A Primer on the NMR of Biological Macromolecules

Nuclear magnetic resonance (NMR) refers to the ability of atomic nuclei to reorient in a magnetic field when exposed to radiation of a particular "resonant" frequency in the radio band.

Certain atomic nuclei ("NMR isotopes") contain charged particles with spin, which according to Maxwell's equations, induces a magnetic field. Though small, the magnetic "moments" of these nuclei makes them sensitive to an external magnetic field. In an NMR magnet, the nuclei act like tiny bar magnets and tend to align themselves preferentially in a particular configuration, while also undergoing spinning motions similar to the gyroscopic precessions of bicycle wheels or spinning tops under an external torque.

Any fluctuating magnetic field orthogonal to that of the NMR magnet will perturb the alignment of the nuclear magnetic moments away from the equilibrium configuration, but only if the frequency of the fluctuating field is precisely equal to the precession frequencies of the nuclear magnetic moments. These are called the resonant, or Larmor, frequencies and are proportional to the field strength of the NMR magnet. The Scripps Research Institute's (TSRI's) new 21 tesla magnet, for instance, causes protons to precess at 900 MHz. Movement of atomic nuclei in the NMR as they go in and out of resonance causes small but measurable induced voltages, and it is this signal which is being measured in the NMR experiment.

An NMR spectrometer will scan a broad range of radio frequencies and record all the resonances as a spectrum. Atoms like 1H, 13C, or 15N, which are ubiquitous in proteins and nucleic acids, have a nuclear spin and give rise to NMR signals, whereas atoms like 12C and 16O have no nuclear spin and therefore no signal. Different spectra can be taken with molecules that have been selectively labeled with isotopes that have or do not have a spin.

In an NMR experiment, a sample in a glass tube is inserted into the magnet, and the resonant responses of the atoms in the sample over a range of frequencies are recorded. These responses are influenced by the shape of the molecule in which the atoms reside—by their proximity to other atoms in the molecule. An NMR spectrum is unique for a particular molecule, and the structure of a molecule can be determined from its spectrum.

There is no question that NMR is one of the fundamental techniques in chemistry and biology today.

In fact, three Nobel prizes have now been awarded for work on the technique. The 1952 Nobel Prize in Physics went to two physicists, Edward Mills Purcell at Harvard University and Felix Bloch at Stanford University, who discovered the NMR effect independently in 1946. The 1991 Nobel Prize in Chemistry was awarded to Richard R. Ernst at ETH in Zürich for "the development of the methodology of high resolution nuclear magnetic resonance (NMR) spectroscopy."

Most recently, the 2002 Nobel Prize in Chemistry was awarded to TSRI's Kurt Wüthrich "for his development of nuclear magnetic resonance spectroscopy for determining the three-dimensional structure of biological macromolecules in solution."




NMR structure of the bovine prion protein, solved by TSRI investigator Kurt Wüthrich. For more images, see the Wüthrich home page.