Cross-correlated chemical shift modulation: A signature of slow internal motions in proteins

A novel NMR experiment allows one to characterize slow motion in macromolec ules. The method exploits the fact that motions, such as rotation about dih edral angles, induce correlated fluctuations of the isotropic chemical shif ts of the nuclei in the vicinity. The relaxation of two-spin coherences inv olving C-alpha and C-beta nuclei in proteins provides information about cor related fluctuations of the isotropic chemical shifts of C-alpha and C-beta . The difference between the relaxation rates of double- and zero-quantum c oherences C-+(alpha) C-+(beta) and C-+(alpha) C-beta is shown to be affecte d by cross-correlated chemical shift modulation. In ubiquitin- evidence for slow motion is found in loops or near the ends of beta -strands and alpha -helices.