Probing interactions by means of pulsed field gradient nuclear magnetic resonance spectroscopy

Molecular self-diffusion coefficients (D) of species in solution are related to size and shape and can be used for studying association phenomena. Pulsed field gradient nuclear magnetic resonance (PFG-NMR) spectroscopy has been revealed to be a powerful analytical tool for D measurement in different research fields. The present work briefly illustrates the use of PFG-NMR for assessing the existence of interactions in very different chemical systems: organic and organometallic compounds, colloidal materials and biological aggregates. The application of PFG-NMR is remarkable for understanding the role of anions in homogenous transition metal catalysis and for assessing the aggregation behaviour of biopolymers in material science.     

Studies of protein hydration in aqueous solution by high-resolution nuclear magnetic resonance spectroscopy

Individual hydration water molecules in aqueous protein solutions have been observed using experimental schemes for homonuclear two-dimensional and heteronuclear three-dimensional NMR experiments in H₂O solution, which do not require suppression of the solvent line by presaturation. In these experiments, the location of the hydration waters is determined from their nuclear Overhauser effects (NOE s) with individual hydrogen atoms of distinct amino acid residues. In the basic pancreatic trypsin inhibitor (BPTI ), four internal water molecules that had been reported in three different crystal forms were also found to be in the same locations in the solution structure, with lifetimes with respect to exchange of the water protons in excess of 0.3 ns. Additional NOE s with polypeptide protons located on the protein surface may involve either hydration water molecules or hydroxyl protons of amino acid side chains. Their total number is small compared to the number of NOE s expected from the hydration water molecules identified in the crystal structures of BPTI .     

Static and pulsed field gradient nuclear magnetic resonance studies of water diffusion in protein matrices

Static field gradient and pulsed field gradient NMR are used to study the temperature dependence of water diffusion in myoglobin and lysozyme matrices for low hydration levels of about 0.3?g/g. We show that in order to determine reliable self-diffusion coefficients D in a broad temperature range, it is very important to consider an exchange of magnetization between water and protein protons, often denoted as cross relaxation. Specifically, upon cooling, the observed stimulated-echo decays, which reflect water diffusion near ambient temperature, become more and more governed by cross relaxation. We demonstrate that comparison of experimental results for inhomogeneous and homogeneous magnetic fields enables successful separation of diffusion and relaxation contributions to the stimulated-echo decays. Making use of this possibility, we find that in the temperature range 230-300?K, the temperature-dependent diffusivities D exhibit a Vogel-Fulcher-Tammann behavior, where water diffusion in the studied protein matrices is substantially slower than in the bulk. By comparing present and previous data, we discuss relations between translational and rotational motions and between short-range and long-range water dynamics in protein matrices. In addition, we critically examine the significance of results from previous applications of NMR diffusometry to the temperature-dependent water diffusion in protein matrices.     

Molecular exchange through vesicle membranes: a pulsed field gradient nuclear magnetic resonance study

The permeability of block copolymer vesicles is studied using pulsed field gradient nuclear magnetic resonance spectroscopy together with a numerical data analysis procedure. Polyethylene oxide molecules of various molecular masses are used to sample the permeability of the vesicle membrane by observing the trans-membrane exchange process under equilibrium conditions. For shorter polyethylene oxide chains, the analysis yields a nearly linear dependence of the logarithmic trans-membrane exchange rate on the hydrodynamic radius of the sample molecules.     

The influence of polymer molecular-weight distributions on pulsed field gradient nuclear magnetic resonance self-diffusion experiments

 The influence of polymer molecular-weight distributions on the outcome of pulsed field gradient (PFG) NMR self-diffusion experiments has been considered. The self-diffusion coefficient, D, of monodisperse poly(ethylene oxide) (PEO) polymers has been determined in order to accurately determine the scaling behavior of D both with molecular weight and concentration. In order to investigate the influence of polydispersity on the PFG NMR signal, a model system consisting of ten reasonably monodisperse PEO polymers was made, and the PFG NMR signal intensities were recorded at a low total concentration. The data were analyzed using both inverse Laplace transformation and nonlinear least-squares fitting to a prescribed distribution function of D. Finally, the molecular-weight distribution was obtained by use of the values of the scaling parameters. We also present some model calculations used to investigate the sensitivity of the degree of polydispersity on the NMR signal decays. Received: 27 May 1999 Accepted in revised form: 19 October 1999     

Boundary effects of molecular diffusion in nanoporous materials: a pulsed field gradient nuclear magnetic resonance study

The boundary conditions of intraparticle diffusion in nanoporous materials may be chosen to approach the limiting cases of either absorbing or reflecting boundaries, depending on the host-guest system under study and the temperature of measurement. Pulsed field gradient nuclear magnetic resonance is applied to monitor molecular diffusion of n-hexane and of an n-hexane-tetrafluoromethane mixture adsorbed in zeolite crystallites of type NaX under either of these limiting conditions. Taking advantage of the thus-established peculiarities of mass transfer at the interface between the zeolite bulk phase and the surrounding atmosphere, three independent routes for probing the crystal size are compared. These techniques are based on (i) the measurement of the effective diffusivity under complete confinement, (ii) the application of the so-called NMR tracer desorption technique, and (iii) an analysis of the time dependence of the effective diffusivity in the short-time limit where, by an appropriate variation of the adsorbate and the measuring conditions, the limiting cases of reflecting and adsorbing boundaries could be considered. All these techniques are found to yield coinciding results, which are in excellent agreement with the crystal sizes determined by microscopy.     

How to compare diffusion processes assessed by single-particle tracking and pulsed field gradient nuclear magnetic resonance

Heterogeneous diffusion processes occur in many different fields such as transport in living cells or diffusion in porous media. A characterization of the transport parameters of such processes can be achieved by ensemble-based methods, such as pulsed field gradient nuclear magnetic resonance (PFG NMR), or by trajectory-based methods obtained from single-particle tracking (SPT) experiments. In this paper, we study the general relationship between both methods and its application to heterogeneous systems. We derive analytical expressions for the distribution of diffusivities from SPT and further relate it to NMR spin-echo diffusion attenuation functions. To exemplify the applicability of this approach, we employ a well-established two-region exchange model, which has widely been used in the context of PFG NMR studies of multiphase systems subjected to interphase molecular exchange processes. This type of systems, which can also describe a layered liquid with layer-dependent self-diffusion coefficients, has also recently gained attention in SPT experiments. We reformulate the results of the two-region exchange model in terms of SPT-observables and compare its predictions to that obtained using the exact transformation which we derived.     

Fast inspection of fruits using nuclear magnetic resonance spectroscopy

High-resolution nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for obtaining chemical structure information. Its quantitative and noninvasive properties have led to its growing popularity as an analytical tool in many fields, including biology, chemistry, medicine, and food science. During transportation and storage, chemical reactions among the many nutrients lead to a loss of food quality. In these circumstances, portable NMR spectrometers can readily be used for food inspection and quality control. Because of the heterogeneous tissue distribution in food, a high-resolution NMR method is required for detailed food inspection. Therefore, in this study, we demonstrated the feasibility of using an intermolecular double-quantum coherence signal to obtain high-resolution metabolic profiles of several fruits, including grape, cantaloupe, tomato, and watermelon. The resulting high-resolution NMR spectra facilitate the identification of important metabolites, which can be used as biomarkers for food quality control. The method established here may be adapted for food inspection using portable NMR equipment.     

Salt-induced micellization of a triblock copolymer in aqueous solution: a 1H nuclear magnetic resonance spectroscopy study

An aqueous micellar solution of a PEO-PPO-PEO triblock copolymer, pluronic F88 (EO103PO39EO103), in the presence of salt (KCl) has been investigated by 1H NMR spectroscopy. The hydrogen-bonding structure in water is directly changed by the strong polarization effect of added salt, which indirectly weakens the interaction of polymer molecules with water. Both EO and PO blocks are dehydrated by the addition of salt in a similar way, whereas the solubility of the PO blocks may be affected in a more pronounced way, which results in the decrease of the critical micellization temperature (CMT). It is found that the addition of salt favors a more compact micellar core, where the water content is decreased and an effective PO-PO interaction is increased. Increasing the salt concentration would result in a decrease in the number of gauche conformers in the PPO chain, which may be the deeper reason for the decreasing solubility of PPO segments in aqueous salt solution. The temperature region over which the micellization occurs is broad, indicating that micelles and unimers coexist over an extended temperature range, whereas this transition region is significantly narrowed by the addition of salt. The addition of salt offers a good substitute way of changing the temperature to induce micellization. The critical micellization salt concentration (CMSC) is determined to be 1.0 mol l-1 for KCl in 2.5% aqueous pluronic F88 solution at 25 degrees C, and the transition region in which both free and associated copolymer molecules coexist is defined to range from 1 to 2 mol L-1.     

Diffusion coefficients of small molecules at the gas-solid interface as measured by the nuclear magnetic resonance pulsed field gradient method

The nuclear magnetic resonance pulsed field gradient method has been used to measure the diffusion coefficients for some low molecular weight compounds adsorbed at the gas-solid interface. The systems studied are ammonia adsorbed upon graphite, methane adsorbed upon graphite, neopentane adsorbed upon graphite and neopentane adsorbed upon titanium dioxide. Results are compared with values obtained from quasi-elastic neutron scattering where available.     

Colloidlike behaviour of a polystyrene-b-poly(ethylene-co-propylene) diblock copolymer dissolved in n-decane investigated by pulsed field gradient nuclear magnetic resonance

The self-diffusion of a polystyrene-b-poly(ethylene-co-propylene) diblock copolymer dissolved in a preferential solvent for the aliphatic block, n-decane, was investigated by pulsed field gradient NMR. The diblock copolymer forms micelles in solution, the structure of the solid polymer being preserved in the native solution because the polystyrene is in the glassy state. The equilibrium state is attained upon heating which again freezes in upon cooling to room temperature. The hydrodynamic radius of the micelles decreases by about 50% during this heating–cooling process. The concentration dependence of the self-diffusivity shows typical colloidlike behaviour, and it can be described by a Vogel–Fulcher–Tammann-like equation. No indications of crystallization at higher concentrations are observed in the micellar solution because the micellar sizes are slightly polydisperse. The self-diffusivity was measured up to the glasslike state, where in-cage- diffusion and dynamic heterogeneities could be detected. Received: 14 April 1999 Accepted in revised form: 14 June 1999     

Quadrupolar nuclear magnetic resonance spectroscopy in solids using frequency-swept echoing pulses

The acquisition of ideal powder line shapes remains a recurring challenge in solid-state wideline nuclear magnetic resonance (NMR). Certain species, particularly quadrupolar spins in sites associated with large electric field gradients, are difficult to excite uniformly and with good efficiencies. This paper discusses some of the opportunities that arise upon departing from standard spin-echo excitation approaches and switching to echo sequences that use low-power, frequency-swept radio frequency (rf) pulses instead. The reduced powers demanded by such swept rf fields allow one to excite spins in different crystallites efficiently and with orientation-independent pulse angles, while the large bandwidths of interest that are needed by the measurement can be covered, thanks to the use of broadband frequency sweeps. The fact that the spins' evolution and ensuing dephasing starts at the beginning of such rf manipulation calls for the use of spin-echo sequences; a number of alternatives capable of providing the desired line shapes both in the frequency and in the time domains are introduced and experimentally demonstrated. Sensitivity- and lineshape-wise these experiments are competitive vis-a-vis current implementations of wideline quadrupolar NMR based on hard rf pulses; additional opportunities that may derive from these ideas are also briefly discussed.     

Theory of covariance nuclear magnetic resonance spectroscopy

Covariance nuclear magnetic resonance (NMR) spectroscopy provides an effective way for establishing nuclear spin connectivities in molecular systems. The method, which identifies correlated spin dynamics in terms of covariances between 1D spectra, benefits from a high spectral resolution along the indirect dimension without requiring apodization and Fourier transformation along this dimension. The theoretical treatment of covariance NMR spectroscopy is given for NOESY and TOCSY experiments. It is shown that for a large class of 2D NMR experiments the covariance spectrum and the 2D Fourier transform spectrum can be related to each other by means of Parseval's theorem. A general procedure is presented for the construction of a symmetric spectrum with improved resolution along the indirect frequency domain as compared to the 2D FT spectrum.