Nuclear magnetic resonance spectroscopy (NMR) is a sophisticated research technique used to obtain detailed information about the structure, dynamics, reaction state, and chemical environment of molecules.
How NMR works
The principle behind NMR is that, in addition to being electrically charged, many nuclei also have spin. When such nuclei are exposed to an external magnetic field, energy transfers from the base energy level to a higher energy level. The reverse happens on removal of the magnetic field and energy is emitted at the same frequency as the energy that was absorbed.
Furthermore, the energy transfer takes place at a wavelength that corresponds to radio frequencies. Since the frequency depends on the intramolecular magnetic field around an atom, measuring the frequency can be used to determine the environment of atoms within the protein and the distance between nuclei, whereby a picture of the structure of a molecule is obtained.
The determination of structure from NMR is a highly technical multi-phase process. Interpretive approaches must be applied to find out which chemical shift corresponds to which nuclei in the spin system, then a structure is calculated based on known and experimentally determined properties of proteins.
Since the quality of the model achieved depends on both the quantity and quality of experimental data used to generate it and the correct interpretation of such data, it is also important that the structure is validated. Validation is essentially a check for errors and is based on established statistical and/or physics principles.
NMR in biology
In biology, NMR is fundamental for determining and exploring the structure of proteins, e.g. enzymes, receptors. It has been used to elucidate the structure and function of numerous biological components. More recent discoveries include determination of the structure of the influenza virus proton transporter and the consequent development of molecules that block this transporter and stall infection by the virus.
Similarly determination of the structure of the CMAT receptor (an oncogene that is involved in cancer metastasis) has allowed a variant to be engineered that has an antagonistic effect. NMR thus plays a key role in the development of vaccines and treatments for a range of diseases, including HIV, influenza, tuberculosis and cancer, by allowing researchers to understand how the disease-causing agents function and to identify potential drug targets.
NMR also provides a powerful tool for studying conformational changes in proteins and chemical kinetic processes, which enables the mode of action of enzymes, transporter proteins etc to be elucidated at a molecular level. For example, it was discovered that dimerization is crucial for the ECAD protein to create adhesion between cells in multicellular organisms.
Similarly, NMR is being used to characterise the transition of proteins in the brain and to investigate the influence of such changes on neurodegenerative disease processes, such as Alzheimer's disease, Parkinson's disease, and Creutzfeldt-Jakob disease. Ultimately, such NMR studies may again lead to the development of tests and treatments for diseases in man.
Professor Michael Summers of The Howard Hughes Medical Institute explained:
NMR is the central tool we use... We've been able to learn a lot using NMR that I don't think you'd be able to learn using any other technology.
The level of detail provided by NMR has made it a fundamental tool for characterising the structure and function of proteins across many areas of research. However, NMR is a highly specialized technique and structure determination by NMR spectroscopy requires in-depth knowledge of chemistry and physics.
Recently, software has been developed (e.g. TopSolids™) making NMR accessible to a broad diversity of users. Furthermore, online NMR libraries are also available that can be searched to see if the sequence needed has already been determined so the measurements need not be repeated.
When conducting NMR it is important to obtain the best spectra possible (within the constraints of the experimental parameters). The key factors impacting the quality of the resultant data are sensitivity, resolution and the number of samples taken.
The development of non-uniform sampling (NUS) methodologies has allowed fewer samples to be taken without compromising resolution. Algorithms, such as NESTA, are used to estimate values for the missing points where measurements were not made. Such techniques allow structures to be determined more quickly and have demonstrated excellent performance sensitivity. However, it is still important to invest sufficient time in sampling to ensure the highest quality data.
The future of NMR
NMR is clearly a powerful tool in biological research, but researchers are still striving for greater sensitivity and resolution to broaden the scope of potential applications. Consequently, new NMR techniques are constantly being developed.
The latest advances (in addition to increased magnetic field strength) include refrigerated magnet technology that allows continuous NMR operation without the need for cryogen refilling, giving greater experiment flexibility and small-angle x-ray scattering (SAXS) NMR that provides more structural detail than NMR alone and can be performed using smaller samples.
Professor Arthur Palmer of Columbia University commented:
We need access to state-of-the art electronics, state-of-the art probes and state-of-the art magnets for everything we do...Each step to a higher and higher magnetic field has opened up new opportunities, and I think this is going to continue.
NMR spectroscopy is continually providing additional information that can be used by pharmaceutical researchers to develop treatments to prevent or combat a range of diseases, and technological advances continue to extend the boundary of capability of this already formidable technique.
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