Measurement of dipole-dipole cross-correlated relaxation in fast rotating CH2D groups

Studies of protein dynamics are key to understanding their biological function. NMR relaxation studies of proteins to date have focused primarily on characterizing backbone dynamics. In this paper, we focus on the aliphatic side-chains (Ala, Thr, Val, Leu, and Ile) with the goal of deriving dynamical information on the motion of terminal methyl groups. Dipole-dipole cross-correlated cross-relaxation is analyzed in a fast rotating CH(2)D group, as found in partially deuteriated proteins. In comparison with previous studies on AMX spin systems (methylene C(beta)H(2) groups), the fast rotation of the methyl group makes a number of relaxation pathways efficient, through the coherence C(+)H(1)(+)H(2)(-)+C(+)H(1)(-)H (2)(+). Several pulse schemes were designed to evaluate these relaxation rates: the measured values are small and well predicted by taking into account the complete relaxation network, but they remain strongly influenced by 1H-1H relaxation with all protons in the neighborhood of the CH(2)D moiety. The prospects and limitations of this method are discussed in comparison with 2H relaxation measurements.