Measurement of dipolar couplings for methylene and methyl sites in weakly oriented macromolecules and their use in structure determination.

A simple and effective method is described for simultaneously measuring dipolar couplings for methine, methylene, and methyl groups in weakly oriented macromolecules. The method is a J-modulated 3D version of the well-known [1H-13C] CT-HSQC experiment, from which the J and dipolar information are most accurately extracted by using time-domain fitting in the third, constant-time dimension. For CH2-sites, the method generally yields only the sum of the two individual 13C-1H couplings. Structure calculations are carried out by minimizing the deviation between the measured sum, and the sum predicted for each methylene on the basis of the structure. For rapidly spinning methyl groups the dipolar contribution to the splitting of the outer 13C quartet components can be used directly to constrain the orientation of the C-CH3 bond. Measured sidechain dipolar couplings are in good agreement with an ensemble of NMR structures calculated without use of these couplings.