The nuclear
overhauser effect (or NOE) is a special case of
nuclear relaxation in
nuclear magnetic resonance (NMR) spectroscopy and
magnetic resonance imaging (MRI). In a typical NMR or MRI experiment, a nucleus in a
magnetic field absorbs
photons of a
frequency mostly determined by the
elemental identity of the nucleus (e.g.
hydrogen,
carbon etc.) and the strength of the applied magnetic field (see
Zeeman effect.) Nuclei so treated are said to be in an
excited nuclear spin state, and they literally spin like a
top in said magnetic field. They can be observed by the
current their spinning generates in wire coils looped around the sample compartment, be it an
NMR tube or the huge cavernous opening in an
MRI device.
But they don't spin forever. One nucleus in an excited spin state can transfer its magnetization to another nucleus in a process called dipolar coupling. The rate at which this happens between a given pair of nuclei is
inversely proportional to the cube of the
distance between. It's also a complicated
function of the
angle between the line formed by the two atoms and the external magnetic field. Since most NMR experiments are performed in solution, and most MRI experiments involve observation of the protons on water molecules in a solution-like environment, this angle varies considerably during the time course of the experiment, as the
molecules being observed are moving and spinning all over the place. This is called
isotropic rotational averaging, and under these conditions the rate of magnetization transfer is independent of the angle between the magnetic field and the internuclear vector, but inversely proportional to the
sixth power of the distance between them. Magnetization transfer by dipolar coupling under the conditions of isotropic rotational averaging is called the
nuclear overhauser effect. This is the principal pathway by which any nucleus being observed in solution loses its magnetization, thus it is the main factor contributing to nuclear relaxation. Note that the magnitude of both the NOE and dipolar coupling are independent of the nature of the intervening
medium, they are
through-space interactions, as opposed to through-bond interactions like
J-coupling.
This may seem pretty esoteric but in fact it forms the basis, along with J-coupling, for the determination of
protein,
DNA and
RNA structures and their interactions with other molecules. One merely excites some particular nucleus, waits a period of time, and observes where the magnetization has been transferred. Because of the aforementioned distance dependence of the magnetization transfer, this allows the 3-dimensional
mapping of the molecule when the experiment is repeated for a sufficient percentage of the nuclei in a molecule. The method of NMR has overtaken that of
x-ray crystallography as the method of choice for
rational drug design not because of its ability to determine structures, but because the nuclear overhauser effect allows one to quickly and easily observe the interaction of these
biomolecules with small molecule drug candidates.