Empirical 3JHN-HA values and angular backbone constraints can be interconverted following well-known Karplus relationships: 93JACS7772 , 94JMRB99-104 .

3JHN-HA=6.64*cos2(fi-60) - 1.43*cos(fi-60) + 1.86 ( 96JACS2483 )
3JHN-HA=6.47*cos2(fi-60) - 1.43*cos(fi-60) + 2.03 ( 97JACS6360 )
3JHN-HA=7.90*cos2(fi-60) - 1.05*cos(fi-60) + 0.65 ( 99JB1-14 )

where fi is the backbone torsion angle.

The 3JHN-HA coupling constants is a valuable NMR restraint to distinguish the secondary structure in proteins. The general trends are: Examples found in the bibliography: lysozyme ( 91BIO986 ) and ubiquitin ( 96JACS2483 ).
The most simple way to measure this coupling constant in unlabeled proteins is fitting the antiphase cross-peak pattern of a phase-sensitive 2D COSY-DQF spectrum to an specific linewidth. However, because the linewidths of large proteins are largely in excess of 4-5 Hz and frequently comparable to 1H-1H coupling constants, accurate values are better measured from E.COSY cross-peaks ( 85JACS6394 ). Other widely used alternatives to be applied on labeled proteins are:
Impact of direct refinement against 3JHN-HA on protein structure determination ( 94JMRB99-104 )

The same dihedral angle information can be obtained from other coupling constants values: