In analytical chemistry, there are several techniques used to decode the structure, composition and properties of molecules. Among these techniques, nuclear magnetic resonance (NMR) spectroscopy has carved out a niche as the ‘gold standard’ of molecular structure determination.
But what exactly makes NMR stand out when compared to other popular techniques such as mass spectrometry, infrared spectroscopy and X-ray crystallography?
Understanding NMR
NMR operates by exposing a sample to a strong magnetic field and radiofrequency radiation, causing the nuclei in the atoms to resonate at specific frequences. These frequencies are influenced by the molecule’s structure, allowing NMR to provide detailed insights into atomic connectivity, stereochemistry and molecular interactions.
NMR vs mass spectrometry
Mass spectrometry (MS) excels in determining the molecular weight and composition of a molecule by ionising and fragmenting it. However, whilst MS is a precise method for identifying molecular mass and detecting small molecules, it struggles to offer complete 3D structural information of a sample.
NMR, on the other hand, excels in providing a full map of the molecule’s structure and has the additional benefit of being non-destructive, whereas MS often alters or fragments the sample during analysis.
MS is beneficial for detecting known substances at trace amounts and its key uses include detecting drug use in professional sports. However, NMR should be used when the substance isn’t known and for more complex research. For example, pharmaceutical companies use NMR to determine how a new drug interacts with its target protein, providing crucial insights into molecular dynamics.
NMR vs infrared spectroscopy
Infrared spectroscopy (IR) detects the functional groups in a molecule by measuring how it absorbs infrared light, which provides insights into the presence of certain bonds, like C=O or N-H. However, IR is limited in its ability to reveal the entire structure of the molecule. NMR offers more comprehensive structural information here, including the connectivity and arrangement of atoms, making it invaluable for detailed analysis.
IR is often utilised in the food industry to analyse the composition of fats, proteins and carbohydrates in different food types. For example, IR can quickly determine the fat content in milk, ensuring quality and consistency during production. However, for more complex food research, such as analysis of the quality of olive oil, NMR is a more reliable method, as it can differentiate between different types of fatty acids and quantify them precisely.
NMR vs X-ray crystallography
X-ray crystallography provides highly accurate 3D structures, particularly for large biomolecules like proteins. However, its major limitation is that it requires crystallisation, which can be difficult and time-consuming in comparison to NMR.
NMR and X-ray crystallography have different uses and benefits but can often be combined for a comprehensive understanding of a sample. For example, in the pharmaceutical industry, both could be combined for developing a new drug.
In this instance, NMR spectroscopy would be used initially to study binding interactions between the drug and its target protein. After optimising the drug based on these findings, X-ray crystallography would be used to determine the crystal structure of the protein and drug to further optimise its effectiveness.
Key advantages of NMR
Non-destructive analysis
NMR is non-invasive, preserving the sample for future analysis, unlike MS and X-ray crystallography, which may alter or destroy the sample.
Comprehensive 3D structure
NMR provides detailed 3D molecular structure information, revealing connectivity, stereochemistry and interactions at the atomic level, making it the go-to technique for understanding organic molecules and biomolecules.
Dynamic information
NMR can analyse molecules in solution, offering insights into how they behave in different environments and revealing molecular flexibility and real-time interactions.
Versatility
NMR is applicable to a wide variety of samples, including liquids, solids and gases and is especially valuable in studying large biomolecules like proteins and nucleic acids without the need for crystallisation.
While other techniques have their strengths, NMR’s ability to provide complete, non-destructive 3D structural information in versatile conditions makes it stand out from the crowd.
Essential in fields from organic chemistry to drug discovery, NMR offers unparalleled insights into molecular structures and dynamics.
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