Evidence of slow motions by cross-correlated chemical shift modulation in deuterated and protonated proteins

Cross-correlated fluctuations of isotropic chemical shifts can provide evidence for slow motions in biomolecules. Slow side-chain dynamics have been investigated in N-15 and C-13 enriched ubiquitin by monitoring the relaxation of C-alpha-C-beta two-spin coherences (Frueh et a]., 2001). This method, which had hitherto been demonstrated only for protonated ubiquitin, has now been applied to both protonated and deuterated proteins. Deuteration reduces the dipole-dipole contributions to the DD/DD cross-correlation, thus facilitating the observation of subtle effects due to cross-correlation of the fluctuations of the isotropic C-13 chemical shifts. The decays of double- and zero-quantum coherences are significantly slower in the deuterated protein than in the protonated sample. Slow motions are found both in loops and in secondary structure elements.