The Karplus relationship is an equation named after its discoverer, Martin Karplus and describes a correlation of the 3J coupling constant and the dihedral angle in the NMR spectroscopy.
Accordingly, the following equation applies:
Here, the coupling constant 3J, and the dihedral angle, and are empirical parameters which are determined by the substitution and functional groups of the constituent atoms. The relationship is used to obtain the magnitude of the 3JH, H coupling (also called vicinal coupling ) estimate. The coupling constant is close to 90 ° at very low torsion angles and at angles of 0 ° and 180 ° is significantly greater than normal. The relationship between the geometry and the coupling constant is valuable for determining and for calculating backbone conformations of proteins.