Solute-solvent contact by intermolecular cross relaxation. I. The nature of the water-hydrophobic interface

Insights into dynamic properties of water in lipidic cubic phases by 2D nuclear Overhauser effect (NOE) NMR spectroscopy

Two-dimensional NOE (nuclear Overhauser effect) NMR spectroscopy was employed to investigate the dynamic properties of water within lyotropic bicontinuous lipidic cubic phases (LCPs) formed by monoolein (MO). Experiments observed categorically different effective residence times of water molecules: (i) in proximity to the glycerol moiety of MO, and (ii) adjacent to the hydrophobic chain towards the hydrocarbon tail of MO, as evidenced by the opposite signs of intermolecular NOE cross peaks between protons of water and those of MO in 2D 1H-1H NOESY spectra. Spectroscopic data delineating the different effective residence times of water molecules within both the gyroid (QIIG) and diamond (QIID) phase groups corresponding to hydration levels of 35 and 40 wt%, respectively, are presented. Additionally, an increase in effective residence time of water molecules in proximity to the glycerol moiety of MO in LCPs was observed upon storage at ambient temperature and in the presence of an additive lipid, cholesterol. Atom-specific NOE build-up curves for protons of water and those of MO are also given. The results presented herein provide new insight into the physicochemical properties and behaviour of water in LCPs, and demonstrate an additional avenue for experimental study of water-lipid interactions and hydration dynamics in model membranes and nanomaterials using 2D NOE NMR spectroscopy.

Proton NMR relaxation from molecular dynamics: intramolecular and intermolecular contributions in water and acetonitrile

Accurate ¹H NMR relaxation rates for protons in pure water and acetonitrile are computed via ab initio and force field molecular dynamics. Dipole–dipole and spin-rotation mechanisms are considered.

Suppressing magnetization exchange effects in stimulated-echo diffusion experiments

Exchange of nuclear magnetization between spin pools, either by chemical exchange or by cross-relaxation or both, has a significant influence on the signal attenuation in stimulated-echo-type pulsed field gradient experiments. Hence, in such cases the obtained molecular self-diffusion coefficients can carry a large systematic error. We propose a modified stimulated echo pulse sequence that contains T2-filters during the z-magnetization store period. We demonstrate, using a common theoretical description for chemical exchange and cross-relaxation, that these filters suppress the effects of exchange on the diffusional decay in that frequent case where one of the participating spin pools is immobile and exhibits a short T2. We demonstrate the performance of this experiment in an agarose/water gel. We posit that this new experiment has advantages over other approaches hitherto used, such as that consisting of measuring separately the magnetization exchange rate, if suitable by Goldman-Shen type experiments, and then correcting for exchange effects within the framework of a two-site exchange model. We also propose experiments based on selective decoupling and applicable in systems with no large T2 difference between the different spin pools.

Water behavior in mesoporous materials as studied by NMR relaxometry

Water in mesoporous materials possessing a two-dimensional hexagonal structure has been studied by the variation of its NMR longitudinal relaxation time T(1) as a function of the static magnetic field value, or equivalently of the NMR measurement frequency. This technique, dubbed relaxometry, has been applied from 5 kHz (measurement frequency) up to 400 MHz with various instruments including a variable-field spectrometer operating between 8 and 90 MHz. Moreover, the range 0-5 kHz could be investigated by transverse relaxation, T(2) denoting the corresponding relaxation time, and relaxation in the rotating frame, T(1ρ) denoting the corresponding relaxation time. Measurements of proton relaxation rates (inverse of relaxation times) have been performed with H(2)O and HOD (residual protons of heavy water) at water volumes of 80%, 60%, and 40% relative to the porous volume. Comparison between H(2)O and HOD shows clearly that, above 1 MHz where both sets of data are superposed, relaxation is purely intermolecular and due to paramagnetic relaxation (dipolar interactions of water protons with unpaired electrons of paramagnetic entities). Below 1 MHz, it is possible to subtract the intermolecular contribution (given by HOD data) from H(2)O data so that one is left with intramolecular relaxation which is solely due to water reorientational motions. The analysis of these low-frequency data (in terms of Lorentzian functions) reveals two types of water within the pores: one interacting strongly with the surface and the other corresponding to a second layer. High-frequency data, which arise from paramagnetic relaxation, exhibit again two types of water. Due to their correlation times, one type is assigned to relatively free water within the pores while the other type corresponds to bulk (interparticular) water. Their proportions, given as a function of the volume fraction, are consistent with the above assignments.

Solute−Solvent Contact by Intermolecular Cross-Relaxation. 2. The Water−Micelle Interface and the Micellar Interior

The intermolecular dipole-dipole cross-relaxation is measured between 19F nuclei of sodium perfluorooctanoate in micelles and 1H nuclei of the water solvent. The cross-relaxation rates for fluorines in the different moieties along the surfactant vary strongly by the resonance frequency in the investigated range of 188-470 MHz. This frequency dependence indicates that the cross-relaxation between water and amphiphilic aggregates is not controlled solely by the fast local water dynamics but significantly contributed to by the long-range translational diffusion of water. The cross-relaxation rates, analyzed in the framework of a model (Nordstierna, L.; Yushmanov, P. V.; Furó, I. J. Chem. Phys. 2006, 125, 074704), provide information about the dynamic retardation of water molecules by the micellar headgroup region and the location of the various moieties along the hydrophobic tail with respect to the water-micelle interface. Both intermolecular cross-relaxation and aggregation-induced 19F chemical shift changes indicate no direct water contact to fluorines except for those closest to the head group.