nuclear magnetic resonance

A rear view of the brain using a technique called magnetic resonance imaging (MRI). This picture can be interpreted to give an understanding of the status of the brain. The radio signals given off by the nuclei of the atoms in the brain indicate each type of tissue present in the brain.

Physical phenomenon allowing the properties of the nuclei of certain atoms to be probed by radio waves with the help of very strong magnetic fields. NMR spectroscopy was originally a method used mainly in organic chemistry, but since the 1990s, the availability of large superconducting electromagnets and of computer-based data analysis made it possible to use it for ever larger molecules, and it now rivals crystallography as a means to determine structure and dynamics of proteins and nucleic acids. NMR is also the basis of magnetic resonance imaging (MRI) in medical diagnosis.

NMR is based on the observation that in atoms with an odd number of protons or neutrons, such as 1H, 13C, 15N, the spin of a nucleus will orient in certain ways when exposed to a strong magnetic field. There will be an extremely small energy difference between two different states, and the higher energy state will be populated by fewer nuclei. Radio waves can flip a spin into the more energetic state. In turn, when it falls back to the ground state, it will emit a radio wave.

As the most common nucleus to be studied by NMR is hydrogen, instruments are classified by the frequency at which hydrogen nuclei can be excited. In 2007, top of the range instruments had a magnetic field of 22.3 Tesla, corresponding to a hydrogen frequency of 950 MHz.

Chemical and biochemical applications of NMR make use of the fact that the precise frequency of the resonance may be shifted by a few millionths depending on the electron density surrounding the nucleus in question. This difference is called chemical shift and measured in ppm (parts per million). A simple NMR spectrum records chemical shift on the X axis and peak intensity on the Y axis. Additionally, there are interactions between resonating nuclei, known as coupling. In a simple spectrum, coupling will split up peaks into characteristic multiplets.