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Chemical Exchange (chemical + exchange)
Selected AbstractsNMR diffusion measurements under chemical exchange between sites involving a large chemical shift differenceCONCEPTS IN MAGNETIC RESONANCE, Issue 2 2010S. Leclerc Abstract This study concerns the thallium-205 cation in aqueous solution in the presence of a calixarene molecule. Although the measurement of the self-diffusion coefficient of pure thallium (without calixarene in the aqueous solution) does not pose any particular problem, major difficulties are encountered with the standard method using gradient strength increment as soon as thallium is partly complexed by calixarene. With static magnetic field gradients, the NMR signal is so weak that it prevents any reliable measurement, whereas radio frequency (rf) field gradients lead to an unrealistic value of the diffusion coefficient. This failure is explained by the fact that thallium is in fast exchange between two sites (complexed and free thallium) thus exhibiting a single NMR signal although, in the course of the experiment, two signals, with an important difference in resonance frequencies (due to the large thallium chemical shift range), are effectively involved. With the objective to understand these quite unexpected observations, the theory underlying NMR diffusion experiments is first reviewed, and criteria of fast exchange are discussed for three parameters: chemical shifts, relaxation rates, and diffusion coefficients. It turns out that off-resonance effects are responsible for unwanted defocusing due to rf pulses in the static magnetic field gradient method and for time-dependent gradients in the rf field gradient method. Concerning the latter, a remedy is proposed which consists in applying the stronger gradient and incrementing the gradient pulse durations. After correction for relaxation, the expected value of the diffusion coefficient is retrieved. © 2010 Wiley Periodicals, Inc. Concepts Magn Reson Part A 36A: 127,137, 2010. [source] NMR in photo-induced chemical exchange systems.CONCEPTS IN MAGNETIC RESONANCE, Issue 5 2006-dipyridyl)ethene photoisomerization kinetics, Double-resonance inverse fractional population transfer application for investigation of Abstract The analytical expression describing the dynamics of nuclear magnetization with the use of double-resonance NMR by the procedure of inverse fractional population transfer in requirements of photo-induced chemical exchange (PICE) is obtained in this article. Laser-induced (, = 308 nm) photoisomerization of 1,2-(2,2,-dipyridyl)ethene was studied by NMR and UV absorption spectroscopy under irreversible and photostationary conditions. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 337,346, 2006 [source] NMR in photoinduced chemical exchange systems: Theoretical basis of double-resonance inverse fractional population transferCONCEPTS IN MAGNETIC RESONANCE, Issue 4 2006S.P. Babailov Abstract The analytical expression describing dynamics of the nuclear magnetization with the use of double-resonance NMR in the transfer of inverse fractional population under photoinduced chemical exchange is obtained. An experimental procedure for the definition of effective rate constants and quantum yield of responses is considered. The technical approach created allows for planning experimental strategies for the study of photoinduced chemical exchange in any solvable substances. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 299,305, 2006 [source] Reconstitution of Photosystem II Reaction Center with Cu-Chlorophyll aJOURNAL OF INTEGRATIVE PLANT BIOLOGY, Issue 11 2006Shuang Liu Abstract An isolated photosystem (PS) II reaction center (RC) with altered pigment content was obtained by chemical exchange of native chlorophyll a (Chl) with externally added Cu-Chl a (Cu-Chl). Pigment composition and spectroscopic properties of the RC exchanged with Cu-Chl were compared with native RC and RC treated with Chl in the same way. High-performance liquid chromatography analysis showed approximately 0.5 Cu-Chl per two pheophytin in the Cu-Chl-reconstituted RC preparation. Insertion of Cu-Chl resulted in a decrease in absorption at 670 nm and an increase at 660 nm, suggesting that the peripheral Chl may have been displaced. Fluorescence emission spectra of the Cu-Chl-reconstituted RC displayed a marked decrease in fluorescence yield and a blue shift of the band maximum, accompanied by the appearance of a broad peak at a shorter wavelength, indicating that energy transfer in the modified RC was disturbed by Cu-Chl, a quencher of the excited state. However, there were few differences in the circular dichroism (CD) spectra, suggesting that the arrangement of pigments and proteins responsible for the CD signal was not significantly affected. In addition, no obvious change in peptide components was found after the exchange procedure. (Managing editor: Ping He) [source] Cooperative interaction of n -butylammonium ion with 1,3-alternate tetrapropoxycalix [4]arene: NMR and theoretical studyMAGNETIC RESONANCE IN CHEMISTRY, Issue 5 2008Jaroslav K Abstract The interaction of 1,3-alternate tetrapropoxycalix[4]arene (1) with n -butylammonium ion (2) in CD2Cl2 was examined using 1H, 13C and 14N NMR spectroscopy and DFT (density functional theory) calculations. NMR shows that 1 forms with 2 an equimolecular hydrogen-bonded complex with the equilibrium constant 5.91 × 103 l/mol at 296 K. The structure of the complex can be shown to be asymmetric at 203 K, with 2 interacting by hydrogen bonds with the two ethereal oxygen atoms of one half of 1 and with the , system of the other half, but is rapidly averaged to an apparent C4h symmetry by chemical exchange at higher temperatures. Using two related but independent techniques based on transverse and rotating-frame proton relaxation, it is shown that only an intermolecular exchange of 2 between the bound and free states takes place, in contrast to previously studied interaction of 1 with H3O+. Its correlation time is 0.169 ms. It is shown by DFT calculations that such swift exchange is not possible without a cooperative interaction of both 2 and 1 with several molecules of water present. Similarities and contrasts between the exchange processes of 2 and H3O+ bound to 1 are discussed, in particular with respect to the apparent quantum tunneling of the latter inside the molecule of the complex. Copyright © 2008 John Wiley & Sons, Ltd. [source] High-resolution magic-angle spinning NMR for the identification of reaction products directly from thin-layer chromatography spotsMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2007Scott A. Bradley Abstract We have investigated the prospect of identifying organic reaction products directly from separated thin-layer chromatography (TLC) spots with high-resolution magic-angle spinning (HRMAS) NMR. The concept is to use the TLC spots for NMR analysis so that spectra can be obtained before the reaction is worked up, but without having to elute the product from the TLC stationary phase. Thus, the separated spot is scraped from the plate, transferred to an HRMAS sample rotor, and suspended with a deuterated solvent. Herein, we describe the effects of having the stationary phase present during NMR acquisition. Using a Varian 4 mm gHX Nanoprobe and rotenone as a test compound, we found that the presence of the stationary phase during NMR acquisition resulted in (i) a large, broad ,background' signal near 4.6 ppm and (ii) a decrease in the signal-to-noise ratio due to the adsorption of the product molecules to the adsorbent. However, both effects could be adequately and conveniently eliminated. The background signal was removed by using either a CPMG pulse sequence or chemical exchange. The adsorption was avoided by using a more polar solvent system. Finally, we found that spectra with good signal-to-noise ratio and resolution could be acquired in a matter of minutes even for cases of limited product concentration. Therefore, we believe the technique has value and provides the organic chemist with another option to obtain NMR data critical for structural elucidation or verification. Copyright © 2007 John Wiley & Sons, Ltd. [source] Competition STD NMR for the detection of high-affinity ligands and NMR-based screeningMAGNETIC RESONANCE IN CHEMISTRY, Issue 6 2004Yu-Sen Wang Abstract The reported competition STD NMR method combines saturation transfer difference (STD) NMR with competition binding experiments to allow the detection of high-affinity ligands that undergo slow chemical exchange on the NMR time-scale. With this technique, the presence of a competing high-affinity ligand in the compound mixture can be detected by the disappearance or reduction of the STD signals of a low-affinity indicator ligand. This is demonstrated on a BACE1 (,-site amyloid precursor protein cleaving enzyme 1) protein,inhibitor system. This method can also be used to derive an approximate value, or a lower limit, for the dissociation constant of the potential ligand based on the reduction of the signal intensity of the STD indicator, which is illustrated on an HSA (human serum albumin) model system. This leads to important applications of the competition STD NMR method for lead discovery: it can be used (i) for compound library screening against a broad range of drug targets to identify both high- and low-affinity ligands and (ii) to rank order analogs rapidly and derive structure,activity relationships, which are used to optimize these NMR hits into viable drug leads. Copyright © 2004 John Wiley & Sons, Ltd. [source] Measurement of spin-lattice relaxation times and chemical exchange rates in multiple-site systems using progressive saturationMAGNETIC RESONANCE IN MEDICINE, Issue 1 2007Craig J. Galbán Abstract A new method for measuring spin-lattice relaxation times and chemical exchange (CE) rate constants in multiple-site exchanging systems is described. The method, chemical exchange and T1 measurement using progressive saturation (CUPS), was applied to determine T1s and analyze phosphorus exchange among phosphocreatine (PCr), ATP, and inorganic phosphate (Pi), mediated by creatine kinase (CK) and ATP synthase, using 31P-MRS. Two-site exchange was analyzed in vitro and in the rat leg, and three-site exchange was analyzed in the rat heart. Data were fitted to a model of progressive saturation incorporating T1 relaxation and CE. For the in vitro system at 8.45T, we found T1(PCr) = 2.86 s and T1(,-ATP) = 1.72 s. For the rat gastrocnemius at 1.9T, we found T1(PCr) = 6.60 s and T1(,-ATP) = 2.06 s. For the rat heart at 9.4T, we found T1(PCr) = 3.35 s, T1(,-ATP) = 0.69 s, and T1(Pi) = 1.83 s. All of these values were within 20% of literature values. Similarly, the determined exchange rates were in the same range as published values. Using simulations, we compared CUPS with transient saturation transfer as a method for measuring T1s and rates. The two methods showed similar sensitivity to noise. We conclude that CUPS is a viable alternative for measuring T1s and CE rates in exchanging systems. Magn Reson Med 58:8,18, 2007. © 2007 Wiley-Liss, Inc. [source] New MRI method with contrast based on the macromolecular characteristics of tissuesMAGNETIC RESONANCE IN MEDICINE, Issue 2 2003Arnon Neufeld Abstract A new MRI method with a contrast that is derived from the macromolecular composition and spin dynamics in the tissue is described and demonstrated on excised mouse brain and rat spinal cord. In the method, magnetization is selectively excited in the macromolecules by using a double quantum filter and subsequently transferred to water. The new imaging method differs from previous methods that rely on magnetization transfer contrast (MTC) in that it enables a separate and independent control of the effect of the macromolecule characteristics, chemical exchange, and water-related parameters on the images. Magn Reson Med 50:229,234, 2003. © 2003 Wiley-Liss, Inc. [source] Backbone dynamics of SDF-1, determined by NMR: Interpretation in the presence of monomer,dimer equilibriumPROTEIN SCIENCE, Issue 11 2006Olga K. Baryshnikova Abstract SDF-1, is a member of the chemokine family implicated in various reactions in the immune system. The interaction of SDF-1, with its receptor, CXCR4, is responsible for metastasis of a variety of cancers. SDF-1, is also known to play a role in HIV-1 pathogenesis. The structures of SDF-1, determined by NMR spectroscopy have been shown to be monomeric while X-ray structures are dimeric. Biochemical data and in vivo studies suggest that dimerization is likely to be important for the function of chemokines. We report here the dynamics of SDF-1, determined through measurement of main chain 15N NMR relaxation data. The data were obtained at several concentrations of SDF-1, and used to determine a dimerization constant of ,5 mM for a monomer,dimer equilibrium. The dimerization constant was subsequently used to extrapolate values for the relaxation data corresponding to monomeric SDF-1,. The experimental relaxation data and the extrapolated data for monomeric SDF-1, were analyzed using the model free approach. The model free analysis indicated that SDF-1, is rigid on the nano- to picosecond timescale with flexible termini. Several residues involved in the dimer interface display slow micro- to millisecond timescale motions attributable to chemical exchange such as monomer,dimer equilibrium. NMR relaxation measurements are shown to be applicable for studying oligomerization processes such as the dimerization of SDF-1,. [source] Strategies for Measurements of Pseudocontact Shifts in Protein NMR SpectroscopyCHEMPHYSCHEM, Issue 16 2007Michael John Dr. Abstract Paramagnetic metal ions bound to proteins generate a dipolar field that can be accurately probed by pseudocontact shifts (PCS) displayed by the protein's nuclear spins. PCS are highly useful for determining the coordinates of individual spins in the molecule and for rapid structure determinations of entire protein,protein and protein,ligand complexes. However, PCS measurements require reliable resonance assignments for the molecule in its paramagnetic state and in a diamagnetic reference state. This article discusses different approaches for pairwise resonance assignments, with emphasis on a strategy which exploits chemical exchange between the two states. [source] | |||||||||||||