Cross-correlated spin relaxation effects in methyl 1H CPMG-based relaxation dispersion experiments: complications and a simple solution.

Artifacts associated with the measurement of methyl (1)H single quantum CPMG-based relaxation dispersion profiles are described. These artifacts arise due to the combination of cross-correlated spin relaxation effects involving intra-methyl (1)H-(1)H dipolar interactions and imperfections in (1)H refocusing pulses that are applied during CPMG intervals that quantify the effects of chemical exchange on measured transverse relaxation rates. As a result substantial errors in extracted exchange parameters can be obtained. A simple 'work-around' is presented where the (1)H chemical shift difference between the exchanging states is extracted from a combination of (13)C single quantum and (13)C-(1)H multiple quantum dispersion profiles. The approach is demonstrated with an application to a folding/unfolding reaction involving a G48M mutant Fyn SH3 domain.