Mechanism Of Hydrogen Exchange Research Articles

Study of hydrogen exchange between the Zr–1%Nb alloy and the gaseous ambient through the oxidized surface under ion and atomic irradiation

In the article, a hydrogen exchange between the Zr–1%Nb alloy (E110) and the gas ambient was experimentally studied when the samples were irradiated with deuterium and argon plasma ions. It has been established that, upon irradiation with deuterium plasma ions with an energy of E = 650 eV/at, the enhanced absorption of deuterium exceeds the release of hydrogen initially contained in the samples, which leads to their loading with hydrogen isotopes. Adding 30 at.% oxygen to the plasma-forming gas or raising the sample temperature from T = 450 K to T = 600 K, significantly reduces the content of hydrogen isotopes in the sample. Based on the aggregate data obtained by atomic and ion irradiation, a mechanism of hydrogen exchange between zirconium alloy and gas ambient is proposed. The process includes three stages: reactions on the oxidized surface of the zirconium alloy (surface hydroxylation and formation of water molecules); reactions at the metal-oxide interface; transfer of hydrogen isotopes through the surface oxide layer in both directions due to hopping between neighboring oxygen ions. Surface reactions caused by irradiation of atoms and ions trigger the hydrogen exchange. The proposed model agrees with the experimental data on the irradiation of the E110 alloy with atoms and ions of hydrogen isotopes.

A RARE CASE MORTALITY OF DIABETIC KETOACIDOSIS- CEREBRAL EDEMA WITH TONSILLAR HERNIATION- A CONTRADICTORY OPINION.IS SODIUM BICARBONATE A REAL CULPRIT?

Diabetic ketoacidosis (DKA) is the most common complication seen in uncontrolled diabetes mellitus. DKA is most commonly seen with patients of type 1 diabetes. Depletion of Insulin leads to high blood sugars which in turn leads osmotic diuresis, production of ketone bodies i.e, βhydroxybutyric acid and acetoacetic acid, dysregulation of sodium hydrogen exchange mechanism[2]. As a consequence to the above stated mechanisms, cerebral edema has been documented as a fatal complication in DKA. Mortality documented due to cerebral edema is 21-25%[4].

A dehydrogenation mechanism through substitution of H by D in LiBH4–MgD2 mixture

In this work, a new substitution mechanism was proposed to describe the dehydrogenation of the hand-mixed LiBH4–MgD2 mixture. Above the melting point of LiBH4, the free D atoms originated from the decomposition of MgD2 would replace the H atoms of liquid LiBH4, leading to the formation of BD bond, and major release of H2 and HD other than D2 gas. The hydrogen substitution mechanism would play the role above the melting point of LiBH4. Similar hydrogen substitution effect was found in the LiBH4–VD2 mixture, not in the NaBH4–MgD2 mixture. Due to the substitution mechanism, the dehydrogenation kinetics of MgD2 in the LiBH4–MgD2 mixture is enhanced, which is contrary to the retarding effect due to hydrogen exchange mechanism previously proposed in the ball-milled LiBH4–MgD2 composite.

The Logic behind a Physical–Organic Chemist’s Research Topics

Although my research has no common theme or defining area, a coherence connects the diverse topics insofar as one project leads logically to another. Thus, studies on mechanisms of hydrogen exchange in amides and amidines led to the influence of hydrogen bonding and to NMR methods for chemical kinetics, including 2D-EXSY spectroscopy. Another connection was the OH--catalyzed NH exchange in amines that had supported the hypothesis of stereoelectronic control. We therefore analyzed that hypothesis critically, tested it, found counterexamples, and proposed an alternative hypothesis. We next addressed one-bond NMR coupling constants in ethers and the reverse anomeric effect. The latter studies required a highly accurate NMR titration method that we developed to measure the additional steric bulk resulting from protonation of a substituent. This method is also applicable to measuring secondary isotope effects on acidity, and we could demonstrate that they arise from n-σ* delocalization, not from an inductive effect. Other studies included kinetic isotope effects for both dissociation and H exchange of aqueous NH4+, for C-N rotation in amides, and for a hydride transfer. The role of hydrogen bonding led us to the rotation of NH4+ within its solvent cage and then to the symmetry of hydrogen bonds.

Comparison of osteoblast proliferation, differentiation and matrix mineralization between thermal-liftoff and enzymatic methods

This work represents the results of detailed studies on cobalt(II) extraction from chloride solutions by 1-(2-pyridyl)tridecan-1-one oxime. The obtained results show that extraction of cobalt(II) ions strongly depended on pH, contact time and concentration of extractant, cobalt(II) and chloride ions. The slope analysis pointed that the cobalt(II) ions’ coordination by 2-pyridinektoxime occurs through a hydrogen exchange mechanism, forming mainly with CoCl+ and Co(NO3)+ a binuclear complex or higher agglomerates. Furthermore, during the extraction cobalt ions’ are also oxidised to the trivalent state, which in the next stage can be stripped off at argon atmosphere using HCl or HCl with metal powder as reductant.

Functional microorganisms and enzymes from anaerobic digestion of plastics and plant waste material

This work represents the results of detailed studies on cobalt(II) extraction from chloride solutions by 1-(2-pyridyl)tridecan-1-one oxime. The obtained results show that extraction of cobalt(II) ions strongly depended on pH, contact time and concentration of extractant, cobalt(II) and chloride ions. The slope analysis pointed that the cobalt(II) ions’ coordination by 2-pyridinektoxime occurs through a hydrogen exchange mechanism, forming mainly with CoCl+ and Co(NO3)+ a binuclear complex or higher agglomerates. Furthermore, during the extraction cobalt ions’ are also oxidised to the trivalent state, which in the next stage can be stripped off at argon atmosphere using HCl or HCl with metal powder as reductant.

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This work represents the results of detailed studies on cobalt(II) extraction from chloride solutions by 1-(2-pyridyl)tridecan-1-one oxime. The obtained results show that extraction of cobalt(II) ions strongly depended on pH, contact time and concentration of extractant, cobalt(II) and chloride ions. The slope analysis pointed that the cobalt(II) ions’ coordination by 2-pyridinektoxime occurs through a hydrogen exchange mechanism, forming mainly with CoCl+ and Co(NO3)+ a binuclear complex or higher agglomerates. Furthermore, during the extraction cobalt ions’ are also oxidised to the trivalent state, which in the next stage can be stripped off at argon atmosphere using HCl or HCl with metal powder as reductant.

Equilibrium and mechanism of cobalt(II) extraction from chloride solution by hydrophobic 2-pyridineketoxime

This work represents the results of detailed studies on cobalt(II) extraction from chloride solutions by 1-(2-pyridyl)tridecan-1-one oxime. The obtained results show that extraction of cobalt(II) ions strongly depended on pH, contact time and concentration of extractant, cobalt(II) and chloride ions. The slope analysis pointed that the cobalt(II) ions’ coordination by 2-pyridinektoxime occurs through a hydrogen exchange mechanism, forming mainly with CoCl+ and Co(NO3)+ a binuclear complex or higher agglomerates. Furthermore, during the extraction cobalt ions’ are also oxidised to the trivalent state, which in the next stage can be stripped off at argon atmosphere using HCl or HCl with metal powder as reductant.

Enhanced dehydrogenation properties of LiBH4 compositing with hydrogenated magnesium-rare earth compounds

LiBH4 is regarded as a promising hydrogen storage material due to its high hydrogen density. In this study, the dehydrogenation properties of LiBH4 were remarkably enhanced by doping hydrogenated Mg3RE compounds (RE denotes La, Ce, Nd rare earth metals), which are composed of nanostructured MgH2 and REH2+x (denoted as H − Mg3RE). For the LiBH4 + H − Mg3La mixture, the component LiBH4 desorbed 6 wt.% hydrogen even at a relatively low temperature of 340 °C, far lower than the desorption temperature of pure LiBH4 or the 2LiBH4 + MgH2 system. This kinetic improvement is attributed to the hydrogen exchange mechanism between the H − Mg3La and LiBH4, in the sense that the decomposition of MgH2 and LaH2+x catalyzed the dehydrogenation of LiBH4 through hydrogen exchange effect rather than mutual chemical reaction requiring higher temperature and hydrogen pressure. However, prior to fast hydrogen release, the hydrogen exchange effect suppressed the dehydriding of MgH2 and elevated its desorption temperature. It is expected to strengthen the hydrogen exchange effect by compositing the LiBH4 with other nanosized metal hydrides and to obtain better dehydrogenation properties.

Destabilizing Mutations Alter the Hydrogen Exchange Mechanism in Ribonuclease A

The effect of strongly destabilizing mutations, I106A and V108G of Ribonuclease A (RNase A), on its structure and stability has been determined by NMR. The solution structures of these variants are essentially equivalent to RNase A. The exchange rates of the most protected amide protons in RNase A (35°C), the I106A variant (35°C), and the V108G variant (10°C) yield stability values of 9.9, 6.0, and 6.8 kcal/mol, respectively, when analyzed assuming an EX2 exchange mechanism. Thus, the destabilization induced by these mutations is propagated throughout the protein. Simulation of RNase A hydrogen exchange indicates that the most protected protons in RNase A and the V108G variant exchange via the EX2 regime, whereas those of I106A exchange through a mixed EX1 + EX2 process. It is striking that a single point mutation can alter the overall exchange mechanism. Thus, destabilizing mutations joins high temperatures, high pH and the presence of denaturating agents as a factor that induces EX1 exchange in proteins. The calculations also indicate a shift from the EX2 to the EX1 mechanism for less protected groups within the same protein. This should be borne in mind when interpreting exchange data as a measure of local stability in less protected regions.

In situ NMR spectroscopy in heterogeneous catalysis: Kinetic study of hydrocarbon conversion mechanisms

The potential of high-resolution solid-state NMR spectroscopy for kinetic and mechanistic studies of hydrocarbon conversion on solid acid catalysis between 20 and 300°C is considered. The use of this technique is illustrated by the elucidation of the mechanisms of hydrogen exchange and 13C label transfer in alkanes and olefins, n-butane isomerization on sulfated zirconia, and ethane aromatization on zinc-containing zeolite beta. The kinetic parameters determined in these studies provide a basis for quantum chemical calculations of possible hydrocarbon activation and conversion pathways and for evaluating the reliability and accuracy of these theoretical calculations.

Hydrogen exchange and ligand binding: Ligand‐dependent and ligand‐independent protection in the Src SH3 domain

Amide hydrogen-deuterium exchange has proven to be a powerful tool for detecting and characterizing high-energy conformations in protein ensembles. Since interactions with ligands can modulate these high-energy conformations, hydrogen exchange appears to be an ideal experimental probe of the physical mechanisms underlying processes like allosteric regulation. The chemical mechanism of hydrogen exchange, however, can complicate such studies. Here, we examine hydrogen exchange rates in a simple model system, the c-Src SH3 domain interacting with a short peptide ligand. Addition of ligand slows the rates of hydrogen exchange at nearly every amide for which we can obtain data. Careful analysis, however, reveals that this slowing is due primarily to a reduction in the population of free protein in the system, and not to any specific property of the complex. We present a method to separate the contributions of free and bound protein to the exchange kinetics that has allowed us to identify the subset of amides where exchange arises directly from the complex. These results demonstrate that the slowing of hydrogen exchange induced by ligand interactions should be interpreted with caution, and more extensive experiments are required to correlate changes in hydrogen exchange with changes in structure or internal dynamics.

Isotope Effects and Hydrogen Exchange Mechanism in Compounds Having Labile Aliphatic CÄH Bonds in Basic Liquid Ammonia Solutions

Specific features of the stepwise hydrogen exchange mechanism and transition state structure in the systems acetophenone-liquid ammonia in the absence of foreign compounds and in the presence of bases and toluene-liquid ammonia in the presence of potassium amide were studied in terms of an approach based primary and secondary kinetic isotope effects of the substrate and the solvent. The mechanisms of reactions involving acetophenone and toluene were compared. In the first case, an elementary act of CH-acid ionization is contributed to a small extent by diffusion-controlled separation of the carbanion and ammonia molecule. hydrogen exchange in toluene is characterized by complete absence of the internal ion pair return effect. The ratio kDNH3/kTNH3 for hydrogen exchange in acetophenone tends to decrease on addition of bases (with simultaneous increase in its rate), which may be explained by formation of an adduct via interaction between the unshared electron pair on the heteroatom in the base molecule and the carbonyl carbon atom. The anomalous temperature dependence of kDNH3/kTNH3 for hydrogen exchange in toluene is interpreted as a result of contribution of side metalation of the CAH bond by potassium amide. The change in the solvent protophilicity due to replacement of the "light" solvent by deuterated one differently affects the kinetics and mechanism of hydrogen exchange in acetophenone and toluene. Measurements of the α-deuterium effect gave information on the mode of angular deformation of CAD bonds in the methyl group of toluene in the hydrogen exchange transition state.

Protein hydrogen exchange mechanism: local fluctuations.

Experiments were done to study the dynamic structural motions that determine protein hydrogen exchange (HX) behavior. The replacement of a solvent-exposed lysine residue with glycine (Lys8Gly) in a helix of recombinant cytochrome c does not perturb the native structure, but it entropically potentiates main-chain flexibility and thus can promote local distortional motions and large-scale unfolding. The mutation accelerates amide hydrogen exchange of the mutated residue by about 50-fold, neighboring residues in the same helix by less, and residues elsewhere in the protein not at all, except for Leu98, which registers the change in global stability. The pattern of HX changes shows that the coupled structural distortions that dominate exchange can be several residues in extent, but they expose to exchange only one amide NH at a time. This "local fluctuation" mode of hydrogen exchange may be generally recognized by disparate near-neighbor rates and a low dependence on destabilants (denaturant, temperature, pressure). In contrast, concerted unfolding reactions expose multiple neighboring amide NHs with very similar computed protection factors, and they show marked destabilant sensitivity. In both modes, ionic hydrogen exchange catalysts attack from the bulk solvent without diffusing through the protein matrix.

Mechanisms of Hydrogen Exchange of Methane with H-Zeolite Y: An ab Initio Embedded Cluster Study

We present ab initio embedded cluster studies on the mechanism of hydrogen exchange of methane with H-Zeolite Y. We found that inclusion of the Madelung field stabilizes the formation of a carbonium-like transition state, and consequently reduces the reaction barrier by 17−23 kJ/mol, relative to the corresponding bare cluster predictions. Using the CCSD(T)/6-31G(d,p) level of theory, including zero-point energy (∼10 kJ/mol) and tunneling (1.6 kJ/mol) corrections, the activation energy is predicted to be 124 ± 5 and 137 ± 5 kJ/mol for hydrogen exchange from two different binding sites. These predictions agree well with the experimental estimate of 122−130 kJ/mol. We also found that it is necessary to include the Madelung potential to find preferrential proton siting at site O1 versus site O4, in agreement with experimental observation.

Mapping the lifetimes of local opening events in a native state protein.

The rate constants for the processes that lead to local opening and closing of the structures around hydrogen bonds in native proteins have been determined for most of the secondary structure hydrogen bonds in the four-helix protein acyl coenzyme A binding protein. In an analysis that combines these results with the energies of activation of the opening processes and the stability of the local structures, three groups of residues in the protein structure have been identified. In one group, the structures around the hydrogen bonds have frequent openings, every 600 to 1,500 s, and long lifetimes in the open state, around 1 s. In another group of local structures, the local opening is a very rare event that takes place only every 15 to 60 h. For these the lifetime in the open state is also around 1 s. The majority of local structures have lifetimes between 2,000 and 20,000 s and relatively short lifetimes of the open state in the range between 30 and 400 ms. Mapping of these groups of amides to the tertiary structure shows that the openings of the local structures are not cooperative at native conditions, and they rarely if ever lead to global unfolding. The results suggest a mechanism of hydrogen exchange by progressive local openings.

Elucidation of Coal Liquefaction Mechanism Using a Tritium Tracer Method. Effect of H2S and H2O on Hydrogen Exchange Reaction of Tetralin with Tritiated Molecular Hydrogen

To elucidate the effect of H2S and H2O on hydrogen exchange reactions between tetralin and gaseous hydrogen, the reactions of tetralin with tritiated hydrogen in the presence of H2S or H2O were performed under practical coal liquefaction conditions. The amounts of hydrogen exchange between tetralin and tritiated hydrogen were estimated from hydrogen and tritium balance. The conversions of tetralin in the presence and absence of H2S at 400 °C for 300 min were 4.1 and 3.9%, respectively, while the hydrogen exchange ratio (HER) of tetralin remarkably increased from 4.6% to 40.4% in the presence of H2S and reached 71% at 400 °C for 600 min. It was suggested that the hydrogen exchange mechanism of tetralin with hydrogen in the presence of H2S would include a different mechanism from that in the absence of H2S. It was considered that the H2S molecule acted as both an initiating species and an HS• radical and rapidly abstracted and added hydrogen atom reversibly, promoting the hydrogen exchange in both an aromatic ring and a naphthene ring. In addition to the radical mechanism, the electrophilic exchange with a proton formed from hydrogen sulfide was proposed for the hydrogen exchange mechanism. On the other hand, the conversion and HER of tetralin in the presence of H2O at 400 °C for 300 min were 1.7 and 1.3%, respectively. These values were smaller than those in the absence of H2O. It was considered that H2O inhibited the formation of tetralyl radicals and the hydrogen transfer from tritiated hydrogen to tetralyl radicals. The tritium distribution in H2S reached equilibrium at the initial stage of the reaction, and the hydrogen exchange reaction between gaseous hydrogen and H2S rapidly occurred. However, it was estimated that the hydrogen exchange reaction between gaseous hydrogen and H2O was slower than that with H2S.

New concepts of the mechanism of hydrogen exchange between organic molecules and strong acidic centers

Ab initio calculations of the activation energy of the reaction of hydrogen exchange between the methane molecule and the H3O+ ion in the gaseous phase have been carried out by using Hartree-Fock methods and Moeller-Plesset second order perturbation theory (MP2) methods. The structure of the transition state of this reaction has been found. The interaction of the H3O+ ion with molecules of aliphatic hydrocarbons and amino acids has been studied by the Serniernpirical AMI method.

A statistical mechanical model for hydrogen exchange in globular proteins.

We develop a statistical mechanical theory for the mechanism of hydrogen exchange in globular proteins. Using the HP lattice model, we explore how the solvent accessibilities of chain monomers vary as proteins fluctuate from their stable native conformations. The model explains why hydrogen exchange appears to involve two mechanisms under different conditions of protein stability: (1) a "global unfolding" mechanism by which all protons exchange at a similar rate, approaching that of the denatured protein, and (2) a "stable-state" mechanism by which protons exchange at rates that can differ by many orders of magnitude. There has been some controversy about the stable-state mechanism: does exchange take place inside the protein by solvent penetration, or outside the protein by the local unfolding of a subregion? The present model indicates that the stable-state mechanism of exchange occurs through an ensemble of conformations, some of which may bear very little resemblance to the native structure. Although most fluctuations are small-amplitude motions involving solvent penetration or local unfolding, other fluctuations (the conformational distant relatives) can involve much larger transient excursions to completely different chain folds.

Nature of the early folding intermediate of ribonuclease A.

A previous study of the folding pathway of the major unfolded species of ribonuclease A by pulsed hydrogen exchange [Udgaonkar, J. B., & Baldwin, R. L. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 8197-8201] showed that there is a major early folding intermediate (Il) that resembles a molten globule species in having stable secondary structure while lacking buried tyrosine side chains. Earlier work showed that there is also a late native-like folding intermediate (IN) that can bind the specific inhibitor 2'CMP and that has buried tyrosine side chains. Results are reported here indicating that Il has a well-developed tertiary structure even though its tyrosine side chains are not buried. First, optical stopped-flow experiments suggest that Il binds 2'CMP. Second, the protection against hydrogen exchange is similar in Il and IN for almost all protected amide protons studied. Third, analysis of the mechanism of hydrogen exchange in Il confirms the large protection factors reported earlier for probes in the beta-sheet of ribonuclease A and indicates that the beta-sheet is formed in Il. Other experiments are also reported that test the interpretation of pulsed hydrogen exchange studies of the folding pathway of ribonuclease A.