Comptes Rendus de l'Académie des Sciences - Series IIC - Chemistry最新文献

Nuclear magnetic relaxation dispersion experiments at different temperatures, using the magnetic field-cycling method, are reported. These experiments allow the obtaining of original information about water or oil molecular dynamics at solid–liquid interface in porous media, such as grain packing or reservoir rocks. These results on molecular dynamics at the pore surface are of real interest for oil-recovery. The water surface diffusion coefficients are compared to the volume self-diffusion coefficients of water in pores, measured by PGSE method, the latter values being more than an order of magnitude higher than the surface ones.

Low field nuclear relaxation measurements applied to porous media can provide a wide variety of information. One important use of NMR measurements in the petroleum industry is the estimation of in-situ permeability as a function of depth. Such information is not available from any other tool and is critical for oil recovery predictions. A large number of empirical relationships have been published without clear explanation of their physical origin. We present some understanding and illustration of the link between NMR relaxation measurements and permeability, which is useful to select the appropriate law as a function of the geological context.

Plant non specific lipid transfer proteins form a superfamily of related proteins composed of type 1 and type 2 LTPs. Type 2 LTPs have been less extensively studied than type 1 and differ in term of primary sequence, molecular mass and transfer efficiency. We have undertaken NMR studies of the wheat 7 kDa type 2 LTP. Whilst the 3D structure determination is in progress, we have studied the protein dynamics. Two zones have been defined within the protein. One remains unperturbed, the LTP being liganded or not; the other one exists in different states, depending on the ligandation, and on the kind of lipid interacting with the protein. S² determination showed a rigid backbone. Multifield analysis allowed the calculation of the chemical exchange rate for each residue.

The electric-field gradients in the supercage of HY and steam-dealuminated Y zeolites have been quantified using one-dimensional nutation NMR applied to physisorbed ¹³¹Xe. Their values are 1.25 and 0.81·10¹⁹ V·m^–2, respectively. The composite-pulse sequence used to detect ¹³¹Xe signal also cancels the ringing signals from the NMR probe head. The fitting program for extracting the quadrupole coupling constant from the nutation NMR data is available in the web site http://www.pascal-man.com.

The principles of the intermolecular relaxation of a nuclear spin by its fluctuating magnetic dipolar interactions with the electronic spins of the paramagnetic surrounding species in solution are briefly recalled. It is shown that a very high dynamic nuclear polarization (DNP) of solvent protons is obtained by saturating allowed transitions of free radicals with a hyperfine structure, and that this effect can be used in efficient Earth field magnetometers. Recent work on trivalent lanthanide Ln³⁺ aqua complexes in heavy water solutions is discussed, including paramagnetic shift and relaxation rate measurements of the ¹H NMR lines of probe solutes. This allows a determination of the effective electronic magnetic moments of the various Ln³⁺ ions in these complexes, and an estimation of their longitudinal and transverse electronic relaxation times T_1e and T_2e. Particular attention is given to Gd(III) hydrated chelates which can serve as contrast agents in magnetic resonance imaging (MRI). The full experimental electronic paramagnetic resonance (EPR) spectra of these complexes can be interpreted within the Redfield relaxation theory. Monte-Carlo simulations are used to explore situations beyond the validity of the Redfield approximation. For each Gd(III) complex, the EPR study leads to an accurate prediction of T_1e, which can be also derived from an independent relaxation dispersion study of the protons of the probe solutes.

Although Pulsed Field Gradient experiments allow measuring a diffusion coefficient at different time and length scales, it may be of some interest to use macroscopic methods. The result of a macroscopic diffusion is viewed here by proton MRI. For water in a limestone, analysing a concentration profile following a superficial coating gives a precise measure. For a divalent ion diffusing in an alginate gel or in the initial alginate solution, the reaction front position allows a tortuosity comparison. The originality of the last part is to visualize the syneresis phenomenon. Moreover, it makes it possible to estimate the maximum reaction time for the chelation (1 ms).

The high xenon solubility in blood and tissues makes hyperpolarized ¹²⁹Xe a potential MR tracer for tissue perfusion studies. Two biocompatible fluids were studied with a view to be used as delivery media for hyperpolarized xenon injection: the carrier agents were ¹²⁹Xe micro-bubbles in Echovist (2–3 μm in diameter), and Intralipid ¹²⁹Xe suspension. Xenon chemical shifts and longitudinal relaxation time T₁ were measured at 2.35 T in both fluids. Xenon chemical shift of the dissolved phase in Echovist was 204.1 ± 0.5 ppm from the micro-bubbles gas phase resonance data (0 ppm). Xenon T₁ was 20.0 s in micro-bubbles in Echovist and 19.0 s for the dissolved phase. Xenon chemical shift in Intralipid was 194.6 ± 0.5 ppm. T₁ was 15.2 ± 4.9 s (n = 5) in Intralipid 20 % and 20.9 ± 2.9 s (n = 4) in Intralipid 30 %. Using an intra-carotid injection of a small volume (0.15 mL) of hyperpolarized xenon dissolved in Intralipid 30 %, cerebral blood flow was measured in rats (160 ± 30 mL·(100 g)^–1·min^–1, n = 10). Rat brain xenon images were performed with 2-D projection–reconstruction pulse sequence, enabling regional blood flow measurements.

In situ ¹H NMR, directly performed on biological fluids is a very powerful tool to study the fate of pollutants in the environment. The biodegradation of 2-aminobenzothiazole by Rhodococcus rhodochrous was monitored by reverse phase HPLC and by in situ ¹H NMR, methods performed directly on culture media without purification. The xenobiotic was biotransformed into a hydroxylated derivative. The chemical structure of this metabolite was determined by a long-range ¹H–¹⁵N heteronuclear shift correlation without any previous ¹⁵N enrichment of the compound. This approach allowed the assignment of the metabolite structure to 2-amino-6-hydroxybenzothiazole.

In order to get deeper insight into the molecular forces responsible for prion pathogenic conversion, conformational properties of a synthetic linear peptide derived from the globular core of sheep prion protein were studied by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. The studied peptide encompassing the 〚142–166〛 (in human numbering) region of sheep prion protein, folds in physiological conditions into a β-hairpin like tertiary structure, whereas, in the non-pathogenic form of protein and in trifuoroethanol (TFE), the region is engaged in largely α-helical conformation. Such structural duality of the fragment indicates a possible transconformational site within prion protein and may explain one of the early structural causes of prion diseases.

The large majority of analytical NMR methods in chemistry or biochemistry are based on the quality of the chemical shift dispersion. The purpose of this work is to show that theoretical chemical shift back calculation starting from structures can be used to select molecular modeling structures in order to differentiate several conformational possibilities. Here we report the result of the chemical shift calculation carried out on two original structures corresponding to two DNA ‘kissing complexes’. The 46 nucleotides sequence corresponds to the RNA deoxyribose analogous implied in the HIV-1_Lai dimerization process. It is interesting to note that, even in cases we are very far from the classical helical structure (loop–loop interaction, AA base pairing, base stacking, misappariement...), the theoretical chemical shift is in very good agreement with the experimental chemical shift (±0.25 ppm). The satisfactory results obtained enable us to conclude that the comparison of the proton chemical shifts is an invaluable tool making it possible to select or to validate oligonucleotides structures.