The main source of geometric information used in NMR structure determination of biomolecules lies in short interproton distance restraints derived from Nuclear Overhauser effect (NOE) measurements.

The NOE observed for two spins A and B at short mixing times is proportional to the inverse sixth power of the distance between them, <1/rAB6>, so its intensity falls off very rapidly with increasing distance between proton pairs. Consequently, NOEs usually are observed only for proton pairs separated by 5-6 A.

2D/3D/4D experiments to measure NOEs in proteins.
Correlation between NOE experiments
Isotope edited/filtered NOE experiments
15N {1H} NOE
13C {1H} NOE

NOEs vs secondary structure
Diagonal Suppression in NOESY experiments