In
nuclear magnetic resonance (NMR), J-coupling is a measure of the effect the spin-state of one nucleus has on the
chemical shift of another. Remember that in
NMR, you're observing the absorbance of
energy coupled to a change in alignment of the
nuclear spin from
parallel to
antiparallel to the external
magnetic field. J-coupling occurs when the amount of energy required to induce this spin-flip changes with the spin orientation of another nucleus. The coupling is measured and expressed (in
Hertz) as the difference between the observed
chemical shifts of the first nucleus when the second nucleus is in both of its possible spin states, and does not change with a change in the strength of the external magnetic field. (For the purposes of this discusion we will
conveniently ignore the possibility that one or both of the nuclei have a spin larger than 1/2 as that is A) too complicated and B) not currently an issue in most comtemporary biomedical NMR.)
J-coupling is a direct measure of the
orbital overlap between the two nuclei (see
Molecular Orbital Theory.) As such it reflects only
through-bond interactions (as opposed to the through-space interactions observed in the
nuclear overhauser effect and dipolar coupling.) The magnitude of the the observed J-coupling depends on the identity of the
nucleus the nucleus you're observing is coupled to (i.e. the second nucleus in the above explanation, but
not the first.) as well as the nature of the
covalent bond or bonds connecting the two.
Orbital overlap is also strongly dependent on
symmetry. This means that the magnitude of the coupling observed between nuclei that are more than one bond apart is dependent on the angles between the bonds. The effect these
torsion angles has on the observed J-coupling can be predicted with reasonable accuracy by a series of equations called the
Karplus Equations. J-coupling is commonly used to help determine the 3-dimensional structures of relatively complicated
molecules such as
folded proteins,
DNA and
RNA with the help of these equations. One just measures the J-coupling between two nuclei a known number of bonds apart, and the Karplus Equations can tell you the possible angles those intervening bonds can take, thus greatly reducing the
degeneracy (i.e. the number of possible structures) left over after the
NOE mapping process.