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 H-1 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 H-1-H-1 dipolar interactions and imperfections in H-1 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 H-1 chemical shift difference between the exchanging states is extracted from a combination of C-13 single quantum and C-13-H-1 multiple quantum dispersion profiles. The approach is demonstrated with an application to a folding/unfolding reaction involving a G48M mutant Fyn SH3 domain.