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Fast Exchange (fast + exchange)
Selected AbstractsMagnetic resonance imaging of spatially resolved acrylamide photopolymerizationMAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2003Tom J. Lees Abstract Magnetic resonance imaging was employed to examine spatially and temporally resolved photopolymerization of acrylamide gels. Fast exchange between free and bound water results in single exponential T2 decay, where 1/T2 scales linearly with polymer concentration. Measured T2s are sensitive to the experimental conditions; however, the 1/T2 relationship to polymer concentration allows a straightforward interpretation of image contrast changes during photopolymerization. The polymer appears to form at a nearly constant rate until the monomer concentration is significantly depleted. Conventional spin-echo images and quantitative CPMG-weighted spin-echo images were acquired. Photopolymerization of a partially masked sample produced a sharp transition (1 mm width) between polymer and monomer regions of the sample. The image intensity is uniform throughout the illuminated region of the sample, indicating uniform polymer formation. Interrupting the illumination quenches polymer formation. Copyright © 2003 John Wiley & Sons, Ltd. [source] NMR 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] Interaction of exchange and differential relaxation in the saturation recovery behavior of the binary spin-bath model for magnetization transferCONCEPTS IN MAGNETIC RESONANCE, Issue 4 2006Gunther Helms Abstract Most closed-form analytical solutions of the binary spin-bath are difficult to interpret in terms of underlying physics. The key notions are the presence of a kinetic and a thermal equilibrium and that the time course of saturation recovery under conditions of fast exchange can be understood as conjoint relaxation and lossless transfer. By introducing a suitable parameter, it is shown how exchange and differential relaxation counteract each other: the amount of transferred saturation (transfer term) is altered and the kinetic equilibrium appears slightly disturbed (difference term). Although the factorization formally represents the general solution of saturation recovery in the binary spin-bath, this interpretation applies only to the case of fast exchange and slow relaxation. By calculating the set of parameters for a wide range of hypothetical relaxation rates, it was shown that the difference term is crucial to describe the transition to the slow-exchange limit. The transfer term vanishes as the two pools appear decoupled in this approximation. © 2006 Wiley Periodicals, Inc. Concepts Magn Reson Part A 28A: 291,298, 2006. [source] Synthesis, CD Spectra, and Enzymatic Stability of ,2 -Oligoazapeptides Prepared from (S)-2-Hydrazino Carboxylic Acids Carrying the Side Chains of Val, Ala, and LeuHELVETICA CHIMICA ACTA, Issue 12 2003Gérald Lelais , -Peptides offer the unique possibility to incorporate additional heteroatoms into the peptidic backbone (Figs.,1 and 2). We report here the synthesis and spectroscopic investigations of ,2 -peptide analogs consisting of (S)-3-aza- , -amino acids carrying the side chains of Val, Ala, and Leu. The hydrazino carboxylic acids were prepared by a known method: Boc amidation of the corresponding N -benzyl- L - , -amino acids with an oxaziridine (Scheme,1). Couplings and fragment coupling of the 3-benzylaza- ,2 -amino acids and a corresponding tripeptide (N -Boc/C -OMe strategy) with common peptide-coupling reagents in solution led to ,2 -di, ,2 -tri-, and ,2 -hexaazapeptide derivatives, which could be N -debenzylated (4,9; Schemes,2,4). The new compounds were identified by optical rotation, and IR, 1H- and 13C-NMR, and CD spectroscopy (Figs.,4 and 5) and high-resolution mass spectrometry, and, in one case, by X-ray crystallography (Fig.,3). In spite of extensive measurements under various conditions (temperatures, solvents), it was not possible to determine the secondary structure of the ,2 -azapeptides by NMR spectroscopy (overlapping and broad signals, fast exchange between the two types of NH protons!). The CD spectra of the N -Boc and C -OMe terminally protected hexapeptide analog 9 in MeOH and in H2O (at different pH) might arise from a (P)- 314 -helical structure. The N -Boc- ,2 -tri and N -Boc- ,2 -hexaazapeptide esters, 7 and 9, were shown to be stable for 48,h against the following peptidases: pronase, proteinase,K, chymotrypsin, trypsin, carboxypeptidase,A, and 20S proteasome. [source] Investigation of penetratin peptides Part 1.JOURNAL OF PEPTIDE SCIENCE, Issue 4 2002The environment dependent conformational properties of penetratin, two of its derivatives Abstract The homeodomain, the DNA-binding domain of Antennapedia homeoprotein, is composed of three ,-helices and one ,-turn between helices II and III. Its third helix from the N -terminal (helix III) can translocate through the cell membrane into the nucleus and can be used as an intracellular vehicle for the delivery of oligopeptides and oligonucleotides. To the best of our knowledge, this helix III, called penetratin, which consists of 16 amino acids, is internalized by cells in a specific, non-receptor-mediated manner. For a better understanding of the mechanism of the transfer, the structure of penetratin was examined in both extracellular matrix-mimetic and membrane-mimetic environments; 1H-NMR and CD spectroscopic measurements were performed in mixtures of TFE/water with different ratios. The molecular conformations of two analogue peptides [(6,14-Phe)-penetratin and a 12 amino acid penetratin derivative (peptide 3)] were also studied. An atomic level comprehensive analysis of penetratin and its two analogues was performed. In a membrane-mimetic solvent system (TFEd2/water = 9 : 1), on the basis of 553 distance restraints, the 4,12 region of penetratin exhibits a bent, irregular helical structure on NMR examination. Interactions between hydrophobic amino acid residues in conjunction with H-bonds stabilize the secondary structure of the molecule. Thus, both derivatives adopt a helix-like conformation. However, while (6,14-Phe)-penetratin displays both ,-helical and 310 -helical features, the structure of peptide 3 is predominantly a 310 -helix. Of the three peptides, surprisingly (6,14-Phe)-penetratin has the largest helical content. An increase in the polarity of the molecular environment gradually disintegrates these helix-like secondary structures. In a highly aqueous molecular system (TFEd2/water = 1 : 9), the fast exchange of multiple conformers leads to too few distance restraints being extracted, therefore the NMR structures can no longer be determined. The NMR data show that only short-range order can be traced in these peptides. Under these conditions, the molecules adopt nascent helix-like structures. On the other hand, CD spectra could be recorded at any TFE/water ratio and the conformational interconversion could therefore be monitored as a function of the polarity of the molecular environment. The CD data were analysed comprehensively by the quantitative deconvolution method (CCA+). All three penetratin peptides display helical conformational features in a low dielectric medium, with significant differences as a function of their amino acid composition. However, these conformational features are gradually lost during the shift from an apolar to a polar molecular environment. Copyright © 2002 European Peptide Society and John Wiley & Sons, Ltd. [source] Long-range substituent and temperature effect on prototropic tautomerism in 2-(acylmethyl)quinolinesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2001Ryszard Gawinecki Abstract Tautomeric equilibria between 2-(cinnamoylmethyl)quinoline, (Z)-1,2-dihydro-2-(cinnamoylmethylene)quinoline and (Z)-4-phenyl-1-(2-quinolyl)-1,3-butadien-2-ol were studied by 1H, 13C and 15N NMR methods. The ,CHCH, fragment conjugated with phenyl and a strong electron donor p -(1-pyrrolidine) substituent were found to favour the enolimine tautomer. This undergoes fast exchange (on the NMR time-scale) with the enaminone form. The amount of the latter tautomer was found to increase at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd. [source] A dedicated small-angle X-ray scattering beamline with a superconducting wiggler source at the NSRRCJOURNAL OF SYNCHROTRON RADIATION, Issue 1 2009Din-Goa Liu At the National Synchrotron Radiation Research Center (NSRRC), which operates a 1.5,GeV storage ring, a dedicated small-angle X-ray scattering (SAXS) beamline has been installed with an in-achromat superconducting wiggler insertion device of peak magnetic field 3.1,T. The vertical beam divergence from the X-ray source is reduced significantly by a collimating mirror. Subsequently the beam is selectively monochromated by a double Si(111) crystal monochromator with high energy resolution (,E/E, 2 × 10,4) in the energy range 5,23,keV, or by a double Mo/B4C multilayer monochromator for 10,30 times higher flux (,1011,photons,s,1) in the 6,15,keV range. These two monochromators are incorporated into one rotating cradle for fast exchange. The monochromated beam is focused by a toroidal mirror with 1:1 focusing for a small beam divergence and a beam size of ,0.9,mm × 0.3 mm (horizontal × vertical) at the focus point located 26.5,m from the radiation source. A plane mirror installed after the toroidal mirror is selectively used to deflect the beam downwards for grazing-incidence SAXS (GISAXS) from liquid surfaces. Two online beam-position monitors separated by 8,m provide an efficient feedback control for an overall beam-position stability in the 10,µm range. The beam features measured, including the flux density, energy resolution, size and divergence, are consistent with those calculated using the ray-tracing program SHADOW. With the deflectable beam of relatively high energy resolution and high flux, the new beamline meets the requirements for a wide range of SAXS applications, including anomalous SAXS for multiphase nanoparticles (e.g. semiconductor core-shell quantum dots) and GISAXS from liquid surfaces. [source] An NMR study of cyclodextrin complexes of the steroidal neuromuscular blocker drug Rocuronium BromideMAGNETIC RESONANCE IN CHEMISTRY, Issue 4 2002Kenneth S. Cameron Abstract The interaction of Rocuronium Bromide, and a model steroid Org 7402, with three cyclodextrins (,-cyclodextrin, ,-cyclodextrin and Org 25969) was studied by solution state NMR experiments. Stoichiometries and binding constants were determined from 1H chemical shift titrations. All of the systems formed 1 : 1 complexes. Most of the complexes were in fast exchange with unbound species on the NMR time scale, but the most tightly bound complex (Rocuronium Bromide,Org 25969) was in the slow exchange regime. The geometry of the complexes was inferred from 1H and 13C NMR shift changes upon complexation and from intramolecular NOE correlations. Rocuronium Bromide forms a weak complex with ,-cyclodextrin (Ka = 3.3 ± 0.5 × 103M,1) and no clear picture of the structure of the complex emerges. The complexes with ,-cyclodextrin (Ka = 1.8 ± 0.2 × 104M,1) and Org 25969 (Ka > 105M,1) are true inclusion complexes with the steroid located inside the central void of the cyclodextrin. Copyright © 2002 John Wiley & Sons, Ltd. [source] In vivo quantification of regional myocardial blood flow: Validity of the fast-exchange approximation for intravascular T1 contrast agent and long inversion time,MAGNETIC RESONANCE IN MEDICINE, Issue 2 2006Marlene Wiart Abstract In the present study we investigated the effects of water exchange between intra- and extravascular compartments on absolute quantification of regional myocardial blood flow (rMBF) using a saturation-recovery sequence with a rather long inversion time (TI, 176 ms) and a T1 -shortening intravascular contrast agent (CMD-A2-Gd-DOTA). Data were acquired in normal and ischemically injured pigs, with radiolabeled microsphere flow measurements used as the gold standard. Five water exchange rates (fast, 6 Hz, 3 Hz, 1 Hz, and no exchange) were tested. The results demonstrate that the fast-exchange approximation may be appropriate for rMBF quantification using the described experimental setting. Relaxation rate change (,R1) analysis improved the accuracy of the analysis of rMBF compared to the MR signal. In conclusion, the current protocol could provide sufficient accuracy for estimating rMBF assuming fast exchange and a linear relationship between signal and tissue concentration when quantification of precontrast T1 is not an option. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc. [source] Solution structure of GOPC PDZ domain and its interaction with the C-terminal motif of neuroliginPROTEIN SCIENCE, Issue 9 2006Xiang Li Abstract GOPC (Golgi-associated PDZ and coiled-coil motif-containing protein) represents a PDZ domain-containing protein associated with the Golgi apparatus, which plays important roles in vesicular trafficking in secretory and endocytic pathways. GOPC interacts with many other proteins, such as the Wnt receptors Frizzled 8 and neuroligin via its PDZ domain. Neuroligin is a neural cell-adhesion molecule of the post-synapse, which binds to the presynapse molecule neurexin to form a heterotypic intercellular junction. Here we report the solution structure of the GOPC PDZ domain by NMR. Our results show that it is a canonical class I PDZ domain, which contains two ,-helices and six ,-strands. Using chemical shift perturbation experiments, we further studied the binding properties of the GOPC PDZ domain with the C-terminal motif of neuroligin. The observations showed that the ensemble of the interaction belongs to fast exchange with low affinity. The 3D model of the GOPC PDZ domain/neuroligin C-terminal peptide complex was constructed with the aid of the molecular dynamics simulation method. Our discoveries provide insight into the specific interaction of the GOPC PDZ domain with the C-terminal peptide of Nlg and also provide a general insight about the possible binding mode of the interaction of Nlg with other PDZ domain-containing proteins. 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