Applications of Nuclear Magnetic Resonance (NMR) Spectroscopy

 

Nuclear Magnetic Resonance (NMR) Spectroscopy is one the most reliable, efficient, and sophisticated technique used in different areas of scientific research.

What is NMR?

NMR is the study of the magnetic properties of a nucleus and its interaction with radio waves. Nuclei having some value of spin are studied in NMR. Some of the most studied nuclei are ¹H, ¹³C, ¹⁹F, ¹⁵N etc., but ¹H and ¹³C are more frequently used by researchers. The NMR active nuclei arrange themselves in all possible orientations under the influence of a strong magnetic field (Superconducting magnet) and starts precessing (movement like a top, just before it falls) at the same time, around their axes, with a particular frequency (Larmour frequency). A strong burst of radio waves is passed through these nuclei, which absorb frequencies that match their precessional frequency (Resonance), and get excited. They relax back by different relaxation mechanisms (longitudinal and transverse) and give signals in the form of peaks in the spectrum. Every different type of nucleus has its own precessional frequency and thus absorb radio waves of different frequencies, thus we can differentiate between different types of nuclei (Chemically or magnetically inequivalent) in one sample. Various techniques developed in NMR are 1D ¹H-NMR, 1D ¹³C-NMR, and 2D-NMR (COSY, NOESY, HMBC, HMQC, and DOESY etc.), each furnishing different type of information.

Structure Elucidation

Chemists are the largest user group of NMR. In chemistry, NMR is employed for the structure determination of compounds. All the above mentioned NMR techniques afford different information about the chemical structure of a compound, the bits and pieces are then combined to get the complete structure. Natural product chemists find it useful in structure elucidation of unknown compounds isolated from natural sources.

Medicine

NMR also finds its applications in biomedical science. Magnetic resonance imaging is a diagnostic tool used for the diagnosis of injuries and other disorders. MRI is actually an NMR machine, with a probe big enough to accommodate the entire human body. Basically it is the study of the ¹H nucleus found in water molecules, present in and around the disordered area. Water molecules bound to the tissues have a different behavior in MRI than those which are lying free. A picture is generated which clearly shows different shades for the two different types of water molecules and thus the radiologist can determine whether any disorder exists in the body part under study.

Structure of Protein

NMR is now widely used in structural biology for determination of protein structure.

Epitope Mapping

The interaction of enzyme with a molecule can be correctly identified by using NMR. This is done by utilizing the Saturation Transfer Difference (STD) technique, which is actually based on NOE studies. The part of the molecule interacting with the active site of the enzyme can be identified by STD-NMR.

Hyphenated techniques

NMR can be coupled with chromatographic techniques. Chromatography is a separation and purification technique. Mixture of compounds can be loaded on to a chromatographic column which separates them and sends them one by one to the NMR machine where they are analyzed. But NMR is a slower technique, therefore it requires an interface, usually SPE, for connectivity with the chromatographic technique used.