Cross-correlated fluctuations of isotropic chem. shifts can provide evidence for slow motions in biomols. Slow side-chain dynamics have been investigated in 15N and 13C enriched ubiquitin by monitoring the relaxation of Ca-Cb two-spin coherences (Frueh et al., 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 13C chem. 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. [on SciFinder (R)]