Triple Quantum Decoherence under Multiple Refocusing: Slow Correlated Chemical Shift Modulations of C' and N Nuclei in Proteins

A new expt. allows the identification of residues that feature slow conformational exchange in macromols. Rotations about dihedral angles that are slower than the global correlation time tc cause a modulation of the isotropic chem. shifts of the nuclei. If these fluctuations are correlated they induce a differential line broadening between three-spin single-quantum and triple-quantum coherences involving three nuclei such as the carbonyl C', the neighboring amide nitrogen N and the amide proton HN belonging to a pair of consecutive amino acids. A cross-correlated relaxation rate RCS/CSC'N can be detd. that corresponds to the sum of the isotropic and anisotropic contributions to the chem. shift modulations of the carbonyl carbon and nitrogen nuclei. Only the isotropic contributions depend on the pulse repetition rate of a multiple-refocusing sequence. An attenuation of the relaxation rate with increasing pulse repetition rate can therefore be attributed to slow motions. The asparagine N25 residue of ubiquitin, located in the first a-helix, is shown to feature significant slow conformational exchange. [on SciFinder (R)]