To determine the spectral density values, one must perform relaxation NMR measurements based on T₁ and T₂ (or T₁(ro)) measurements of ¹H and ¹⁵N and heteronuclear ¹H-¹⁵N NOE. Extraction of relaxation rates involves recording a series of 2D heteronuclear correlation HSQC-based or related experiments in which a relaxation building block of duration T is applied just before the S-evolution t₁ period. This building block must be carefully designed in order to avoid other relaxation mechanisms to contribute to the detected signal. The processing usually involves a fitting procedure on the cross-peaks intensities to a single-exponential decay in order to extract the corresponding autorelaxation rate.

The relaxation of protonated ¹⁵N nuclei is mediated primarily by dipole-dipole interactions with the attached protons and secondarily by the ¹⁵N chemical shift anisotropy. Typically, ¹⁵N T₁, ¹⁵N T₂ and ¹⁵N{¹H} NOE data are usually determined by modified 2D proton-detected HSQC-like NMR experiments on ¹⁵N-labeled proteins :

¹⁵N T₁ Measurements
¹⁵N T₂ Measurements
¹⁵N T_1ro Measurements
¹⁵N{¹H} NOE Measurements
Others