A novel NMR experiment allows one to characterize slow motion in macromolecules. The method exploits the fact that motions, such as rotation about dihedral angles, induce correlated fluctuations of the isotropic chemical shifts of the nuclei in the vicinity. The relaxation of two-spin coherences involving C(alpha) and Cbeta nuclei in proteins provides information about correlated fluctuations of the isotropic chemical shifts of C(alpha) and Cbeta. The difference between the relaxation rates of double- and zero-quantum coherences and is shown to be affected 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.
Isomerization kinetics of lanthanide(III) complexes with the pendantarm macrocyclic ligand 1,4,7,10-tetrakis(2-hydroxyethyl)-1,4,7,10tetraazacyclododecane †
A method is described to determine long-range cross-correlations between the modulations of an anisotropic chemical shift (e.g., of a C' carbonyl carbon in a protein) and the fluctuations of a weak long-range dipolar interaction (e.g., in cross-correlation between the same C' carbonyl and the H(N) proton of the neighboring amide group). Such long-range correlations are difficult to measure because the corresponding long-range scalar couplings are so small that Redfield's secular approximation is often violated. The method, which combines features of single- and double-quantum NMR spectroscopy, allows one to cancel the effects of dominant short-range dipolar interactions (e.g., between the CSA of the amide nitrogen N and the dipolar coupling to its attached proton H(N)) and is designed so that the secular approximation is rescued even if the scalar coupling between the long-range dipolar coupling partners is very small. The cross-correlation rates thus determined in ubiquitin cover a wide range because of local motions and variations of the CSA tensors.
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