A novel NMR expt. allows one to characterize slow motion in macromols. The method exploits the fact that motions, such as rotation about dihedral angles, induce correlated fluctuations of the isotropic chem. shifts of the nuclei in the vicinity. The relaxation of two-spin coherences involving Ca and Cb nuclei in proteins provides information about correlated fluctuations of the isotropic chem. shifts of Ca and Cb. The difference between the relaxation rates of double- and zero-quantum coherences C+a C+b and C+a C-b is shown to be affected by cross-correlated chem. shift modulation. In ubiquitin, evidence for slow motion is found in loops or near the ends of b-strands and a-helixes. [on SciFinder (R)]