| Abstract: |
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 μs-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 15N, 13Cα and 13C′. 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 χ which represents the extent of correlated motion on the slow (μs-ms) timescale. This methodology has been applied to 15N,13C,REDPRO-2H-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 α-helix and in the loop connecting the strands β4 and β5. 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 13Cα chemical shift tensor on local secondary structure of the protein backbone. |
