Hashim Farooq

Comprehensive multiphase NMR spectroscopy: Basic experimental approaches to differentiate phases in heterogeneous samples

Heterogeneous samples, such as soils, sediments, plants, tissues, foods and organisms, often contain liquid-, gel- and solid-like phases and it is the synergism between these phases that determine their environmental and biological properties. Studying each phase separately can perturb the sample, removing important structural information such as chemical interactions at the gel-solid interface, kinetics across boundaries and conformation in the natural state. In order to overcome these limitations a Comprehensive Multiphase-Nuclear Magnetic Resonance (CMP-NMR) probe has been developed, and is introduced here, that permits all bonds in all phases to be studied and differentiated in whole unaltered natural samples. The CMP-NMR probe is built with high power circuitry, Magic Angle Spinning (MAS), is fitted with a lock channel, pulse field gradients, and is fully susceptibility matched. Consequently, this novel NMR probe has to cover all HR-MAS aspects without compromising power handling to permit the full range of solution-, gel- and solid-state experiments available today. Using this technology, both structures and interactions can be studied independently in each phase as well as transfer/interactions between phases within a heterogeneous sample. This paper outlines some basic experimental approaches using a model heterogeneous multiphase sample containing liquid-, gel- and solid-like components in water, yielding separate (1)H and (13)C spectra for the different phases. In addition, (19)F performance is also addressed. To illustrate the capability of (19)F NMR soil samples, containing two different contaminants, are used, demonstrating a preliminary, but real-world application of this technology. This novel NMR approach possesses a great potential for the in situ study of natural samples in their native state.

HR-MAS NMR Spectroscopy: A Practical Guide for Natural Samples

High Resolution Magic Angle Spinning (HR-MAS) NMR spectroscopy is a versatile technique that provides high resolution NMR data on samples containing solutions, gels and swellable solids. In addition to providing structural information HR-MAS can also be used to investigate interfaces (for example organic structures at the solid-aqueous soil interface), processes such as swelling/flocculation, kinetic transfer between gel/liquid phases, as well as conformation and molecular interactions in situ. As such, HRMAS has potential in a diversity of fields including organic, biological, environmental, and medical research. This manuscript focuses on the application of HR-MAS to intact natural samples. A range of 1D and 2D NMR experiments are reviewed and compared in terms of general performance on a range of samples. Therefore, this practical guide and review should be a useful reference and starting point for those wishing to apply HR-MAS NMR. While this article focuses on natural samples, common key 1D and 2D experiments are considered which may be of interest to readers with diverse research interests.

Soil Organic Matter in Its Native State: Unravelling the Most Complex Biomaterial on Earth

Since the isolation of soil organic matter in 1786, tens of thousands of publications have searched for its structure. Nuclear magnetic resonance (NMR) spectroscopy has played a critical role in defining soil organic matter but traditional approaches remove key information such as the distribution of components at the soil-water interface and conformational information. Here a novel form of NMR with capabilities to study all physical phases termed Comprehensive Multiphase NMR, is applied to analyze soil in its natural swollen-state. The key structural components in soil organic matter are identified to be largely composed of macromolecular inputs from degrading biomass. Polar lipid heads and carbohydrates dominate the soil-water interface while lignin and microbes are arranged in a more hydrophobic interior. Lignin domains cannot be penetrated by aqueous solvents even at extreme pH indicating they are the most hydrophobic environment in soil and are ideal for sequestering hydrophobic contaminants. Here, for the first time, a complete range of physical states of a whole soil can be studied. This provides a more detailed understanding of soil organic matter at the molecular level itself key to develop the most efficient soil remediation and agricultural techniques, and better predict carbon sequestration and climate change.

Rapid estimation of nuclear magnetic resonance experiment time in low‐concentration environmental samples

Nuclear magnetic resonance (NMR) spectroscopy is an essential tool for studying environmental samples but is often hindered by low sensitivity, especially for the direct detection of nuclei such as(13) C. In very heterogeneous samples with NMR nuclei at low abundance, such as soils, sediments, and air particulates, it can take days to acquire a conventional(13) C spectrum. The present study describes a prescreening method that permits the rapid prediction of experimental run time in natural samples. The approach focuses the NMR chemical shift dispersion into a single spike, and, even in samples with extremely low carbon content, the spike can be observed in two to three minutes, or less. The intensity of the spike is directly proportional to the total concentration of nuclei of interest in the sample. Consequently, the spike intensity can be used as a powerful prescreening method that answers two key questions: (1) Will this sample produce a conventional NMR spectrum? (2) How much instrument time is required to record a spectrum with a specific signal-to-noise (S/N) ratio? The approach identifies samples to avoid (or pretreat) and permits additional NMR experiments to be performed on samples producing high-quality NMR data. Applications in solid- and liquid-state(13) C NMR are demonstrated, and it is shown that the technique is applicable to a range of nuclei.

Rapid parameter optimization of low signal-to-noise samples in NMR spectroscopy using rapid CPMG pulsing during acquisition: application to recycle delays.

A method is presented that combines Carr–Purcell–Meiboom–Gill (CPMG) during acquisition with either selective or nonselective excitation to produce a considerable intensity enhancement and a simultaneous loss in chemical shift information. A range of parameters can theoretically be optimized very rapidly on the basis of the signal from the entire sample (hard excitation) or spectral subregion (soft excitation) and should prove useful for biological, environmental, and polymer samples that often exhibit highly dispersed and broad spectral profiles. To demonstrate the concept, we focus on the application of our method to T1 determination, specifically for the slowest relaxing components in a sample, which ultimately determines the optimal recycle delay in quantitative NMR. The traditional inversion recovery (IR) pulse program is combined with a CPMG sequence during acquisition. The slowest relaxing components are selected with a shaped pulse, and then, low‐power CPMG echoes are applied during acquisition with intervals shorter than chemical shift evolution (RCPMG) thus producing a single peak with an SNR commensurate with the sum of the signal integrals in the selected region. A traditional 13C IR experiment is compared with the selective 13C IR‐RCPMG sequence and yields the same T1 values for samples of lysozyme and riverine dissolved organic matter within error. For lysozyme, the RCPMG approach is ~70 times faster, and in the case of dissolved organic matter is over 600 times faster. This approach can be adapted for the optimization of a host of parameters where chemical shift information is not necessary, such as cross‐polarization/mixing times and pulse lengths. Copyright

Tailoring 1H Spin Dynamics in Small Molecules via Supercooled Water: A Promising Approach for Metabolite Identification and Validation

Metabolic mixtures are often analyzed via NMR spectroscopy as it provides a metabolic profile without sample alteration in a noninvasive manner. These mixtures however tend to be very complex and demonstrate considerable spectral overlap resulting in assignments that are sometimes ambiguous given the range of current NMR methods available. De novo molecular identification in these mixtures is generally accomplished using chemical shift information and J-coupling based experiments to determine spin connectivity information, but these techniques fall short when a molecule of interest contains nonrelaying centers. A method is presented here that enhances intramolecular spatial interactions via supercooled water and uses the resulting spatial correlations to edit mixtures. This is accomplished by utilizing nuclear Overhauser effect spectroscopy (NOESY) at subzero temperatures in capillaries to enhance NOE and provide more complete spin systems. This technique is applied to a standard mixture of three known molecules in D(2)O with overlapping resonances and is further demonstrated to assign molecules in a worm tissue extract. The current method proves to be a powerful complement to existing methods such as total correlation spectroscopy (TOCSY) to expand the range of molecules that can be assigned in situ without physical separation of mixtures.

Characterisation of oil contaminated soils by comprehensive multiphase NMR spectroscopy

Environmental context Novel technology is used to examine oil contaminated soil to better understand this longstanding problem. The data indicate that oil forms a non-discriminant layer over all the soil components, which in their natural state would be exposed to water, and that it retains certain polar compounds while contributing other oil contaminants to the surrounding porewater and groundwater. Such molecular level information helps to better understand the reoccurrence of hydrophobicity in remediated soil, and could lead to novel clean-up methods. Abstract Comprehensive multiphase (CMP) NMR spectroscopy is a novel NMR technology introduced in 2012. CMP NMR spectroscopy permits the analysis of solid, gel and liquid phases in unaltered natural samples. Here the technology is applied to control and oil contaminated soils to understand the molecular processes that give rise to non-wettable soils. 13C solid-state NMR spectroscopy is found to be excellent for studying the bulk rigid components of the soils whereas 1H solution and gel-state NMR provide a complimentary overview to subtleties occurring at the soil–water interface. Considered holistically the NMR data support the finding that the oil forms a non-discriminant layer over all the soil components, which in the natural state, would be exposed to water. Specifically, the oil was found to preferentially coat aliphatics and carbohydrates that normally stick out at the soil–water interface. In addition, it was shown that the oil forms a barrier that keeps small polar molecules such as formic acid inside the soil. At the soil–water interface selective oil components, such as asphaltenes, were found to exhibit unrestricted diffusion, suggesting that these components could leach into surrounding groundwater.

Water-mediated NOE: a promising tool for interrogating interfacial clay–xenobiotic interactions

BackgroundThe sorption of anthropogenic compounds on clay minerals is a complex molecular process with important implications for the fate of agrochemicals and organic pollutants in the environment.ResultsThe present study illustrates the use of a water-mediated NOE approach to study clay binding interactions. This method exploits the interfacial water layer on clay surfaces as a hydrogen reservoir for magnetization transfer. The interactions of four different xenobiotics with clay suspension were investigated through this method to demonstrate its capability to screen for the clay–xenobiotic molecular affinity. Further, based on the NOE build-up rates, epitope map of clay–xenobiotic interactions can be generated, explaining the orientation and mechanism of the interactions.ConclusionsThe water-mediated NOE approach has the potential to reveal key insights into the role that interfacial water plays in the binding process, providing a better understanding of the partitioning of anthropogenic compounds from bulk water into aqueous clay suspensions.Graphical abstract.