13C and 77Se NMR spectra of thienoisoselenazoles,selenoloisothiazoles, selenoloisoselenazoles and thienoisothiazoles

Measurements are reported of NH exchange rates of urea, thiourea, acetamide and thioacetamide, using line-shape analysis with natural-abundance ¹⁵N NMR spectra. Base-catalyzed NH-proton exchange of thiourea is 30–40 times faster than for urea, while the corresponding acid-catalyzed exchange is 150 times slower. The base-catalyzed proton exchange for thioacetamide is about 1000 times faster than that of acetamide at 25°C, while the corresponding acid-catalyzed exchange is some 260 times slower.     

An NMR study of hydrogen exchange in p-hydroxybenzoic acid

The compound has been used over a temperature range in dimethylsulfoxide, with trichloroacetic acid and triethylamine as catalysts. The rate constant for uncatalyzed bimolecular exchange at 25 C is k_N=131 M^–1 sec^–1; the constant for acid-catalyzed exchange is k_A=0.9 · 10⁴ M^–1 sec^–1; and that for base-catalyzed exchange is k_B=0.5 · 10⁴ M^–1 sec^–1. The activation energy for uncatalyzed exchange is 5.75 kcal/mole. The exchange rates in dimethylformamide and acetone are higher, on account of differences in the hydrogen bond. The exchange rates of the isomers fall in the sequence ortho > para > meta, which is due to conjugation and intramolecular hydrogen bonding. The exchange mechanism is discussed. Simple relationships in dimensionless parameters are given for calculating the exchange rate from the shape of the NMR signal.     

The kinetics of hydrogen-deuterium exchange of methylisothiazoles

The rates of hydrogen-deuterium exchange of 3-methyl, 4-methyl, 5-methyI and 3,5-dimethyl-isothiazole were determined and compared with the rate of exchange of 2-methylpyridine. The rates were measured in dimethylsulfoxide-d₆ -deuterium oxide (90:10), using sodium deuteroxide as catalyst. The relative rates for exchange of both ring protons and methyl protons were 5 > 3 > 4. For exchange of methyl hydrogens, the approximate ratio of rate constants is 100:1:< 10^?4 for positions 5,3, and 4, respectively. Activation parameters are reported for 5-methylisothiazole.     

PMR spectroscopic study of the kinetics of H/D exchange in methyl groups in a series of heterocyclic azines

The rate of H/D exchange among methyl group protons in a series of substituted 3-hydroxypyridines, 5-hydroxypyrimidines, and their N-oxides has been shown to increase with increasing acidity of the medium. The most reactive form of these molecules is the cationic form at pH<2. The rate of H/D exchange of CH₃ group protons in 3-hydroxypyridine derivatives has also been found to be several orders of magnitude lower than the rates of exchange for methyl-substituted 5-hydroxypyrimidine and its N-oxide. Effective rate constants for methyl group proton exchange have been estimated. In the case of methyl-substituted 5-hydroxypyrimidine N-oxide derivatives it has been established that the rate of proton exchange is greater for an ortho-methyl group than for a methyl group in the para-position relative to the N-oxide site.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 93–96, January, 1991.     

Kinetics of isotopic exchange between chloramine-B and radioactive chloride

The kinetics of chlorine isotope exchange between chloramine-B /CAB/ and chloride has been studied using ion-exchange separation and tracer technique. McKay's plot are linear. The exchange reaction is fast in acidic medium, very slow in neutral medium and does not take place in alkaline medium. In the acidic range the exchange is maximum at pH 3.3. The rate of exchange decreases at pH >3.3 and <3.3. The order with respect to CAB and chloride is unity. The order with respect to [H⁺] is unity at pH>5. Addition of neutral salt or parent compound has no effect on the rate of exchange. Activation energy and activation entropy for this exchange reaction have been calculated.     

Anion Exchange-Adsorption on Low Capacity Anion Exchangers: Separation of Organic Acids,Amino Acids,Small Chain Peptides

Abstract

The variables that influence the retention of organic analyte anions on a macroporous, high surface area polystyrenedivinyl-benzene copolymer that is chemically modified by attaching tetraalkylammonium groups to the copolymer surface are identified and studied as a function of anion exchange capacity. A combined adsorption-anion exchange retention of the organic analyte anion is possible providing the analyte has both a hydrophophic center and an anionic charge site. As the column anion exchange capacity (0 to 173 μeq of anion exchange sites/column was studied) increases, analyte retention due to adsorption decreases and retention due to anion exchange increases. The factors influencing organic analyte anion retention by adsorption are low anion exchange capacity and mobile phase solvent composition, type of organic modifier, and pH for analytes that are weak organic acids. For anion exchange the major factors are a high anion exchange     

Determination of iodine in organic iodine compounds by the non-equilibrium isotopic exchange method

A new type of the isotope exchange method of analysis is proposed. The method can be applied to the exchange system in which the rate of the exchange reaction is rather slow but measurable, before the exchange equilibrium is attained. The validity of the principle of the method is verified experimentally with several exchange systems of RyI+KI^*⇌RyI^*+KI (RyI stands for alkyl, alkylene or benzyl iodide) type. The iodine content of the organic iodine compounds can be determined with an error of ±4%.     

Quantitative Assessment of Protein Structural Models by Comparison of H/D Exchange MS Data with Exchange Behavior Accurately Predicted by DXCOREX

Peptide amide hydrogen/deuterium exchange mass spectrometry (DXMS) data are often used to qualitatively support models for protein structure. We have developed and validated a method (DXCOREX) by which exchange data can be used to quantitatively assess the accuracy of three-dimensional (3-D) models of protein structure. The method utilizes the COREX algorithm to predict a protein’s amide hydrogen exchange rates by reference to a hypothesized structure, and these values are used to generate a virtual data set (deuteron incorporation per peptide) that can be quantitatively compared with the deuteration level of the peptide probes measured by hydrogen exchange experimentation. The accuracy of DXCOREX was established in studies performed with 13 proteins for which both high-resolution structures and experimental data were available. The DXCOREX-calculated and experimental data for each protein was highly correlated. We then employed correlation analysis of DXCOREX-calculated versus DXMS experimental data to assess the accuracy of a recently proposed structural model for the catalytic domain of a Ca²⁺-independent phospholipase A₂. The model’s calculated exchange behavior was highly correlated with the experimental exchange results available for the protein, supporting the accuracy of the proposed model. This method of analysis will substantially increase the precision with which experimental hydrogen exchange data can help decipher challenging questions regarding protein structure and dynamics.     

Application of Br-82 as a radioactive tracer isotope to study bromide ion-isotopic exchange reaction in strongly basic anion exchange resin Duolite A-161

In the present investigation Br-82 radioactive isotope was used as a tracer to study the kinetics of exchange reaction taking place between the ion exchange resin and the external bromide ion solution. In an attempt to study the reversible bromide ion-isotopic exchange reaction kinetics, it was expected that whether the initial step was the exchange of radioactive bromide ions from the solution on to the ion exchange resin (forward reaction), or from the ion exchange resin in to the solution (reverse reaction), the two ion-isotopic exchange reactions should occur simultaneously, which was further confirmed from the values of specific reaction rate 0.122 and 0.123 min⁻¹ respectively obtained under identical experimental conditions. The radioactive tracer technique employed in the present investigation will be useful for characterization of various industrial grade ion exchange resins.     

The heats of exchange of transition metal ions on the Na form of clinoptilolite

Direct calorimetric measurements were used to determine the heats of exchange of the Mn²⁺, Co²⁺, Cu²⁺, and Ni²⁺ cations on the Na form of clinoptilolite over the entire range of solid phase fillings with sorbed cations. In parallel, ion exchange isotherms for the systems were measured by the sorption-analytic method. The integral free energies and entropies of ion exchange were calculated. It was shown that the solution phase of the clinoptilolite-electrolyte solution two-phase system contributed significantly to the total thermodynamic characteristics of ion exchange. The differentiation of the dependence of the integral enthalpy on the degree of filling was performed to show that the clinoptilolite structure contained at least two types of exchange sites having different interaction energies with transition metal ions.     

Development of the spin crossover model for compressible and nonuniform chains of exchange clusters

A model of a compressible chain of exchange clusters, which retains the cooperative behavior during thermal crossover, was developed. The model was generalized for periodical chains with two and more exchange clusters per unit cell. A general expression was obtained for the effective magnetic moment of the unit cell with several exchange clusters. A two-position approximation for the partition function of the Jahn-Teller paramagnetic center Cu^II (spin 1/2) was proposed for chains with the “head-head” motif, which makes it possible to derive the equation for the effective magnetic moment in quadratures. The model developed describes the cases of both smooth and sharp (cooperative) spin crossover for chains of three-spin exchange clusters.     

The gramicidin dimer shows both EX1 and EX2 mechanisms of H/D exchange

We describe the use of H/D amide exchange and electrospray ionization mass spectrometry to study, in organic solvents, the pentadecapeptide gramicidin as a model for protein self association. In methanol-OD, all active H’s in the peptide exchange for D within 5 min, indicating a monomer/dimer equilibrium that is shifted towards the fast-exchanging monomer. H/D exchange in n-propanol-OD, however, showed a partially protected gramicidin that slowly converts to a second species that exchanges nearly all the active hydrogens, indicating EX1 kinetics for the H/D exchange. We propose that this behavior is the result of the slower rate of unfolding in n-propanol compared with that in methanol. The rate constant for the unfolding of the dimer is the rate of disappearance of the partially protected species, and it agrees within a factor of two with a value reported in literature. The rate constant of dimer refolding can be determined from the ratio of the rate constant for unfolding and the affinity constant for the dimer, which we determined in an earlier study. The unfolding activation energy is 20 kcal mol⁻¹, determined by performing the exchange experiments as a function of temperature. To study gramicidin in an even more hydrophobic medium than n-propanol, we measured its H/D exchange kinetics in a phospholipids vesicle and found a different H/D amide exchange behavior. Gramicidin is an unusual peptide dimer that can exhibit both EX1 and EX2 mechanisms for its H/D exchange, depending on the solvent.     

Mass transport in a boundary layer and in an ion exchange membrane: Mechanism of concentration polarization and water dissociation

In an unforced flowing NaCl solution in bulk, gravitational or electro convection supplies ions from bulk toward the membrane surface through a boundary layer. In a boundary layer formed on an anion exchange membrane, the convection converts to migration and diffusion and carries an electric current. In a boundary layer formed on a cation exchange membrane, the convection converts to migration and carry an electric current. In a forced flowing solution in bulk, the boundary layer thickness is reduced and gravitation or electro convection is disappeared. An electric current is carried by diffusion and migration on the anion exchange membrane and by migration on the cation exchange membrane. Ion transport in a boundary layer on the cation exchange membrane immersed in a NaCl solution is more restricted comparing to the phenomenon on the anion exchange membrane. This is due to lower counter-ion mobility in the boundary layer and the restricted water dissociation reaction in the membrane. The water dissociation reaction is generated in an ion exchange membrane and promoted due to the increased forward reaction rate constant. However, the current efficiency for the water dissociation reaction is generally low. The intensity of the water dissociation is more suppressed in the strong acid cation exchange membrane comparing to the phenomenon in the strong base anion exchange membrane due to lower forward reaction rate constant in the cation exchange membrane. In the strong acid cation exchange membrane, the intensity of electric potential is larger than the values in the strong base anion exchange membrane. Accordingly, the stronger repulsive force is developed between ion exchange groups (SO ₃ ^? groups) and co-ions (OH^? ions) in the cation exchange membrane, and the water dissociation reaction is suppressed. In the strong base anion exchange membrane, the repulsive force between ion exchange groups (N⁺(CH₃)₃ groups) and co-ions (H⁺ ions) is relatively low, and the water dissociation reaction is not suppressed. Violent water dissociation is generated in metallic hydroxides precipitated on the desalting surface of the cation exchange membrane. This phenomenon is caused by a catalytic effect of metallic hydroxides. Such violent water dissociation does not occur on the anion exchange membrane.     

Influence of non-bridging donor atoms on the value of exchange interaction in binuclear Cu<Superscript>II</Superscript> complexes with bis-hydrazones based on 2,6-diformylphenol

The quantum chemical simulation of exchange interaction according to the broken symmetry approach was performed for a series of binuclear Cu^II complexes containing the asymmetric exchange moiety based on bis-hydrazones of 2,6-diformylphenol. The calculated values were compared between themselves and with the experimental values of the exchange parameter 2J for the binuclear copper(II) complexes with bis-hydrazones based on 2,6-diformylphenol in order to reveal the influence pattern of the nature of nonbridging donor atoms X (O, N, S, and Se) onto the exchange interaction. The strength of exchange interaction of the antiferromagnetic type depends significantly on the nature of nonbridging donor atoms, increasing in the series of X = O, NH, S, and Se.     

Site specificity in the H–D exchange reactions of gas-phase protonated amino acids with CH3OD

H–D exchange reactions of methanol-d₁ with protonated amino acids were performed in an external-source Fourier transform mass spectrometer. Absolute rate constants were determined for the group which included glycine, alanine, valine, leucine, isoleucine and proline. By comparing reactivities with selected methyl esters, it was found that exchange on the carboxylic acid occurs 3–10 times faster than exchange on the amino group. No simple correlation is observed between the rates of H–D exchange on the acid group and the size of the alkyl group. However, the rates of exchange on the amine decrease with increasing gas-phase basicity. Glycine, the least basic amino acid, exchanges its amine hydrogens the fastest. These results are useful for determining the interaction of methanol with protonated amino acids and can provide insight into the H–D exchange reactions observed with polyprotonated proteins produced by electrospray ionization.     

Metal exchange reaction of cadmium 5-monoaza-2,3,7,8,12,13,17,18-octamethylporphyrinate with zinc(II) and copper(II) chlorides in dimethyl sulfoxide

The metal exchange reactions of cadmium 5-monoaza-2,3,7,8,12,13,17,18-octamethylporphyrinate with CuCl₂ and ZnCl₂ in dimethyl sulfoxide (DMSO) were studied by spectrophotometry. The kinetic and energy parameters of the metal exchange reaction were determined. The possible stoichiometric mechanism of the metal exchange reaction was proposed. A comparative analysis of the reaction rate constants obtained for the metal exchange and for the complex formation of 5-monoaza-2,3,7,8,12,13,17,18-octamethylporphyrin with the same salts in DMSO was performed.     

Measurement of Very Fast Exchange Rates of Individual Amide Protons in Proteins by NMR Spectroscopy

NMR spectroscopy is a pivotal technique to measure hydrogen exchange rates in proteins. However, currently available NMR methods to measure backbone exchange are limited to rates of up to a few per second. To raise this limit, we have developed an approach that is capable of measuring proton exchange rates up to approximately 10⁴ s⁻¹. Our method relies on the detection of signal loss due to the decorrelation of antiphase operators 2N_xH_z by exchange events that occur during a series of pi pulses on the ¹⁵N channel. In practice, signal attenuation was monitored in a series of 2D H(CACO)N spectra, recorded with varying pi-pulse spacing, and the exchange rate was obtained by numerical fitting to the evolution of the density matrix. The method was applied to the small calcium-binding protein Calbindin D_9k, where exchange rates up to 600 s⁻¹ were measured for amides, where no signal was detectable in ¹⁵N−¹H HSQC spectra. A temperature variation study allowed us to determine apparent activation energies in the range 47–69 kJ mol⁻¹ for these fast exchanging amide protons, consistent with hydroxide-catalyzed exchange.     

Minimizing 18O/16O back‐exchange in the relative quantification of ribonucleic acids

The use of isotopically labeled endonuclease digestion products allows for the relative quantification of ribonucleic acids (RNAs). This approach utilizes ribonucleases such as RNase T1 to mediate the incorporation of ¹⁸O onto the 3′‐terminus of the endonuclease digestion product from a solution containing heavy water (H₂¹⁸O). The accuracy and precision of relative quantification are dependent on the efficiency of isotope incorporation and minimizing any possible ¹⁸O to ¹⁶O back‐exchange before or during mass spectral analysis. Here, we have investigated the stability of ¹⁸O‐labeled endonuclease digestion products to back‐exchange. In particular, the effects of pH, temperature and presence of RNase on the back‐exchange process were examined using matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS). We have found that back‐exchange depends on the presence of the RNase—back‐exchange was not observed once the enzyme was removed from the sample. With RNase present, at all pH values examined (from acidic to basic pH), back‐exchange was detected at incubation above room temperature. The rates and extent of back‐exchange were similar at all pH values. In contrast, back‐exchange in the presence of RNase was found to be especially sensitive to incubation temperature—at temperatures below room temperature, minimal back‐exchange was detected. However, back‐exchange increased as the incubation temperature increased. Based on these findings, appropriate sample‐handling and sample storage conditions for isotopically labeled endonuclease digestion products have been identified, and these conditions should improve the accuracy and precision of results from the relative quantification of RNAs obtained by this approach. Copyright © 2009 John Wiley & Sons, Ltd.     

NMR Study of the Ligand Interchange of Hexacyclen and Hexamethylhexacyclen Complexes with TI+ Ion in 70% Methanol Solution

The exchange kinetics of TI⁺ complexes with hexacyclen and hexamethylhexacyclen were studied in 70% methanol solution by proton NMR line-shape analysis. In TI⁺-hexacyclen system, a dissociative mechanism is found to be predominant at higher temperatures, while a bimolecular pathway may become more important at lower temperatures. In contrast, the TI⁺-hexamethylhexacyclen system solely prefers a dissociative exchange mechanism in entire temperature range studied. The corresponding exchange rates and the activation parameters E_a, ΔH^?, ΔS^? and ΔG^? for the exchange have been determined, and the possible parameters controlling the exchange rates and mechanisms are discussed.     

Understanding and Practical Use of Ligand and Metal Exchange Reactions in Thiolate‐Protected Metal Clusters to Synthesize Controlled Metal Clusters

It is now possible to accurately synthesize thiolate (SR)‐protected gold clusters (Au_n(SR)_m) with various chemical compositions with atomic precision. The geometric structure, electronic structure, physical properties, and functions of these clusters are well known. In contrast, the ligand or metal atom exchange reactions between these clusters and other substances have not been studied extensively until recently, even though these phenomena were observed during early studies. Understanding the mechanisms of these reactions could allow desired functional metal clusters to be produced via exchange reactions. Therefore, we have studied the exchange reactions between Au_n(SR)_m and analogous clusters and other substances for the past four years. The results have enabled us to gain deep understanding of ligand exchange with respect to preferential exchange sites, acceleration means, effect on electronic structure, and intercluster exchange. We have also synthesized several new metal clusters using ligand and metal exchange reactions. In this account, we summarize our research on ligand and metal exchange reactions.