The experimental relaxation data obtained from NMR measurements are interpreted on a residue-by-residue basis and they are commonly related to motional parameters using several approaches ( 99JPCB83 ):

• Model-free approach. ( 82JACS4546 and 82JACS4559 ). This approach allows for a separation between the relatively slow (ns) overall reorientational motion and the relatively fast (ps-ns) internal motions. The equations for the relaxation rates are fitted using three parameters, namely:
• The generalized order parameter (S²) that describes the degree of spatial restriction of the bond vector in the molecular frame, which can take on a value between o and 1, corresponding to complete isotropic disorder and fixed orientation, respectively, of this relevant vector
• The overall molecular rotational correlation time (t_c)
• The effective correlation time for internal motion (t_i).
C(t)=[ S² + (1-S²) exp(-t/t_i) ] exp(-t/t_c)

Some drawbacks: loss of information on nano-second time-scale motions and application on anisotropic molecules. However, novel parametrization incorporating motional restrictions occurring on relatively slow and fast time scales have been reported:

• Extended model free approach ( 90JACS4989 ). Application to the analysis of slow interdomain motion ( 01JACS3953 and 01JACS11484)
• Anisotropic molecular diffusion ( 99JACS6829 ).
• Comparison of different models (03JB25-25)
• Assessment of accuracy and model selectino protocols (04JB243-29)
• Spectral density mapping. ( 92BIO8571 ) Six independent relaxation rates are required to determine the spectral density J(w), at the five frequencies 0, w_x, w_H-w_x, w_H, and w_H+w_x and the proton-proton dipolar term.
• Reduced spectral density mapping. ( 95JB153 , 00JB83-18 , 01JMR32-151 ) Three relaxation rates are required corresponding to the measurement of T₁, T₂ and heteronuclear NOEs. Some examples in: 98JMB221-283 and 01BIO2743 .
• A Bayesian Statistical Method ( 99JMR408-139 )
• Other Methods
  A new approach to visualizing spectral density functions and deriving motional correlation time distributions (01JMR132-152)
  Slowly Relaxing Local Structure or SRLS ( 01JACS3055 )
  Reorientational eigenmode dynamics to be applied in mobile protein parts ( 01JACS7305 )
  Potential bias in nmr relaxation data introduced by peak i ntensity analysis and curve fitting methods (01JB1-21)
  MUNIN: use of three-way decomposition (01JB263-21)
  iRED: isotropic Reorientational eigenmode dynamics, to be applied to folded and unfolded structures (02JACS4522)
  Accurate description of global tumbling (02JB215-24)