamental Kinetic Rates of SOFC Cathode Materials from Oxygen Isotope Exchange Experiments

A fundamental understanding of the oxygen reduction reaction (ORR) mechanisms on SOFC cathodes is essential for the development of lower temperature SOFCs with high performance. Isotope exchange is a powerful tool for analyzing the oxygen exchange process (1,2,3,4,5). In this study, we attempt to determine the ORR mechanism and extract fundamental kinetic rates for La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3-x (LSM) using isothermal isotope exchange (IIE)(6,7). IIE experiments were conducted at various temperatures and oxygen partial pressures. To extract fundamental kinetic rates from in-situ isotope exchange experiments on LSM and LSCF, a model was been developed based on a two-step reaction mechanism across the heterogeneous gas-solid interface. The coupled reactions can be shown in two elementary steps: dissociative adsorption and incorporation. The relation between the fundamental kinetic rates and surface exchange coefficient (kex) can be directly linked (8,9). In this paper we have developed a mathematic model to unify extracted surface exchange rates between techniques, including isotope exchange, conductivity relaxation experiments, etc. These results can lead to a better understanding of the ORR mechanisms and help design better cathode materials for SOFCs.

Fluorescence recovery after photobleaching investigation of protein transport and exchange in chromatographic media

A fully-mechanistic understanding of protein transport and sorption in chromatographic materials has remained elusive despite the application of modern continuum and molecular observation techniques. While measuring overall uptake rates in proteins in chromatographic media is relatively straightforward, quantifying mechanistic contributions is much more challenging. Further, at equilibrium in fully-loaded particles, measuring rates of kinetic exchange and diffusion can be very challenging. As models of multicomponent separations rely on accurate depictions of protein displacement and elution, a straightforward method is desired to measure the mobility of bound protein in chromatographic media. We have adapted fluorescence recovery after photobleaching (FRAP) methods to study transport and exchange of protein at equilibrium in a single particle. Further, we have developed a mathematical model to capture diffusion and desorption rates governing fluorescence recovery and investigate how these rates vary as a function of protein size, binding strength and media type. An emphasis is placed on explaining differences between polymer-modified and traditional media, which in the former case is characterized by rapid uptake, slow displacement and large elution pools, differences that have been postulated to result from steric and kinetic limitations. Finally, good qualitative agreement is achieved predicting flow confocal displacement profiles in polymer-modified materials, based solely on estimates of kinetic and diffusion parameters from FRAP observations.

Dynamics of Inter-heavy Chain Interactions in Human Immunoglobulin G (IgG) Subclasses Studied by Kinetic Fab Arm Exchange

Interdomain interactions between the CH3 domains of antibody heavy chains are the first step in antibody assembly and are of prime importance for maintaining the native structure of IgG. For human IgG4 it was shown that CH3-CH3 interactions are weak, resulting in the potential for half-molecule exchange ("Fab arm exchange"). Here we systematically investigated non-covalent interchain interactions for CH3 domains in the other human subclasses, including polymorphisms (allotypes), using real-time monitoring of Fab arm exchange with a FRET-based kinetic assay. We identified structural variation between human IgG subclasses and allotypes at three amino acid positions (Lys/Asn-392, Val/Met-397, Lys/Arg-409) to alter the strength of inter-domain interactions by >6 orders of magnitude. Each substitution affected the interactions independent from the other substitutions in terms of affinity, but the enthalpic and entropic contributions were non-additive, suggesting a complex interplay. Allotypic variation in IgG3 resulted in widely different CH3 interaction strengths that were even weaker for IgG3 than for IgG4 in the case of allotype G3m(c3c5*/6,24*), whereas G3m(s*/15*) was equally stable to IgG1. These interactions are sufficiently strong to maintain the structural integrity of IgG1 during its normal life span; for IgG2 and IgG3 the inter-heavy chain disulfide bonds are essential to prevent half-molecule dissociation, whereas the labile hinge disulfide bonds favor half-molecule exchange in vivo for IgG4.

Analysis of the Magnetic Exchange Interaction in Halide-Bridged Cu(II) Binuclear Complexes: Deciphering the Paths

Following a recently reported synthesis of linearly halide-bridged Cu(II) complexes [Inorg. Chem. 2012, 51, 7966-7968] characterized by strong antiferromagnetic exchange couplings (J), we applied Difference Dedicated Configuration Interaction (DDCI) and Broken-Symmetry Density Functional Theory (BS-DFT) approaches in order to analyze theoretically the trend observed, in which the decreasing electronegativity of the central halide induces an exacerbed magnetic coupling. The importance of the magnetic orbitals in DDCI calculations is acknowledged. The use of reduced molecular models is shown to lead to significant differences in J, in the case of chloride and bromide bridges. Finally, the BS-DFT decomposition of J in its main physical contributions confirms the importance of the kinetic exchange mediated by the halide bridge, but also points towards an increasing core polarization contribution when going from fluoride to bromide. Keywords: Broken-Symmetry DFT, Difference Dedicated CI, Magnetic exchange coupling.

Use of ¹⁹F NMR methods to probe conformational heterogeneity and dynamics of exchange in functional RNA molecules.

Functional RNA molecules are often very plastic and often undergo changes in base-pairing patterns to achieve alternative secondary and tertiary conformations associated with their roles in multiple events in gene expression. Solution NMR techniques are an excellent tool for the analysis of conformational heterogeneity and dynamic exchange. In this work, we measure the rates associated with spontaneous interconversion between major conformers in folded RNA sequences by use of a (19)F-(19)F EXSY NMR experiment, taking advantage of RNA samples carrying a single 5-(19)F-pyrimidine label. We first utilize this approach to determine kinetic exchange rates between conformers in a model RNA stem loop capable of adopting two conformations. We then probe the dynamics of conformational rearrangements in a larger RNA construct, the U2-U6 snRNA complex of the human spliceosome. In the case of the U2-U6 snRNA complex, such a rearrangement in the context of the intact spliceosome may have critical implications in splicing activity.

A Kinetic Isotope Effect and Isotope Exchange Study of the Nonenzymatic and the Equine Serum Butyrylcholinesterase-Catalyzed Thioester Hydrolysis

Formylthiocholine (FTC) was synthesized and found to be a substrate for nonenzymatic and butyrylcholinesterase (BChE)-catalyzed hydrolysis. Solvent (D2O) and secondary formyl-H kinetic isotope effects (KIEs) were measured by an NMR spectroscopic method. The solvent (D2O) KIEs are (D2O)k = 0.20 in 200 mM HCl, (D2O)k = 0.81 in 50 mM HCl, and (D2O)k = 4.2 in pure water. The formyl-H KIEs are (D)k = 0.80 in 200 mM HCl, (D)k = 0.77 in 50 mM HCl, (D)k = 0.75 in pure water, (D)k = 0.88 in 50 mM NaOH, and (D)(V/K) = 0.89 in the BChE-catalyzed hydrolysis in MES buffer at pH 6.8. Positional isotope exchange experiments showed no detectable exchange of (18)O into the carbonyl oxygen of FTC or the product, formate, under any of the above conditions. Solvent nucleophile-O KIEs were determined to be (18)k = 0.9917 under neutral conditions, (18)k = 1.0290 (water nucleophile) or (18)k = 0.989 (hydroxide nucleophile) under alkaline conditions, and (18)(V/K) = 0.9925 for BChE catalysis. The acidic, neutral, and BChE-catalyzed reactions are explained in terms of a stepwise mechanism with tetrahedral intermediates. Evidence for a change to a direct displacement mechanism under alkaline conditions is presented.

Ground-state and dynamical properties of two-dimensional dipolar Fermi liquids

We study the ground-state properties of a two-dimensional spin-polarized fluid of dipolar fermions within the Euler–Lagrange Fermi-hypernetted-chain approximation. Our method is based on the solution of a scattering Schrödinger equation for the “pair amplitude” g(r), where g(r) is the pair distribution function. A key ingredient in our theory is the effective pair potential, which includes a bosonic term from Jastrow–Feenberg correlations and a fermionic contribution from kinetic energy and exchange, which is tailored to reproduce the Hartree–Fock limit at weak coupling. Very good agreement with recent results based on quantum Monte Carlo simulations is achieved over a wide range of coupling constants up to the liquid-to-crystal quantum phase transition. Using the fluctuation–dissipation theorem and a static approximation for the effective inter-particle interactions, we calculate the dynamical density–density response function, and furthermore demonstrate that an undamped zero-sound mode exists for any value of the interaction strength, down to infinitesimally weak couplings.

Surfactant transfer across a water/oil interface: A diffusion/kinetics model for the interfacial tension evolution

The transfer of amphiphilic solutes through an interface between water and an organic solvent has been studied numerically by simulations based on a general diffusion/reaction model. This description predicts the evolution of the solute concentrations and the transfer across the liquid/liquid interface. It especially focuses on the dynamic interfacial tension evolution that can be measured by a pendant drop tensiometer. Both the bulk diffusion and the adsorption/desorption rate on both sides of the liquid interface are assumed to contribute to the global transfer kinetic. The calculations provide an understanding on how kinetic exchange at the interface and diffusion transport through the bulk determine the dynamic interfacial tension evolution. In particular, complex interfacial tension evolutions with a non-trivial behavior are predicted in some special cases when diffusion and kinetic exchange are of the same order of magnitude. Finally, this model is used to re-interpret experimental data about Triton X-100 transfer at a water–oil interface.

Unveiling the Physics of the Doped Phase of thet−JModel on the Kagome Lattice

We investigate the ground state properties of the kagome lattice t - J model at low doping by variational Monte Carlo calculations. The resulting state possesses an interesting balance of spin exchange and kinetic exchange through the building blocks of stars which are linked by triangles and their internal hexagons. While the spin exchange is taking place mainly on the stars, hopping is favored on the hexagons. There is a density modulation, resulting in the holes having an effective static contribution. From this observation, how holes lead to dimerization in this model and why a particular valence bond crystal pattern is formed can be understood. Furthermore, we argue the optimal doping for this state. We discuss our result in connection with static impurities, and show the likely relevance to the diluted kagome lattice Heisenberg model, describing actual compounds.

Kinetic exchange models: From molecular physics to social science

We discuss several multi-agent models that have their origin in the kinetic exchange theory of statistical mechanics and have been recently applied to a variety of problems in the social sciences. This class of models can be easily adapted for simulations in areas other than physics, such as the modeling of income and wealth distributions in economics and opinion dynamics in sociology.

Additive Decomposition of the Physical Components of the Magnetic Coupling from Broken Symmetry Density Functional Theory Calculations.

The procedure to extract and identify from broken-symmetry density functional theory (BS-DFT) calculations the various components of the magnetic couplings in diradicals [J. Chem. Phys. 2012, 137, 114106] is re-examined. It is shown that this previous decomposition scheme fails for systems exhibiting large core polarization effects and hence becomes not additive in such cases. At variance, the new scheme which differs from the previous one in the assessment of the polarization effects is perfectly additive. As done previously, the direct exchange is calculated from the MS = 1 and MS = 0 restricted solutions. We show that allowing first the delocalization of the magnetic orbitals in the field of the closed shell frozen core furnishes a good evaluation of the kinetic exchange contribution to the magnetic exchange coupling, i.e. the intersite delocalization of the magnetic electrons in the low-spin state. In a second step, allowing the polarization of the core to take place in the field of the so-revised magnetic orbitals practically leads to the same total value of the magnetic coupling obtained by the brute-force BS-DFT calculation. The success of this decomposition is illustrated on a representative series of inorganic and organic diradicals. The obtained quasi-additivity of the effects is rationalized thanks to a careful theoretical analysis of the broken-symmetry solutions.

Bimodality in the firm size distributions: a kinetic exchange model approach

Firm growth process in the developing economies is known to produce divergence in their growth path giving rise to bimodality in the size distribution. Similar bimodality has been observed in wealth distribution as well. Here, we introduce a modified kinetic exchange model which can reproduce such features. In particular, we will show numerically that a nonlinear retention rate (or savings propensity) causes this bimodality. This model can accommodate binary trading as well as the whole system-side trading thus making it more suitable to explain the non-standard features of wealth distribution as well as firm size distribution.

Theoretical Model on the Interplay of Disorder and Electron Correlations

In the present review paper we discuss a general theoretical treatment to investigate the interplay between the effects of disorder and electron-electron interaction in the Anderson-localized regime. In constructing the model Hamiltonian, the behaviour of Anderson-localized states with envelope function is explicitly taken into account. It is shown that, in the approximation of neglecting the interstate interactions, the rival effects of disorder and electron correlation can be described by two fundamental quantities: the average intrastate interaction energy and the distribution width of one-electron energies. On the basis of this model the spin susceptibility and electronic specific heat as well as density of states are calculated. It is shown that the intrastate interaction gives rise to the occurrence of singly-occupied states below the Fermi energy and that this results in a Curie-type spin susceptibility at low temperatures. It is shown that the configuration interaction which is one of the effects of interstate interactions gives rise to a kinetic type exchange interaction between singly-occupied states which favours spin-singlet states and that the competition between direct and kinetic exchanges leads to the spin pair model in which each localized spin of singly occupied state forms a ferromagnetic or antiferromagnetic pair with its nearest neighbour. Specific heat anomaly, in particular its magnetic field dependence, and magnetic susceptibility at very low temperatures are calculated by taking into account the coexistence of ferromagnetic and antiferromagnetic interactions within a spin pair model. It is shown that the experimental results on Si:P can be well explained by the present treatment. Finally the effects of electron-electron interactions on the magnetoresistance of the variable range hopping conduction are investigated. It is shown that magnetoresistance is positive as far as the change of localization length due to a magnetic field is neglected Taking into account the latter effect which is responsible for negative magneto-resistance, the total magnetoresistance is calculated for the systems of 1T-TaS2 and their observed peculiar features of magnetoresistance can be explained well by the present theory.

Applicability of approximated effective Hamiltonian method to first-principle study on magnetic clusters

In the first-principle study on the magnetic interactions in transition metal dimer complexes, we have shown that the effective Hamiltonian method can be effectively utilized (J. Phys.: Condens. Matter 23 375502). To extend our method to clusters with higher nuclearity, some approximations are needed since the number of the parameters of the effective Hamiltonian is too large. In the present paper, how to decrease the number of parameters of our effective Hamiltonian and how to determine the parameters from the calculations based on the density functional theory are shown. Then, approximated Hamiltonians are examined for its applicability in the first-principle study on the magnetic interactions in transition metal clusters. From the calculations of the Ti2Cl3−9 cluster whose parameters of the effective Hamiltonian are already determined, it is found that small differences caused by the symmetry of the each orbital can be neglected and not only kinetic exchange but also potential exchange terms are needed for quantitative calculation of the magnetic susceptibility. The appropriately approximated Hamiltonian reproduces the global structure of the lower energies so as to realize the nature of the magnetic interaction.

Axial Ligand Exchange of N-heterocyclic Cobalt(III) Schiff Base Complexes: Molecular Structure and NMR Solution Dynamics

The kinetic and thermodynamic ligand exchange dynamics are important considerations in the rational design of metal-based therapeutics and therefore, require detailed investigation. Co(III) Schiff base complex derivatives of bis(acetylacetone)ethylenediimine [acacen] have been found to be potent enzyme and transcription factor inhibitors. These complexes undergo solution exchange of labile axial ligands. Upon dissociation, Co(III) irreversibly interacts with specific histidine residues of a protein, and consequently alters structure and causes inhibition. To guide the rational design of next generation agents, understanding the mechanism and dynamics of the ligand exchange process is essential. To investigate the lability, pH stability, and axial ligand exchange of these complexes in the absence of proteins, the pD- and temperature-dependent axial ligand substitution dynamics of a series of N-heterocyclic [Co(acacen)(X)(2)](+) complexes [where X = 2-methylimidazole (2MeIm), 4-methylimidazole (4MeIm), ammine (NH(3)), N-methylimidazole (NMeIm), and pyridine (Py)] were characterized by NMR spectroscopy. The pD stability was shown to be closely related to the nature of the axial ligand with the following trend toward aquation: 2MeIm > NH(3) ≫ 4MeIm > Py > Im > NMeIm. Reaction of each [Co(III)(acacen)(X)(2)](+) derivative with 4MeIm showed formation of a mixed ligand Co(III) intermediate via a dissociative ligand exchange mechanism. The stability of the mixed ligand adduct was directly correlated to the pD-dependent stability of the starting Co(III) Schiff base with respect to [Co(acacen)(4MeIm)(2)](+). Crystal structure analysis of the [Co(acacen)(X)(2)](+) derivatives confirmed the trends in stability observed by NMR spectroscopy. Bond distances between the Co(III) and the axial nitrogen atoms were longest in the 2MeIm derivative as a result of distortion in the planar tetradentate ligand, and this was directly correlated to axial ligand lability and propensity toward exchange.

Platinum(ii) and platinum(iv) complexes stabilized by abnormal/mesoionic C4-bound dicarbenes

Platinum(II) complexes comprising abnormal diimidazolylidene ligands were synthesized from cis-PtMe(2)(DMSO)(2) using microwave-assisted double C-H bond activation. NMR analysis revealed an unusual solvolysis process, induced by coordinating solvents such as DMSO and MeCN, which has not been observed in related normal dicarbene complexes. NMR and IR spectroscopy and crystallographic analysis of the mono-substituted DMSO complex indicate a sulfur-bonding of the DMSO ligand to the platinum(II) center. Analysis of the DMSO exchange kinetics provided for the first time a quantitative measure of the trans effect of abnormal carbene ligands. The kinetic exchange rate in these bidentate abnormal dicarbene complexes is 0.050(±2) s(-1) and thus similar to analogous platinum(II) complexes containing phenylpyridine, yet significantly slower than that induced by pyridylidene pyridine. Reaction of the dicarbene platinum(II) complexes with PhICl(2), Br(2) and I(2) afforded the corresponding platinum(IV) complexes. Linkage isomerism of the Pt(IV)-bound DMSO was observed when the bromination reaction was performed in DMSO solution. Moreover, solvolysis was less pronounced in the platinum(IV) complexes than in the corresponding platinum(II) analogues.