Preferential solvation of carbohydrates in water-trifluoroethanol mixtures: a solvent detected heteronuclear NMR approach.

The present study aims to establish a simple approach involving multi-field multinuclear longitudinal relaxation (R1) analysis of the solvents to decipher solute-solvent interactions during the solvation of model carbohydrates in aqueous trifluoroethanol (TFE) co-solvent systems (TFE:D2O). The behavior of D2O and TFE is monitored around ß-CD (ß-cyclodextrin) and glucose through R1D (2H) and R1F (19F), respectively. Correlation times (τc) are estimated for D2O and TFE for various % (v/v) compositions of TFE:D2O mixtures. The differential trends of the R1 or τc ratio for D2O and TFE (in the presence and absence of carbohydrates) revealed that both ß-CD and glucose undergo selective solvation by TFE in comparison to D2O. Owing to its encapsulation properties, ß-CD exhibited a comparatively higher tendency to undergo solvation by TFE than glucose. The maximum transfer of solute bound water to bulk solvent appears in the 20-30% (v/v) TFE range. The current approach emerges as being straightforward in contrast to traditional methods that primarily focus on solute behavior to unravel the preferential solvation dynamics.

Magnetic resonance imaging of enhanced mobility of light non aqueous phase liquid (LNAPL) during drying of water wet porous media.

Visualization of NAPLs in multiphase systems in porous media is important for determining contaminant transport in the environment. In this study, magnetic resonance imaging (MRI) was used to confirm the recent observations of mobilisation of a light non aqueous phase liquid (LNAPL) trapped in wet sand under natural drying conditions of the wet porous medium. Visualization of LNAPL (motor oil) and water mobility during the drying of wet glass beads (0.5 mm) in a cylindrical glass column (15 mm ID, 45 mm long) was obtained using spin echo-based NMR microimaging performed at 500 MHz, corresponding to a field of ca. 11.75 T. Sagittal and axial images of LNAPL and water in the porous medium were obtained at a spatial resolution of 59 µm/pixel at different time intervals. A rise of 15-20 mm was observed in the presence of evaporation of water as compared to a 2-3 mm rise in the absence of evaporation in a time span of about 1400 min. The spatio-temporal MRI scans of the water and LNAPL in the glass column reveals that LNAPL rise occurs when the water evaporation front reaches the LNAPL layer. This implied that the enhanced LNAPL rise was strongly linked to the process of water evaporation. A linear correlation of the MRI signal intensities of LNAPL and water with reference to different saturation levels of LNAPL and water in the porous media was obtained. This calibration information was used to quantify the saturation levels of the LNAPL and water during the drying process. These findings show the application of non-invasive techniques such as MRI in quantifying and understanding the mechanism of fate and transport of LNAPLs in porous media, towards effective environmental quality assessment.

Transferred Overhauser DNP: A Fast, Efficient Approach for Room Temperature 13C ODNP at Moderately Low Fields and Natural Abundance.

Overhauser dynamic nuclear polarization (ODNP) is investigated at a moderately low field (1.2 T) for natural abundance 13C NMR of small molecules in solution state at room temperature. It is shown that ODNP transferred from 1H to 13C by NMR coherence transfer is in general significantly more efficient than direct ODNP of 13C. Compared to direct 13C ODNP, we demonstrate over 4-fold higher 13C sensitivity (signal-to-noise ratio, SNR), achieved in one-eighth of the measurement time by transferred ODNP (t-ODNP). Compared to the 13C signal arising from Boltzmann equilibrium in a fixed measurement time, this is equivalent to about 1500-fold enhancement of 13C signal by t-ODNP, as against a direct 13C ODNP signal enhancement of about 45-fold, both at a moderate ESR saturation factor of about 0.25. This owes in part to the short polarization times characteristic of 1H. Typically, t-ODNP reflects the essentially uniform ODNP enhancements of all protons in a molecule. Although the purpose of this work is to establish the superiority of t-ODNP vis-à-vis direct 13C ODNP, a comparison is also made of the SNR in t-ODNP experiments with standard high resolution NMR as well. Finally, the potential of t-ODNP experiments for 2D heteronuclear correlation spectroscopy of small molecules is demonstrated in 2D 1H-13C HETCOR experiments at natural abundance, with decoupling in both dimensions.

Slow Molecular Motions in Ionic Liquids Probed by Cross-Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization.

Solution-state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron-nuclear cross-relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross-relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three-spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the 19 F/1 H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub-megahertz (MHz) to tens of MHz range-motions which are not accessed at high fields.

Quantification in Hyperpolarized NMR.

Quantitative aspects of hyperpolarized NMR are analyzed in the present work, and it is shown theoretically and experimentally that measured "apparent" signal enhancements could deviate significantly from real enhancements of polarization. Expressions are given as a function of spin count to deduce real enhancements from measured "apparent" enhancements, and vice versa. While the findings are of particular relevance to high-field work employing high-Q probes, and to analytical applications of hyperpolarized NMR whose objective is the measurement of spin count, our experiments demonstrate their significance even for low- and moderate-field work with probes of moderate Q-factor.

Two-dimensional nuclear magnetic resonance: exploiting spin echoes to maximize information content by suppression of diagonal peaks in homonuclear experiments.

Two-dimensional nuclear magnetic resonance (2D NMR) correlation spectra help visualize inter- or intra-molecular spin connectivity through space or through bonds. This is accomplished by magnetization transfer between interacting ("connected") spins located at different sites in molecules. In homonuclear 2D experiments, cross peaks which demonstrate spin connectivity and result from magnetization transfer between sites are unfortunately invariably accompanied by other peaks that result from magnetization that has not undergone any transfer, viz., diagonal peaks. The latter can often mask close-lying cross peaks. We report here the general principles that constitute a design strategy for diagonal suppression, relying on echo formation. Next, a novel experiment that effects diagonal suppression in the high-resolution mode is demonstrated. Pure phase capability is also introduced. Examples from both 2D exchange and high-resolution 2D correlation spectroscopy are included, and the proposed method is compared with other established as well as recent attempts to accomplish diagonal suppression.

Chemical-shift-resolved ¹9F NMR spectroscopy between 13.5 and 135 MHz: Overhauser-DNP-enhanced diagonal suppressed correlation spectroscopy.

Overhauser-DNP-enhanced homonuclear 2D (19)F correlation spectroscopy with diagonal suppression is presented for small molecules in the solution state at moderate fields. Multi-frequency, multi-radical studies demonstrate that these relatively low-field experiments may be operated with sensitivity rivalling that of standard 200-1000 MHz NMR spectroscopy. Structural information is accessible without a sensitivity penalty, and diagonal suppressed 2D NMR correlations emerge despite the general lack of multiplet resolution in the 1D ODNP spectra. This powerful general approach avoids the rather stiff excitation, detection, and other special requirements of high-field (19)F NMR spectroscopy.

(119/117/115)Sn low-abundance single-transition correlation spectroscopy (LASSY): sensitivity-enhanced homonuclear correlation experiments for Sn NMR.

We report the implementation of our novel rare-spin homonuclear correlation experiment, namely, Low-Abundance Single-transition correlation SpectroscopY (LASSY), for (119/117/115)Sn NMR at natural abundance. Our pulse sequence results in diagonal suppressed COSY-style display and outperforms the optimal homonuclear correlation experiment for rare spins, which involves double quantum evolution (INADEQUATE CR). The new experiment maximizes efficiency both in respect of pulse transformations as well as relaxation effects, and gives rise to a simplified two-dimensional (2D) spectrum with considerably reduced crowding, exhibiting only one transition in each cross peak, instead of four. Performance optimization of LASSY is carried out in light of the relatively 'large' line widths typical of Sn NMR in solution state. The superior performance of the sequence is demonstrated on dimeric tetraorganodistannoxane samples.

Postharvest ripening study of sweet lime (Citrus limettioides) in situ by volume-localized NMR spectroscopy.

Spatially resolved NMR--especially volume-localized spectroscopy (VLS)is useful in various fields including clinical diagnosis, process monitoring, etc. VLS carries high significance because of its ability to identify molecular species and hence track molecular events. This paper reports the application of VLS at 200 MHz to study the postharvest ripening of sweet lime ( Citrus limettioides ) in situ, including a comparative study of normal and acetylene-mediated ripening. Localization to a cubic voxel of 64 microL was achieved with point-resolved spectroscopy (PRESS). Glucose, sucrose, fructose, and citric acid are found to be among the main constituents in the fruit. In the natural process, the sugar to acid ratio increases with ripening. Ethanol generation is seen to occur at a faster rate in acetylene-mediated ripening. Whereas NMR imaging experiments including parametric imaging (e.g., T(1) or T(2) maps) may be employed for "macro" monitoring of processes such as these, this work demonstrates that the molecular imprint of the process may be tracked noninvasively by VLS.

1H detected (13)C echo planar imaging.

Cyclic J cross polarisation (CYCLCROP) is a sensitive method for the noninvasive monitoring of (13)C distributions and fluxes. The PRAWN rotating frame Hartmann-Hahn mixing sequence ameliorates problems associated with sensitivity to Hartmann-Hahn mismatch and reduces RF power deposition. The combination of CYCLCROP with echo planar imaging (EPI) for spatial encoding of the proton detected carbon signal allows efficient use of the available signal to be made, permitting a significant improvement in the temporal resolution of any study. We report here on some initial experiments to demonstrate the feasibility of echo planar proton detected (13)C imaging using CYCLCROP based upon the PRAWN module, including the application of the technique to the measurement of transport and accumulation of (13)C-labelled sucrose in a castor bean seedling. Two methods that can be used to eliminate the effect of the J-splitting in the EP images are presented. In addition, a fast, image-based B(1) field-mapping method which may be used to quantitatively map the low frequency RF field in a dual resonant ((13)C/(1)H) probe is presented. The technique utilises the above described imaging method, permitting fully quantitative, 64x64 axial field maps to be generated in about a minute.