Dipolar dynamic frequency shifts in multiple-quantum spectra of methyl groups in proteins: correlation with side-chain motion

Small deviations from the expected relative positions of multiplet components in double- and zero-quantum H-1-C-13 methyl correlation maps have been observed in spectra recorded on a 7-kDa protein. These dynamic frequency shifts (DFS) are the result of dipolar cross-correlations that derive from fields produced by the spins within the methyl groups. The shifts have been quantified and compared with values calculated from a Redfield analysis. Good agreement is noted between the signs of the predicted and experimentally observed relative shifts of lines in both F-1 and F-2 dimensions of spectra, as well as between the magnitudes of the calculated and observed shifts in the F-2 (H-1) dimension. The experimental DFS values show a reasonable correlation with H-2 relaxation-derived measures of methyl side-chain dynamics, as expected from theory. This suggests that in cases where such shifts can be quantified, they can serve as qualitative measures of motion. Copyright (C) 2006 John Wiley & Sons, Ltd.