Chromatographic separation of neodymium isotopes by using chemical exchange process

2011 ◽  
Vol 1218 (20) ◽  
pp. 2923-2928 ◽  
Author(s):  
I.M. Ismail ◽  
M. Ibrahim ◽  
H.F. Aly ◽  
M. Nomura ◽  
Y. Fujii
Keyword(s):  
Chromatographic Separation ◽  
Chemical Exchange ◽  
Exchange Process ◽  
Neodymium Isotopes

scholarly journals Study of the coordination of quinuclidine to a chiral zinc phthalocyanine dimer

2016 ◽  
Vol 20 (08n11) ◽  
pp. 1224-1232 ◽  
Author(s):  
Nelson Giménez-Agulló ◽  
Gemma Aragay ◽  
José Ramón Galán-Mascarós ◽  
Pablo Ballester
Keyword(s):  
Chemical Exchange ◽  
Variable Temperature ◽  
Exchange Process ◽  
Zinc Phthalocyanine ◽  
Hydrogen Atoms ◽  
Dimerization Constant ◽  
H Nmr ◽  
Dimerization Process ◽  
Nmr Titrations ◽  
Millimolar Concentration

We attempted the calculation of an accurate equilibrium constant for the dimerization process of enantiomerically pure Zn-1 using UV-vis dilution experiments. At millimolar concentration Zn-1 is involved in a chemical exchange process between its monomeric and dimeric state that is slow on the 1H NMR timescale. We performed variable-temperature 1H NMR experiments in CDCl3 solution to determine the dimerization constant value at different temperatures and performed a van’t Hoff plot to derive the thermodynamic parameters of the process. The calculated thermodynamic data revealed that the dimerization process is entropy-driven and enthalpically opposed. We also probed the coordination of quinuclidine, 1-azabicyclo[2.2.2]octane, 2, to the Zn-1 using UV-vis and 1H NMR titrations in CDCl3 solution. At micromolar concentration the Zn-1 exclusively exists in solution as a monomer and forms a simple 1:1, [Formula: see text], complex with quinuclidine having a stability constant of [Formula: see text]([Formula: see text]) [Formula: see text] 106 M[Formula: see text]. On the other hand, the 1H NMR titrations carried out at 298 K and at millimolar concentration showed that Zn-1 was present in solution as the dimer and formed 1:2, [Formula: see text], and 2:2, [Formula: see text] complexes by coordination to 2. In addition, the 1:1 complex, [Formula: see text] showed a reduced dimerization constant compared to the uncoordinated parent monomer Zn-1. At high quinuclidine concentration, the 1:1 complex, [Formula: see text], derived from the coordinated dimer dissociation was also detected. The 1H NMR spectra of the titrations displayed separate signals for some hydrogen atoms of the Zn-phthalocyanine in each one of the four species. Remarkably, the chemical exchange processes involving free and bound quinuclidine in the monomeric and dimeric complexes showed different kinetics on the NMR timescale.

The A39G FF domain folds on a volcano-shaped free energy surface via separate pathways

2021 ◽  
Vol 118 (46) ◽  
pp. e2115113118
Author(s):  
Ved P. Tiwari ◽  
Yuki Toyama ◽  
Debajyoti De ◽  
Lewis E. Kay ◽  
Pramodh Vallurupalli
Keyword(s):  
Free Energy ◽  
Chemical Exchange ◽  
Exchange Process ◽  
Conformational Dynamics ◽  
Chemical Exchange Saturation Transfer ◽  
Folding Kinetics ◽  
Free Energy Surface ◽  
Wide Range ◽  
Order Of Magnitude ◽  
Energy Surfaces

Conformational dynamics play critical roles in protein folding, misfolding, function, misfunction, and aggregation. While detecting and studying the different conformational states populated by protein molecules on their free energy surfaces (FESs) remain a challenge, NMR spectroscopy has emerged as an invaluable experimental tool to explore the FES of a protein, as conformational dynamics can be probed at atomic resolution over a wide range of timescales. Here, we use chemical exchange saturation transfer (CEST) to detect “invisible” minor states on the energy landscape of the A39G mutant FF domain that exhibited “two-state” folding kinetics in traditional experiments. Although CEST has mostly been limited to studies of processes with rates between ∼5 to 300 s−1 involving sparse states with populations as low as ∼1%, we show that the line broadening that is often associated with minor state dips in CEST profiles can be exploited to inform on additional conformers, with lifetimes an order of magnitude shorter and populations close to 10-fold smaller than what typically is characterized. Our analysis of CEST profiles that exploits the minor state linewidths of the 71-residue A39G FF domain establishes a folding mechanism that can be described in terms of a four-state exchange process between interconverting states spanning over two orders of magnitude in timescale from ∼100 to ∼15,000 μs. A similar folding scheme is established for the wild-type domain as well. The study shows that the folding of this small domain proceeds through a pair of sparse, partially structured intermediates via two discrete pathways on a volcano-shaped FES.

Modeling a Chemical Exchange Process for the 13C Isotope Enrichment

2015 ◽  
Vol 772 ◽  
pp. 27-32 ◽  
Author(s):  
Zoltan Kovendi ◽  
Vlad Mureşan ◽  
Mihail Abrudean ◽  
Iulia Clitan ◽  
Mihaela Ligia Ungureşan ◽  
...  
Keyword(s):  
Carbon Dioxide ◽  
Process Modeling ◽  
Process Simulation ◽  
Control Structure ◽  
Chemical Exchange ◽  
Exchange Process ◽  
Open Loop ◽  
Isotope Enrichment ◽  
Isotope Concentration ◽  
Made In

This paper presents a solution for modeling a chemical exchange process carbon dioxide (CO2) – carbamate for the 13C isotope enrichment. A big difficulty in the process modeling procedure is the fact that it is a non-linear one. In order to solve this problem, an original modeling solution that permits the process simulation for the entire domain of the values of the input signal is used. The process modeling is made in order to include it in an automatic control structure of the 13C isotope concentration. Some relevant simulations of the open loop process are presented, both in the case when the disturbances do not occur and the case when they occur in the system.

Optimization of primary enrichment section of mono-thermal ammonia–hydrogen chemical exchange process

2008 ◽  
Vol 142 (3) ◽  
pp. 285-300 ◽  
Author(s):  
M.R. Sawant ◽  
K.V. Patwardhan ◽  
A.W. Patwardhan ◽  
V.G. Gaikar ◽  
M. Bhaskaran
Keyword(s):  
Chemical Exchange ◽  
Exchange Process

Nuclear magnetic resonance studies of ketone.BF3 complexes. I. Exchange reactions in methylene chloride solution

1968 ◽  
Vol 46 (12) ◽  
pp. 2147-2157 ◽  
Author(s):  
R. J. Gillespie ◽  
J. S. Hartman
Keyword(s):  
Chloride Solution ◽  
Methylene Chloride ◽  
Chemical Exchange ◽  
Exchange Process ◽  
Exchange Reactions ◽  
Methylene Chloride Solution ◽  
Donor Acceptor ◽  
Group 3 ◽  
Proton Resonances ◽  
Increasing Temperature

1H and 19F n.m.r. studies of BF3 adducts of some simple methyl ketones in methylene chloride solution have led to the following conclusions. (1) Only 1:1 adducts are formed which are not appreciably dissociated. (2) The down-field shifts of the proton resonances caused by complexation with BF3 are essentially independent of the ketone and depend only on the distance of the proton from the carbonyl group. (3) BF3 exchange is rapid on the n.m.r. time scale at room temperature, but the exchange process can be slowed sufficiently by lowering the temperature that separate signals due to free and complexed species can be observed. (4) Collapse of the 10B−11B isotope shift with increasing temperature showed that a second chemical exchange process, which exchanges fluorine among boron atoms, occurs in addition to the process of rapid breaking and re-forming of donor–acceptor bonds. A possible mechanism for this fluorine scrambling reaction is discussed.

THE DESCRIPTION AND USE OF A SPIN-ECHO SPECTROMETER TO STUDY CHEMICAL EXCHANGE

1966 ◽  
Vol 44 (14) ◽  
pp. 1685-1698 ◽  
Author(s):  
K. H. Abramson ◽  
P. T. Inglefield ◽  
E. Krakower ◽  
L. W. Reeves
Keyword(s):  
High Resolution ◽  
Chemical Shift ◽  
Internal Rotation ◽  
Chemical Exchange ◽  
Spin Echo ◽  
Exchange Process ◽  
Approximate Expression ◽  
Closed Expression ◽  
Selective Deuteration ◽  
Resolution Study

A spin-echo spectrometer, suitable for studies of chemical exchange, is described. The performance of the apparatus is described and illustrated. From a Carr–Purcell sequence of pulses T2 measurements have been made using two alternative schemes for the correction of cumulative error in the width of π pulses. Agreement is satisfactory and the standard errors in each method are ± 5% of the average T2. The Meiboom and Gill method of phase-shifting the r.f. in the first pulse has the advantage of more flexibility in the choice of pulse intervals.A theory developed by Bloom, Reeves, and Wells for the dependence of T2 measured in a Carr–Purcell experiment on pulse intervals in the presence of a suitable exchange process has been tested experimentally for two molecules. The rate constants for the hindered internal rotation about the N=N bond in N,N -dimethylnitrosamine agree for the following sets of data: (a) a previous high-resolution study by Phillips, (b) a repeated high-resolution study made in this work, (c) spin-echo studies using the complete closed expression for a two site exchange process, and (d) the approximate expression derived for the limit of fast pulse repetition rates.A similar study by the spin-echo method of the rates of internal rotation about the C=N bond in N,N-dimethyltrifluoroacetamide does not agree with a previous high-resolution study. Since the chemical shift between the inequivalent methyl groups is so small, it is suggested that the rates of rotation in this molecule are much better evaluated by the spin-echo method.The spin-echo method can more easily take account of variations with temperature of the natural relaxation time and the chemical shift. The non-selective nature of an experiment with such a large H1 field remains a disadvantage at this time. The selective deuteration of sites unaffected by exchange is in general difficult enough to limit the scope of the spin-echo method when protons are observed.

Intermolecular exchange in a triarylborane-phosphine complex: a multinuclear magnetic resonance study

1994 ◽  
Vol 348 (1687) ◽  
pp. 315-322 ◽  
Keyword(s):  
Rate Coefficient ◽  
Chemical Exchange ◽  
Exchange Process ◽  
Nuclear Spin Relaxation ◽  
Magnetic Resonance Study ◽  
Temperature Dependent ◽  
A Value ◽  
Selective Inversion ◽  
Phosphine Complex ◽  
Dissociation Step

It has been shown using 1 H, 19 F, and 31 P NMR spectroscopy that the tris -(pentafluorophenyl) borane-phosphine adduct undergoes intermolecular exchange in toluene solution involving transfer of the phosphine group. The rate coefficient (k) for the exchange process was measured using the 31 P NMR selective inversion experiment on a series of samples containing excess phosphine. At 254 K, k is 3.60 ± 0.15 s -1 and is independent of the excess phosphine concentration. The mechanism for the exchange process is proposed to be two step, a slow initial dissociation of the adduct followed by a rapid recombination step. The rate coefficient is strongly temperature dependent in the range 243-263 K and a value of 126 kJ mol -1 is obtained for the activation energy of the dissociation step. The appearance of the 1 H two-dimensional exchange spectrum (NOESY) is described in terms of the competing effects of nuclear spin relaxation and chemical exchange.

scholarly journals Research on chemical exchange process of boron isotope separation

2011 ◽  
Vol 18 ◽  
pp. 151-156 ◽  
Author(s):  
Yiping Huang ◽  
Shuang Cheng ◽  
Jiao Xu ◽  
Weijiang Zhang
Keyword(s):  
Chemical Exchange ◽  
Isotope Separation ◽  
Exchange Process ◽  
Boron Isotope
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