Interference between cross-correlated relaxation and the measurement of scalar and dipolar couplings by Quantitative J.

The effects of cross-correlated relaxation in Quantitative J methods are analyzed. One-bond 1H-13C scalar and dipolar couplings of protein methine and methylene sites are obtained by monitoring proton and carbon magnetization in Quantitative J experiments. We find that scalar and dipolar couplings of the same pair of nuclei vary depending on the type of magnetization involved. These discrepancies can be as large as several Hz for methylene moieties. The contribution of dynamic frequency shifts, which are known to affect J couplings, is too small to explain the observed differences. We show that processes of magnetization transfer originated by cross-correlated relaxation are largely responsible for these discrepancies. We estimate the error transferred to methylene J values by cross-correlation interference, and show that is close to the experimentally observed one. Furthermore, this analysis indicates that cross-correlated relaxation effects under isotropic and anisotropic media differ, indicating that errors are not cancelled in residual dipolar coupling measurements.