NMR relaxation studies can provide detailed insight into internal motions in biomolecules ( 96JPC13293). In the past decade, ¹⁵N spin relaxation methods have been developed which utilize the ¹⁵N-¹H bond vector as a probe to sense protein backbone dynamics. However, a single probe vector cannot properly characterize the anisotropy of local motions. It is thus appropiate to employ relaxation probes to monitor such local motions from different alternatives, such as ¹³CO relaxation data or cross-correlation data.

The study of dynamic motion in proteins relies upon the accurate determination of several relaxation rates. Thus, heteronuclear ¹³C and ¹⁵N relaxation provides useful information on internal dynamics of labeled proteins because of the dependence of their longitudinal (T₁) and transverse (T₂) relaxation times on rotational diffusion anisotropy ( 98PROG207). The relaxation behavior of an heteronuclear S spin is basically governed by two mechanisms:

1. The heteronuclear dipole-dipole interaction (DD) in which the S spins experience the local dipolar field of the directly bonded I spin.
2. The S spin chemical shift anisotropy (CSA) in which the relaxation is caused by fluctuations in the local shielding of the S spin.

Useful relaxation data can be extracted from the following spin orders:

Autorelaxation

Transverse Relaxation Rates: R_S(S_x) and R_I(I_x)

Longitudinal Relaxation Rates: R_S(S_z) and R_I(I_z)

Longitudinal spin-order : R_IS(2I_zS_z)

Single-quantum antiphase coherence : R_IS(2I_zS_x) and R_IS(2I_xS_z)

Multiple-quantum coherence: R_IS(2I₁S₁) and R_IS(2I₁S_-1)

Cross-relaxation

Heteronuclear dipolar cross-relaxation: R_S(I_z ^__> S_z)

Proton longitudinal cross-relaxation or NOE: R_I(I_z ^__> I'_z)

Proton transverse cross-relaxation or ROE: R_I(I_x ^__> I'_x)

Longitudinal and transverse CSA-DD cross-correlated cross-relaxation: R_S(S_z ^__> 2I_zS_z) and R_S(S_x ^__> 2I_zS_x)