Probing slow backbone dynamics in proteins using TROSY-based experiments to detect cross-correlated time-modulation of isotropic chemical shifts.

The difference in the relaxation rates of zero-quantum (ZQ) and double-quantum (DQ) coherences is the result of three principal mechanisms. These include the cross-correlation between the chemical shift anisotropies of the two participating nuclei, dipolar interactions with remote protons as well as interference effects due to the time-modulation of their isotropic chemical shifts as a consequence of slow micros-ms dynamics. The last effect when present, dominates the others resulting in large differences between the relaxation rates of ZQ and DQ coherences. We present here four sets of TROSY-based (Salzmann et al., 1998) experiments that measure this effect for several pairs of backbone nuclei including (15)N, (13)C(alpha) and (13)C'. These experiments allow the detection of the presence of slow dynamic processes in the protein backbone including correlated motion over two and three bonds. Further, we define a new parameter chi which represents the extent of correlated motion on the slow (micros-ms) timescale. This methodology has been applied to (15)N,(13)C,REDPRO-(2)H-labeled (Shekhtman et al., 2002) human ubiquitin. The ubiquitin backbone is seen to exhibit extensive dynamics on the slow timescale. This is most pronounced in several residues in the N-terminal region of the alpha-helix and in the loop connecting the strands beta(4) and beta(5). These residues which include Glu24, Asn25, Glu51 and Asp 52 form a continuous surface. As an additional benefit, the measured rates confirm the dependence of the (13)C(alpha) chemical shift tensor on local secondary structure of the protein backbone.