Jean Grandjean

Sodium complexation by the calcium binding site of parvalbumin

Parvalbumins are a class of muscular proteins with calcium binding properties, distributed in all classes of vertebrates [ 11. The amino acid sequence is known for a number of parvalbumins. The tertiary structure has been established by X-ray crystallography for carp parvalbumin B [2]: it shows two calcium sites; each of these has an octahedral arrangement of oxygen atoms from carboxyl, carbonyl and hydroxyl groups coordinating the Ca2+ cations. The calcium binding sites are the same in parvalbumin III from pike, with mol. wt 11 780 [3 3, chosen for this study. 23Na nuclear magnetic resonance is establishing itself as an extremely powerful method for studying the interactions between cations and organic or biological molecules [4]. A major impetus for our study is the finding that half-decalcified parvalbumin (PAB-I) of pike or carp will return reversibly to the native form (PAB-2) when the missing calcium is restored to it [5,6]. Furthermore, internal mobility remains similar in PAB-1 and PAB-2 [7]. Therefore, we set upon an exploration of the vacant calcium binding site in PAB-1, using the sodium cation as our probe. We do find indeed that Na’ will bind to PAB-1; it affixes itself to the calcium-binding site, albeit much more weakly than calcium, by some five orders of magnitude. However, and rather surprisingly, the sodium cation has a residence time on the protein long with respect to the PAB-I reorientational

Deuterium nuclear magnetic resonance studies of water molecules restrained by their proximity to a clay surface

Observation of quadrupolar splittings for 2H and 17O nuclei from D2O solvent molecules in suspensions of the Ecca Gum BP bentonite points unambiguously to the ordering of water molecules with respect to a static magnetic field. The application of the magnetic field apparently orients the clay platelets as tactoids. Some water molecules adhere to the platelets and are oriented. The magnitude of the quadrupolar splitting depends strongly on the nature of the interstitial cations. The sign of the splitting changes as the ratio of divalent to monovalent cation increases, reflecting a switch in the microdynamics of water molecules next to the plates. With the entry of divalent cations, the clay becomes more cohesive due to the vertical electrostatic attraction of the counterions to the charged plates. More of these hydrated divalent ions compared with monovalent ions condense onto the charged sheet, thereby increasing metal coordination to those water molecules in contact with the clay surface. The dominant reorientation mode of the water molecules switches from rotation of water molecules around hydrogen bonds to the charged sheets to rotation around the electrostatic bond to the divalent metal cation. The effect is spectacular and important in understanding the chemical properties of cation-exchanged montmorillonites.

23Na 2D 3QMAS NMR and 29Si, 27Al MAS NMR investigation of Laponite and synthetic saponites of variable interlayer charge

Abstract 29Si, 27Al MAS NMR is used to characterize Laponite RD and synthetic saponites of variable interlayer charge. The Si/Al ratios are in good agreement with the calculated charge from chemical analysis except for the lowest-charged saponite. In contrast to the 29Si MAS NMR spectra in which resolved signals are detected, the 27Al MAS NMR spectra show one signal whose linewidth increases with the clay charge. The water content of the clay samples was obtained from 1H MAS NMR. The 2D MQMAS NMR technique is required to obtain a high-resolution spectrum of nuclei with strong quadrupolar interaction. This method was applied to the 23Na nucleus of clay counterions and to the 27Al structural nucleus. One well-defined 23Na NMR signal is observed for all the clays studied except the highest-charged saponite. Possible explanations for this different behaviour are discussed. The calculated isotropic chemical shift evolves progressively with the clay charge whereas the deduced quadrupolar interaction does not change significantly. The 27Al 2D 3QMAS technique was not able to resolve more than one signal.

Theory of nuclear spin relaxation in heterogeneous media and application to the cross correlation between quadrupolar and dipolar fluctuations of deuterons in clay gels

Abstract The fictitious-spin - 1 2 operator formalism allows a comprehensive study of the longitudinal and transverse nuclear relaxations of a nuclear spin I = 1 in heterogeneous media, in the presence of a residual time-averaged quadrupolar interaction. An extension of this formalism makes possible the calculation of the cross correlation between the quadrupolar and dipolar (paramagnetic) fluctuations. Although this cross correlation is second order relative to the quadrupolar relaxation, its long range of effectiveness ( ∞ 1 r 3 ) is sufficient to provide the sole asymmetric component to the fine broadening of the deuterium doublet. This differential line broadening depends linearly on the applied magnetic induction field and on the paramagnetic spin concentration, as checked experimentally from a lineshape analysis of the 2H doublet observed for D20 in clay gels. The reorientational correlation time of the adsorbed water at the clay interface is obtained from the measured 2H spin-relaxation rates.

Multinuclear and pulsed gradient magnetic resonance studies of sodium cations and of water reorientation at the interface of a clay

Knowledge of the microdynamics of adsorbates is an important prerequisite to the understanding ofcatalysis of organic reactions by inorganic solids, such as phyllosilicates. We have studied gels formed by a bentonite suspended in an aqueous solution of p-cresol. We have applied the techniques of 1H, 2H, 13C, 17O, and 23Na NMR to this system. We have also determined by proton spin echoes, using pulsed gradients, the self-diffusion coefficients under the same conditions. A description by a two-site system with fast chemical exchange accounts for the measurements and for their temperature dependence from 278 to 320 K. One site is the bulk solvent. The other site, for which we have obtained the limiting parameters, is at the clay interface. A third contribution from exchange between two sites close to the solid interface is relatively minor.

Solid-state NMR study of modified clays and polymer/clay nanocomposites

Abstract Intercalation of surfactant and polymer chains between the clay platelets gives rise to molecular ordering that changes both the chain conformation and mobility with respect to the bulk phase. As a local probe, nuclear magnetic resonance is particularly suited for such investigations, and this review reports the main results obtained in the solid state. The properties of the modified clays are studied as a function of the surfactant loading, the nature of the head group, and the length of the hydrocarbon chain(s). The structure and charge of the mineral also influence the behaviour of molecules in the gallery space. Among papers on polymer/clay nanocomposites, those dealing with poly(ethylene oxide) have been studied in particular, using a multinuclear approach. Natural clays often contain paramagnetic species such as Fe(III) that perturb the nuclear magnetic resonance (NMR) relaxation processes and can prevent observation of appropriate NMR data. Accordingly, most organic/clay hybrids are studied with hectorite or synthetic smectites. However, the paramagnetic effect has also been found useful in characterizing clay dispersion within the polymer matrix of the nanocomposites.

Orientational correlation times for globular hydrocarbons as neat liquids

Spin-lattice relaxation times have been measured for various non-equivalent carbon-13 nuclei on the surface of quasi-spherical hydrocarbon molecules, and the results indicate the isotropy of molecular reorientation in the pure liquids. Variations in carbon-hydrogen bond lengths affect significantly the corresponding dipolar relaxation times as expected. At ambient temperatures, the molecular dynamics conform to a diffusional model. Translations and rotations are only weakly coupled. The Kivelson parameter % has values in the range 0·07–0·10. The reorientational correlation times, τe, extrapolated to zero viscosity agree with those calculated under the assumption of free rotation. The reduced orientational correlation times display a very good linear dependence upon the reduced temperatures derived from the boiling points for norbornadiene, 2-methylene norbornene, α- and β-pinene. This empirical relationship consists of a single line for these four hydrocarbons. The points corresponding to other globular s...

Solution structure and cation-binding abilities of two quasi-isomorphous antibiotic ionophores, M 139603 and tetronomycin

Abstract The 200 and 400 MHz 1 H nmr spectra of the title antibiotics are fully attributed and analyzed with the help of COSY and DEPT sequences. The solution conformation, the same for the free acid and the sodium salt, is very similar to that in the solid.

XRD and NMR characterization of synthetic hectorites and the corresponding surfactant-exchanged clays

Abstract Synthetic hectorites and the corresponding surfactant-exchanged clays have been characterized by X-ray diffraction and 1H, 7Li, 13C, 23Na and 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy. The low-charge clays retain water more efficiently, forming aggregates without extensive drying. The hydroxylated hectorite exhibits two 1H NMR signals near 0 ppm whereas the fluorohectorites are characterized by a single peak in the same region. The 23Na 2D 3Q magic angle spinning (MAS) spectra of the low-charge hectorites show a single peak. The 29Si NMR shift depends on the interlayer charge. Tactoids formed by the low-charge hectorites reduce the rate of surfactant incorporation. The population of the all-trans conformer of the hydrocarbon chain, determined by 13C MASNMR, varies with the surfactant content. 13C NMR relaxation data show an increase in mobility with the surfactant loading and along the long alkyl chain, from the polar head to the terminal group. Complexity of the motional behaviour precludes any detailed analysis. These modified clays are not useful in preparing poly(ε-caprolactone) nanocomposites by in situ polymerization.

A synthetic crown ether carboxylic acid ionophore displays synergistic transport of Pr3+ in conjunction with lasalocid.

Transport of Pr3+ across phosphatidyl choline vesicles, as monitored by 31P nmr, is second-order in the crown ether carboxylic acid 2, as it is with respect to lasalocid (X-537 A). When the synthetic (2) and the natural (lasalocid) ionophores are incorporated together in approximately 3:1 ratio into the lipidic phase, the transport velocity is markedly enhanced.