Correlated dynamics between protein HN and HC bonds observed by NMR cross relaxation.

Although collective dynamics of atom groups steer many biologically relevant processes in biomacromolecules, most atomic resolution motional studies focus on isolated bonds. In this study, a new method is introduced to assess correlated dynamics between bond vectors by cross relaxation nuclear magnetic resonance (NMR). Dipole-dipole cross correlated relaxation rates between intra- and inter-residual H(N)-N and H(alpha)-C(alpha) in the 56 residue protein GB3 are measured with high accuracy. It is demonstrated that the assumption of anisotropic molecular tumbling is necessary to evaluate rates accurately and predictions from the static structure using effective bond lengths of 1.041 and 1.117 A for H(N)-N and H(alpha)-C(alpha) are within 3% of both experimental intra- and inter-residual rates. Deviations are matched to models of different degrees of motional correlation. These models are based on previously determined orientations and motional amplitudes from residual dipolar couplings with high accuracy and precision. Clear evidence of correlated motion in the loops comprising residues 10-14, 20-22, and 47-50 and anticorrelated motion in the alpha helix comprising 23-38 is presented. Somewhat weaker correlation is observed in the beta strands 2-4, which have previously been shown to exhibit slow correlated motional modes.