Chemical Exchange Kinetics Research Articles

Insight into Details of Chemical Exchange Kinetics Studied by NMR CPMG Method

A more detailed insight into the chemical kinetics and dynamics of chemical exchanges within a molecule or between molecules in liquids is made possible by the NMR CPMG method, which, in addition to the exchange rate, gives its power spectrum, which contains information about the underlying processes of chemical exchange. The applicability of the method is demonstrated by measuring the chemical exchange in an aqueous solutions of sucrose, whose rate spectra have shapes that cannot be explained as transitions in a double potential well, but after interpretation using the chemical Langevin equations, it can be explained as a cascading chemical transition across several intermediate potential walls.

How Digestive Processes Can Affect the Bioavailability of PCBs Associated with Microplastics: A Modeling Study Supported by Empirical Data.

The transfer kinetics of plastic-associated chemicals during intestinal digestive processes is unknown. Here, we assessed whether digestive processes affect chemical exchange kinetics on microplastics, using an in vitro gut fluid digestive model mimicking the human upper intestinal tract. Chemical exchange kinetics of microplastics were measured for 10 polychlorinated biphenyls (PCBs) as proxies for the broad class of hydrophobic organic chemicals. Following earlier studies, olive oil was used as a proxy for digestible food, under high and low digestive enzyme activities. The micelle-water and oil-water partition coefficients of the 10 PCBs were also determined to evaluate the relative contribution of each gut component to sorb PCBs. A new biphasic and reversible chemical exchange model, which included the digestion process, fitted well to the empirical data. We demonstrate that the digestive processes that break down contaminated food can lead to a substantial increase in chemical concentration in microplastics by a factor of 10-20, thereby reducing the overall chemical bioavailability in the gastrointestinal tract when compared to a scenario without microplastics. Higher enzyme activities result in more chemicals being released by the digested food, thereby resulting in higher chemical concentrations in the microplastics. While the model-calibrated kinetic parameters are specific to the studied scenario, we argue that the mechanism of the reduced bioavailability of chemicals and the modeling tool developed have generic relevance. These digestive processes should be considered when assessing the risks of microplastics to humans and also biomagnification in aquatic food webs.

Detecting water-protein chemical exchange in membrane-bound proteins/peptides by solid-state NMR spectroscopy

Water plays an important role in many essential biological processes of membrane proteins in hydrated lipid environments. In general, the 1H polarization transfers between water molecules and site-specific protons in proteins can be classified as coherent (via dipolar spin diffusion) and incoherent (via chemical exchange and nuclear Overhauser effect) transfers. Solid-state NMR is the technique of choice for studying such water-protein interactions in membrane-bound proteins/peptides through the detection of 1H polarization transfers from water to the proteins. These polarization transfer mechanisms often exist simultaneously and are difficult to quantify individually. Here, we review water-protein polarization transfer techniques in solid-state NMR, with a focus on the recent progress for the direct detection of site-specific kinetic water-protein chemical exchange processes on the sub-millisecond time scale in membrane-bound proteins. The measurements of the pure chemical exchange kinetics provide a unique opportunity to understand the role that water plays in the structure-function relationships of membrane-bound species at the water-bilayer interface. In addition, the perspective of chemical exchange saturation transfer (CEST) experiments in membrane-bound proteins/peptides is further discussed.

Binding site exchange kinetics revealed through efficient spin-spin dephasing of hyperpolarized 129Xe.

Spin exchange between different chemical environments is an important observable for characterizing chemical exchange kinetics in various contexts, including protein folding, chelation chemistry, and host–guest interactions. Such spins experience effective spin–spin relaxation rate, R2,eff, that typically shows a dispersive behavior which requires detailed analysis. Here, we describe a class of highly simplified R2,eff behavior by relying on hyperpolarized 129Xe as a freely exchanging ligand reporter. It provides large chemical shift separations that yield reduced expressions of both the Swift–Connick and the Carver–Richards treatment of exchange-induced relaxation. Despite observing a diamagnetic system, R2,eff is dominated by large Larmor frequency jumps and thus allows detection of otherwise inaccessible analyte concentrations with a single spin echo train (only 0.01% of the overall hyperpolarized spins need to be transiently bound to the molecule). The two Xe hosts cryptophane-A monoacid (CrA-ma) and cucurbit[6]uril (CB6) represent two exemplary families of container molecules (the latter one also serving as drug delivery vehicles) that act as highly efficient phase shifters for which we observed unprecedented exchange-induced relaxivity r2 (up to 866 s−1 mM−1). By including methods of spatial encoding, multiple data points can be collected simultaneously to isolate the exchange contribution and determine the effective exchange rate in partially occupied binding sites with a single delivery of hyperpolarized nuclei. The relaxivity is directly related to the guest turnover in these systems and temperature-dependent measurements yield an activation energy of EA = 41 kJ mol−1 for Xe@CrA-ma from simple relaxometry analysis. The concept is transferable to many applications where Xe is known to exhibit large chemical shifts.

Initial investigation of glucose metabolism in mouse brain using enriched 17 O-glucose and dynamic 17 O-MRS.

In this initial work, the in vivo degradation of 17 O-labeled glucose was studied during cellular glycolysis. To monitor cellular glucose metabolism, direct 17 O-magnetic resonance spectroscopy (MRS) was used in the mouse brain at 9.4T. Non-localized spectra were acquired with a custom-built transmit/receive (Tx/Rx) two-turn surface coil and a free induction decay (FID) sequence with a short TR of 5.4ms. The dynamics of labeled oxygen in the anomeric 1-OH and 6-CH2 OH groups was detected using a Hankel-Lanczos singular value decomposition (HLSVD) algorithm for water suppression. Time-resolved 17 O-MRS (temporal resolution, 42/10.5s) was performed in 10 anesthetized (1.25% isoflurane) mice after injection of a 2.2M solution containing 2.5mg/g body weight of differently labeled 17 O-glucose dissolved in 0.9% physiological saline. From a pharmacokinetic model fit of the H217 O concentration-time course, a mean apparent cerebral metabolic rate of 17 O-labeled glucose in mouse brain of CMRGlc =0.07±0.02μmol/g/min was extracted, which is of the same order of magnitude as a literature value of 0.26±0.06μmol/g/min reported by 18 F-fluorodeoxyglucose (18 F-FDG) positron emission tomography (PET). In addition, we studied the chemical exchange kinetics of aqueous solutions of 17 O-labeled glucose at the C1 and C6 positions with dynamic 17 O-MRS. In conclusion, the results of the exchange and in vivo experiments demonstrate that the C6-17 OH label in the 6-CH2 OH group is transformed only glycolytically by the enzyme enolase into the metabolic end-product H217 O, whereas C1-17 OH ends up in water via direct hydrolysis as well as glycolysis. Therefore, dynamic 17 O-MRS of highly labeled 17 O-glucose could provide a valuable non-radioactive alternative to FDG PET in order to investigate glucose metabolism.

Simple models for disequilibrium fractional melting and batch melting with application to REE fractionation in abyssal peridotites

Disequilibrium melting arises when the kinetics of chemical exchange between a residual mineral and partial melt is sluggish compare to the rate of melting. To better understand the role of a finite crystal–melt exchange rate on trace element fractionation during mantle melting, we have developed a disequilibrium melting model for partial melting in an upwelling steady-state column. We use linear kinetics to approximate crystal–melt mass exchange rate and obtain simple analytical solutions for cases of perfect fractional melting and batch melting. A key parameter determining the extent of chemical disequilibrium during partial melting is an element specific dimensionless ratio (ε) defined as the melting rate relative to the solid–melt chemical exchange rate for the trace element of interest. In the case of diffusion in mineral limited chemical exchange, ε is inversely proportional to diffusivity of the element of interest. Disequilibrium melting is important for the trace element when ε is comparable to or greater than the bulk solid–melt partition coefficient for the trace element (k). The disequilibrium fractional melting model is reduced to the equilibrium perfect fractional melting model when ε is much smaller than k. Hence highly incompatible trace elements with smaller mobilities in minerals are more susceptible to disequilibrium melting than moderately incompatible and compatible trace elements. Effect of chemical disequilibrium is to hinder the extent of fractionation between residual solid and partial melt, making the residual solid less depleted and the accumulated melt more depleted in incompatible trace element abundances relative the case of equilibrium melting.Application of the disequilibrium fractional melting model to REE and Y abundances in clinopyroxene in abyssal peridotites from the Central Indian Ridge and the Vema Lithospheric Section, Mid-Atlantic Ridge revealed a positive correlation between the disequilibrium parameter ε and the degree of melting, which can be explained by an increase in melting rate and a decrease in REE diffusion rate in the upper part of the melting column. Small extent of disequilibrium melting for LREE and equilibrium melting for HREE in the upper part of the melting column can explain the elevated LREE abundances or spoon-shaped REE patterns in clinopyroxene in more refractory abyssal peridotites. The latter has often been attributed to melt refertilization.

Kinetics of exchange between zero-, one-, and two-hydrogen-bonded states of methyl and ethyl acetate in methanol.

It has recently been shown that the ester carbonyl stretching vibration can be used as a sensitive probe of local electrostatic field in molecular systems. To further characterize this vibrational probe and extend its potential applications, we studied the kinetics of chemical exchange between differently hydrogen-bonded (H-bonded) ester carbonyl groups of methyl acetate (MA) and ethyl acetate (EA) in methanol. We found that, while both MA and EA can form zero, one, or two H-bonds with the solvent, the population of the 2hb state in MA is significantly smaller than that in EA. Using a combination of linear and nonlinear infrared measurements and numerical simulations, we further determined the rate constants for the exchange between these differently H-bonded states. We found that for MA the chemical exchange reaction between the two dominant states (i.e., 0hb and 1hb states) has a relaxation rate constant of 0.14 ps(-1), whereas for EA the three-state chemical exchange reaction occurs in a predominantly sequential manner with the following relaxation rate constants: 0.11 ps(-1) for exchange between 0hb and 1hb states and 0.12 ps(-1) for exchange between 1hb and 2hb states.

Beryllium fluoride exchange rate accelerated by Mg²⁺ as discovered by ¹⁹F NMR.

Beryllium fluoride is widely used as a phosphoryl analogue in macromolecular studies, which are not only fluoride-sensitive but also magnesium-dependent. The beryllium fluorides are a mixture of different species including BeF3(-) and BeF4(2-) exchanging under thermodynamic equilibrium in neutral aqueous solutions. In the cases of mimicking phosphate group transfer, both beryllium fluoride and the magnesium ion are generally needed. However, the impact of magnesium on the bioactivity of beryllium fluoride is not clear. We have found by (19)F NMR spectroscopy that Mg(2+) can severely affect the chemical exchange kinetics between BeF3(-) and BeF4(2-). When the F(-) concentration is relatively low, the presence of 10.0 mM Mg(2+) can accelerate the exchange rate 3-4 fold. However, when the F(-) concentration is relatively high, the Mg(2+) effect on the chemical exchange vanishes. On the basis of these findings, we proposed a possible mechanism that BeF4(2-) and Mg(2+) form an ion pair that affects the distribution of beryllium fluoride species and thus the activity in the solution.

Characterization of Chemical Exchange Using Residual Dipolar Coupling

NMR line shape analysis and relaxation dispersion measurements for N,N-dimethyltrichloroacetamide (DMTCA) weakly aligned using poly-γ-benzyl-l-glutamate, which forms a lyotropic nematic phase when dissolved in chloroform, were used to characterize chemical exchange kinetics for the rotation around the C−N amide bond. At low temperatures (<312 K), slow-exchange 1H-coupled 13C NMR spectra show two resolved 13C methyl quartets with different residual dipolar coupling constants (RDCs). At high temperatures (>312 K), a single population-averaged 13C methyl quartet is observed; in this regime, the differences in RDCs contribute to differential line broadening of the quartet components. Self-consistent measurements of the difference in RDCs are obtained from both line shape and relaxation dispersion techniques. The results show that NMR spectroscopy of weakly aligned molecules allows complete characterization of chemical exchange processes using RDCs, even if exchange broadening is absent in isotropic samples.

A Markov Model for Relaxation and Exchange in NMR Spectroscopy

A two-state Markov noise process for lattice fluctuations and chemical exchange dynamics is used to derive a stochastic Liouville equation describing the evolution of the spin-density operator in nuclear magnetic resonance spectroscopy. Relaxation through lattice fluctuations and chemical exchange processes is incorporated into the theory at the same fundamental level, and the results are valid for all time scales provided that lattice fluctuations are much faster than chemical exchange kinetics. Time-scale separation emerges as an essential feature from the lowest-order perturbation expansion of the average resolvent in the Laplace domain.

On the use of the stochastic Liouville equation in nuclear magnetic resonance: Application to R1ρ relaxation in the presence of exchange

AbstractThe nuclear magnetic resonance (NMR) spin relaxation rate constant in the rotating frame of reference R1ρ provides information on microsecond to millisecond (μs‐ms) timescale kinetic processes that stochastically modulate the resonance frequencies of nuclear spins in molecules. Expressions for R1ρ are presented for two‐site chemical exchange using both the conventional perturbation approach for evolution of the density operator in the interaction frame of reference and the stochastic Liouville equation (SLE) for evolution of the average density operator in the rotating frame of reference. The former approach is limited to the fast‐exchange regime, in which the chemical exchange kinetics are faster than the frequency difference between spins in the two sites. The latter approach provides approximate expressions for R1ρ that are accurate when chemical exchange is not fast. Expressions for R1ρ that are accurate over the widest range of experimental conditions facilitate the interpretation of chemical exchange phenomena. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part A 19A: 134–148, 2003.

Speciation equilibria, clustering, and chemical-exchange kinetics in nonoxide glasses and melts. High-temperature phosphorus-31 NMR study of the system phosphorus-selenium

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSpeciation equilibria, clustering, and chemical-exchange kinetics in nonoxide glasses and melts. High-temperature phosphorus-31 NMR study of the system phosphorus-seleniumRobert Maxwell and Hellmut EckertCite this: J. Am. Chem. Soc. 1993, 115, 11, 4747–4753Publication Date (Print):June 1, 1993Publication History Published online1 May 2002Published inissue 1 June 1993https://doi.org/10.1021/ja00064a039RIGHTS & PERMISSIONSArticle Views116Altmetric-Citations22LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (719 KB) Get e-Alerts Get e-Alerts

Solid-state cesium-133 NMR studies of cations in cesium/lithium/sodium zeolite A: an example of cation dynamics involving three sites

{sup 133}Cs NMR spectra show marked changes in the line shape as a function of temperature for a fully hydrated zeolite. A sample containing a mixture of Li{sup +}, Na{sup +}, and Cs{sup +} ions. A single Lorentzian peak is observed at temperature above 285 K, and three resolvable peaks are observed at temperatures below 268 K. These three peaks arise from the Cs{sup +} ion at the sites of the single eight-membered ring (S8R), the single six-membered ring (S6R), and the {alpha}-cage sites near the four-membered rings. The experimental line shapes are satisfactorily simulated by the modified Bloch equations that include the chemical exchange kinetics between these three sites. The analysis yields a lifetime, {tau}, of 0.12 ms for the Cs{sup +} at S8R site at 297 K. The Arrhenius plot yields an activation energy of 44 kJ/mol for the site-exchange process of the charge-balancing Cs{sup +} ions. The exchange between the S6R and S8R sites is considered to be the elementary step of the Cs{sup +} diffusion process.

1H‐NMR studies of the interaction between a self‐complementary deoxyoligonucleotide duplex and indolo[2,3‐b]quinoxaline derivatives active against herpes virus

1H NMR has been used to study the interactions of ellipticine and the ellipticine analogues 2-3-dimethyl-6-(2-dimethylaminoethyl)6H-indolo-[2,3-b]quinoxaline and 6-(2-dimethylaminoethyl)6H-indolo-[2,3-b]quinoxaline with the self-complementary decadeoxyribonucleotide d(CGCGATCGCG)2. The Watson-Crick H-bonded imino proton resonances were studied. The drugs were shown to bind to the duplex by intercalation involving slow exchange kinetics for the imino proton resonances on the NMR time scale (500 MHz). Ellipticine and the 2,3-dimethyl analogue were found not to show strong base preferences, while the other analogue was found to have a preferred primary binding site between the A.T base pairs with a probable minor secondary binding site between the A.T and adjacent G.C base pairs. The new drug-shifted imino proton resonances were assigned through saturation transfer experiments. The base-specific interactions were accompanied by drug-induced non-uniform broadening of the resonances (due to intermediate chemical exchange kinetics), in the spectral region of the non-exchangeable aromatic and sugar H1' proton resonances of the oligonucleotide at 25 degrees C.

Dunitification of mantle lithosphere below the society archipelago: Evidence for magma-mantle thermal and chemical transfers through xenoliths from the Papenoo Valley, Tahiti

Chemical and thermal transfers between mantle lithosphere and O.I.B. from French Polynesia are described in an inclusion suite from the Papenoo Valley, Tahiti. The methods used consider chemical exchange kinetics in geothermometry and olivine deformation study. The mineralogical study shows the extent of the chemical and thermal exchanges between a lithosphere equilibrated at 1000–1050°C and plume basalts whose liquidus temperature is about 1250°C. The dunitification process along with the spinel and olivine “fertilization” is revealed in samples which preserved features of their former mantle deformation. The deformation study also shows that interaction took place within the mantle itself. The whole of the results implies a significant metasomatism of the mantle lithosphere and alteration of the initial geochemical signature of the deep originating O.I.B. beneath French Polynesia.

Application of two-dimensional NMR to kinetics of chemical exchange

Application of two-dimensional NMR to kinetics of chemical exchange

Studies of the chemical exchange kinetics of glycinate ions in mixed ligand complexes of copper (II) and glycylglycine

Nuclear magnetic resonance studies of the chemical exchange kinetics of glycinate ions in binary and ternary complexes of copper(II) and glycylglycine indicate the increase in the exchange rate of glycinate ions in the ternary complex. This phenomenon is assumed to be due to the glycinate ion coordination in the axial—equatorial position.