Recent Developments in Analytical Science - NMR Spectroscopy

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  • Published: Jul 18, 2016
  • Channels: Electrophoresis / Ion Chromatography / Gas Chromatography / Proteomics & Genomics / HPLC / Proteomics / Base Peak / X-ray Spectrometry / Raman / MRI Spectroscopy / Infrared Spectroscopy / Atomic / NMR Knowledge Base

NMR

NMR spectroscopy is a versatile, non-destructive technique that is often used to complement structural information gleaned by mass spectrometry and other techniques, as well as operating in its own right. It has the flexibility to analyse gases, liquids and solids and new advances are constantly being introduced to steer it forward.

2D NMR is one of the many operating modes, the addition of the second dimension simplifying complex spectra such as those from proteins which display many overlapping signals. Both homonuclear and heteronuclear spectra can be recorded, following one or more nuclei, respectively. The homonuclear experiments can be operated in different forms. 2D Correlation spectroscopy (COSY) determines the correlations between coupled spins whereas 2D total correlation spectroscopy (TOCSY) measures correlations between all spins in the molecule. Nuclear Overhauser effect spectroscopy (NOESY) measures resonances between nuclei that are close to each other rather than bonded together. The three types have all been used to determine the structure of a fungal plant pathogen.32

Two modes applied in heteronuclear studies are heteronuclear single quantum correlation (HSQC) and heteronuclear multiple-bond correlation spectroscopy (HMBC). The former measures the correlations between protons and atoms such as 13C and 15N to which they are directly bonded. This is especially important for protein structural studies.33 HMBC measures long-range correlations between protons and carbon atoms that are generally 2-3 bonds apart.

The majority of 2D NMR experiments have exploited proton resonances because they provide greater sensitivity but instrument improvements mean that carbon is becoming more competitive.34 Better probes, higher field magnets and greater computer memory and storage are all contributing to its resurgence.

A technique called comprehensive multiphase NMR has also been developed that will allow the components in solution, the gel phase and the solid state of one sample to be analysed together in situ. It combines the characteristics of low-power probes with the properties of solid-state probes, eliminating the need for time-consuming exchange and calibration of individual probes. The performance was evaluated on intact Arabidopsis thaliana seedlings and stems to reveal cell wall metabolites.35

NMR has been employed in metabolomics studies of plants or biological fluids where it is often compared to mass spectrometry. Its principal advantages are that sample preparation is simpler, derivatisation is not required, and the technique is quantitative. However, it fails to reach the sensitivity of mass spectrometry, despite recent innovations in scanning techniques and hardware.36 The same deficiency has been noted for other biomolecules and a number of potential improvements have been considered. They included the optimisation of pulse sequences, better probes and coils, stronger magnetic fields and amended spin alignment modes.37

In that vein, a new superconducting magnet has been designed with a field of 1020 MHz (24.0 T), the highest magnetic field available in high resolution NMR superconducting magnets. The temporal field stability and spatial field homogeneity proved to be adequate for both solid-state and solution NMR.38 However, small magnets are also in demand so that the move towards benchtop NMR spectrometers can be supported. Rare earth magnets can provide sufficient resolution and sensitivity for many applications and the benchtop systems offer 1D and 2D measurements on a number of nuclei.

NMR has been coupled with different separation and analytical techniques to provide more comprehensive analyses in a shorter time. HPLC coupled directly to NMR is limited by low sensitivity and shorter available times for spectral acquisition. So, more intricate systems such as HPLC-UV-MS-SPE-NMR have been developed which split the HPLC fractions for analysis by mass spectrometry and collection by SPE for subsequent NMR study. Combined techniques like this will accelerate plant and human metabolomics studies.39

Electrochemistry-NMR was devised to study reactions such as oxidation and reduction and can be deployed in online and in situ modes. A number of different cell and electrode designs have been successful and future developments may include coupling an HPLC column to prevent signal overlap. More complex NMR sequences and ultrafast 2D NMR which can be coupled with LC-NMR experiments will allow different nuclei to be studied.40

Nuclear magnetic resonance imaging, better known as MRI, was born out of NMR spectroscopy and has become firmly established in the clinical arena for disease detection, diagnosis and treatment. Examining patients in vivo, it exploits the proton resonances from water molecules in the body to provide detailed anatomical images. Diffusion MRI and the recently developed diffusion tensor imaging follow the diffusion of water through the tissue whereas functional MRI measures blood flow in the brain and other organs. MRI is non-invasive although some patients feel claustrophobic within the machine.

High-field MRI systems have been developed for research and they might provide the ability to visualise changes at the cellular level. They would be able to assist in drug development studies and reveal the composition of living tissue in greater detail. MRI performance will be improved by the development of cell-specific contrast agents that will improve sensitivity at local levels.

Technological advances and an increasingly aging population will help to increase the global MRI system market from USD 5.61 billion in 2016 to USD 7.19 billion by 2021.

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